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Sample chapter
1.1 Electrical energy 1.2 Voltage and current 1.3 Resistance
1.4 Electric circuits
1.5 Electricity and safety
1
Electrical appliances
Exploring Verkennen
You have forgotten your mobile phone charger! This is not a good start to your holiday. Might you be able to use your laptop charger to charge up your phone? Maybe you can use batteries or a dynamo. Can you try out all of these options, or will this break your phone? How can you find out?
This chapter is about electrical appliances. You will learn about the energy consumption of electrical appliances, voltage and current, and circuits and safety.
I
Do you know this already?
A kettle uses electrical energy to heat water. a Name another three household appliances that heat something. b Name three appliances where something rotates. c Name an appliance that uses a large amount of electrical energy.
II
True or false?
a When no electricity is flowing there is no voltage. b All of the electrical current supplied by the electricity company flows
c Lights are less bright when more lights are connected to a plug socket. d A plug socket which has no appliances plugged into it does not use any
For each of the following statements answer true or false.
back to that company via a cable.
energy.
III Flow comparison
urrent flows from the positive to the negative terminal in an electrical C appliance. Table 1 shows other situations where something flows from one point to another. Copy and complete Table 1. Situation
What is flowing?
milk shake current through wire students in corridor
When does it flow easiest? wide road
TABLE 1
IV Household appliances All electrical appliances in the home use electrical energy. They are all connected to the same power supply and they all use electric current. a What is the voltage of the domestic power supply? Some appliances use more energy than others. b How can you see this from the appliance? c What is different with an appliance that uses more electricity: the voltage or the current?
Ch1 Electrical appliances
6
7
Discovering
Earning money
2 Give one reason why this could provide a greater saving than Nick’s plan. 3 Also give a reason why this may not be the case and Ivy’s plan could instead provide a smaller saving. Andrew has noticed that when using the kettle more water is boiled than is actually needed. He does a test and observes that if you heat exactly the amount of water you need, the kettle switches off 20 seconds sooner than with the generous amount of water that he usually uses. The kettle is used to boil water for tea five times a day on average. 4 Why could this measure provide a large saving?
FIG 1
The electricity bill for the Cash family increases every year. It is time to start saving energy. John and Elisabeth Cash tell their children that they can keep the savings from one energy-saving measure each, which they themselves think of, for the first year. The three children, Nick, Ivy and Andrew, decide to turn this into a competition.
Investigate! Nick promises that from now on, when he goes to school, he will remember to turn off his bedroom light. On a website about saving energy he has found out how much this will save. Leaving a 60 W light on for 8 hours a day, 100 times in a year, costs about € 10.
5 Why might the saving not be as large as expected? Table 2 shows the power rating for each appliance with which the children are trying to save energy. This gives how much energy an appliance uses per second. For Nick’s plan the amount of time that the light is now not switched on is also given. Name
Uses
Power rating
Time
Cost
Nick light 60 W 800 hours € 10 Ivy five chargers 4 W per charger Andrew kettle 1 800 W TABLE 2
6 Calculate how much Ivy and Andrew will save per year, using Table 2. 7 Which of the three children will save the most money?
1 Give two reasons why 100 days is a better estimate of the number of days of savings, and not 365 days. Ivy searches the web and finds some information on a Belgian website.
Read Source 1. Ivy keeps a close eye on the chargers used for the five telephones they have at home. The chargers are normally left plugged in. From now on Ivy is going to unplug each charger after the phone has recharged.
8
8 Most people have the following appliances at home that use a large amount of electricity: washing machine, tumble dryer, dishwasher, lighting, refrigerator and television. a Why is it a good idea to look at these particular appliances if you want to save energy? b Choose one of these appliances and think of a way to save energy when using it. c Estimate the amount that you would then save in one year. d Name another appliance which uses a lot of electricity. This can be an appliance that not many people have at home.
1.1
Electrical energy
Discovering
FIG 2
End results Draw up the complete savings plan for the Cash family. This should include: • the various ways in which energy can be saved; • the advantages and disadvantages of each method; • what can be done to minimise the disadvantages; • an estimate of the amount saved with each energysaving measure. Write a script for a sketch about the topic of saving energy. Film the sketch and hand it in to your teacher. The sketch must show why it is important to save energy and in which ways energy can be saved around the home.
Source 1
Power-hungry devices
Some appliances use electricity even when the appliance is not being used, or it appears to be switched off. This is stand-by, no-load or continuity consumption. This dormant power usage can easily reach 10% of the total electricity usage per year. Half of this stand-by usage can be prevented by taking the following measures. • Use the on/off switch of an appliance instead of the stand-by mode. • Remove the plug from the plug socket with appliances which you do not use often. • If several appliances are next to each other, plug these into a multi-way socket with a switch. Then switch the multi-way socket off after use. • Halogen lights often have a switch between the lamp and the transformer. The transformer continues to use power after the light is switched off. This is easily avoided if you put the switch between the transformer and the plug socket. • Chargers for a hand-held vacuum cleaner, toothbrush or mobile phone use electricity even when they are no longer charging the device. Remove them from the plug socket when they are not charging or when the appliance is fully charged.
www.positieve-economie.be
FIG 3
Ch1 Electrical appliances
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Understanding Onderdeel
Electrical appliances are increasingly more economical. Tekst... However, households now use more energy than just a few years ago, as yet more electrical appliances appear in homes. This is not good for the environment and is also expensive. So when purchasing electrical appliances you should look at the energy consumption.
Energy loss
A light bulb is used to provide light. However, the filament converts just 5% of the electrical energy into light. The remaining 95% is converted into heat. A light bulb is therefore not very efficient. Energy is lost in almost every energy conversion, mostly in the form of heat. Power sources and electrical wires as well as appliances lose energy in the form of heat. This is unavoidable. Batteries get hot when they are used to power an appliance. Current-carrying wires also get hot. Try touching the lead of a washing machine when it has been running for a while.
Efficiency
An incandescent light bulb which converts just 5% of the electrical energy into light is said to have an efficiency of 5%.
FIG 4 This flying wing uses electrical energy economically.
Energy-saving light bulbs and LED bulbs have a high efficiency. As electrical energy becomes increasingly expensive, manufacturers develop appliances with higher efficiency.
Key terms
Energy converter, efficiency, power, watt, kilowatt, kilowatt-hour, joule.
Appliances convert energy
An electric current provides an appliance with energy. This energy comes from a battery or mains plug socket. The electrical appliances convert the electrical energy into other forms of energy such as heat, light, motion or radiation. FIG 6 These batteries have clearly become too hot.
electrical energy
motion
Economical cars
sound
A car is also an energy converter. The engine runs using the energy released by the combustion of petrol. This also releases a lot of heat, so the engine has to be cooled with water. Hybrid cars and electric cars are more economical with energy. They generate electrical energy during braking which is then stored in an accumulator. This means that a large amount of energy is recovered, particularly when driving in built-up areas when a driver must brake often. For city driving a hybrid car runs approximately double the distance compared to an ordinary car using the same amount of petrol.
heat
FIG 5 Some, but not all of the energy is used for vacuum cleaning.
Figure 5 shows the various forms of energy drawn as arrows entering and leaving the appliance. The appliance is an energy converter. The vacuum cleaner converts the electrical energy into motion, sound and heat.
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1.1 0.0
Electrical energy Titel paragraaf
The energy label
The energy label shows how efficient an appliance is. Figure 7 shows the energy label of an air conditioning unit. This is an A rating label, which means that the air conditioning unit is very efficient. The ratings A+ and A++ are for appliances which are even more efficient. Ratings B to G are for less efficient appliances.
Energy
Air conditioner
Manufacturer Outside unit Inside unit
Understanding Onderdeel
Energy and power Activity 1, page 20. For all appliances the energy consumption depends upon the number of hours that the appliance is in use. The power of an appliance is how much energy the appliance uses per second. The symbol for power is P and the unit of power is the watt (W). With many electrical appliances the power rating is given on a type plate or sticker. An 11 W lamp uses 11 J every second (J = joule). For higher power appliances the unit kilowatt (kW) is used. 1 kW = 1 000 W. A 2,5 kW oven uses 2 500 J every second.
More efficient
Type:/7
7400 230 V Serie Nr.
50 Hz
240 W
08/04/119338
Less efficient Annual energy consumption kWh in cooling mode
FIG 8 The type plate shows the power of the appliance.
(Actual consumption will depend on how the appliance is used and climate)
Cooling output kW Energy efficiency ratio (EER) At full load (The higher the better)
Type Cooling only Cooling/heating
Air cooled Water cooled
Heat output kW Heating performance A = more efficient G = less efficient
Noise (dB(A) re 1p/W)
The power of an appliance is a measure of how powerful it is. A 300 W kitchen mixer is more powerful than a 100 W mixer, and a 1 000 W drill is more powerful than a 400 W drill. With many appliances the power is not constant. A washing machine uses a large amount of power when heating, but uses much less power when the laundry is just turning in the drum. A refrigerator switches on and off continuously to keep the temperature constant. The power consumption of an MP3 player is higher when the music is played louder.
Further information contained in product brochures. Norm EN 14511 Air-conditioner Energy label Directive 2002/31/EC
FIG 7 The energy label indicates how economical an appliance is.
Ch1 Electrical appliances
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Understanding Onderdeel
Questions: Understanding
Tekst...
1 For each of the following statements answer true or false. If the statement is false, change it to make a statement which is true. a Electrical appliances convert heat and motion into electricity. b With electrical appliances energy is also ‘lost’ in the wire from the appliance to the plug socket. c With an incandescent light bulb 5% of the electrical energy is lost as heat. d The power of an appliance is how much energy the appliance uses in a year. e The unit of power is the kilowatt-hour. f The energy label tells you how much energy the appliance uses in a year with average use.
FIG 9 You can measure the power of an appliance using an energy meter.
An energy meter enables you to measure the power consumption of an appliance. Then you can see that the power used by an electric drill increases when the drill is turning faster, for example. To measure the energy consumption you start a measurement. You can see the number of kilowatt-hours (kWh) gradually increasing on the display. The kilowatt-hour is an energy unit which is used for electrical energy consumption. The standard unit for measuring energy is the joule. This can also be used for other types of energy.
Summary
• Electricity carries energy to an appliance. • An appliance converts the electrical energy into other forms of energy such as light, heat or sound. • An energy label shows how energy-efficient an appliance is. • The power is the amount of energy converted by the appliance per second. • The power of an appliance is expressed in watts (W) or kilowatts (kW). • The energy consumption of an appliance depends on the power rating of the appliance and the number of hours that the appliance is in use. • The efficiency of an appliance gives the percentage of energy used usefully.
2 A vacuum cleaner is marked 230 V – 1 200 W. a What is the power rating of this vacuum cleaner? b How much energy does the vacuum cleaner use per second?
3 You need to replace a broken 25 W incandescent light
bulb. You can choose from three bulbs: an 8 W energysaving bulb, a 15 W energy-saving bulb and a 15 W incandescent light bulb. a Which bulb produces the most light? b Which bulb uses the least energy? c Explain which bulb is the best choice to replace the broken light bulb.
4 Copy Table 3. For each appliance fill in which type of energy is required and which type of energy is undesirable. Appliance
light bulb kitchen mixer hair-dryer soldering iron radio TABLE 3
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Type of energy required
Unwanted energy type
1.1 0.0
Electrical energy Titel paragraaf
Understanding Onderdeel
5 Put the following words in the correct places in the
a In which circumstances is a hybrid car much more
paragraph below. Copy the paragraph and fill in the correct words. hair-dryer, heat, useful, little, energy, light, heat, light bulb, electrical
b Which headline appeared in which magazine? c Explain how the vehicle could be tested fairly to
... is converted in an electrical appliance. ... energy enters the appliance and other types of energy leave the appliance. The energy that is required is called ... energy. A lot of energy is lost during conversion in the form of ... A light bulb generates relatively little ... and a large amount of ... You could therefore use a ... to incubate eggs. On the other hand some appliances are designed to generate heat. With these appliances relatively ... energy is lost. A ... is an appliance which generates heat from electrical energy.
6 A carpenter has a powerful electric drill and an electric
screwdriver. He hardly uses the powerful drill, but uses the screwdriver almost all day long. a Explain which appliance has a higher power rating. b Which appliance will use more electricity in a year?
7 An air conditioning unit can both heat and cool. Figure 7
shows the energy label of an air conditioning unit. a How much energy does the unit use per second when cooling? b How much energy does the unit use per second when heating? The power rating of the unit is very high. c Explain why it is still possible for this air conditioning unit to have a green energy label.
8 The bill from the electricity supply company shows the
number of kilowatt-hours (kWh) of electricity supplied. The kilowatt-hour meter in the consumer unit measures the energy usage. a Is the kilowatt-hour a unit associated with power or with energy? b How many kWh is given on the energy label for the air conditioning unit in figure 7? c In which time period does the air conditioning unit use this amount of kWh?
economical than an ordinary petrol car?
establish the energy usage.
10 The unit kilowatt-hour is only used for electrical energy. a What is the standard unit for energy? b How do you convert kWh into this standard unit? c Convert the energy usage of the air conditioning unit in figure 7 to the standard unit.
d Can you now think of a reason why the energy
company does not use the standard unit, but uses kWh instead?
11 Modern electrical appliances often have a higher efficiency than older appliances. a Explain what is meant by the term efficiency. b How can you tell that energy is lost by touching the battery of a mobile phone? With a particular appliance three-quarters of the energy is lost in (non-useful) heat. c What is the efficiency of this appliance? Someone replaces incandescent light bulbs with an efficiency of 5% by energy-saving bulbs with an efficiency of 40%. d Explain which of the following statements is true: 1 The cost of energy for the lights falls by 35%. 2 The cost of energy for the lights is then 8 times less.
12 A rechargeable AA battery can store about 12 kJ of energy and costs about € 4. An alkaline battery costs about € 1,20 and contains 8 kJ of energy. a Convert the energy stored in each battery into kWh. b Name an appliance or other application where it is better to use rechargeable batteries than alkaline batteries. c Calculate the price of 1 kWh energy with the alkaline battery. d Calculate how often the rechargeable battery must be charged until the cost of the stored energy is the same as the purchase price of the battery. Assume a cost of € 0,20 per kWh.
9 Two car magazines test the energy usage of a new hybrid car. The ‘Car plus’ magazine drives the car through Amsterdam. The ‘New Car’ magazine drives the car on the motorway. The headlines ’50 km on 1 litre of petrol!’ and ’New hybrid is not economical after all’ appear above the articles in the magazines.
Ch1 Electrical appliances
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Comprehending Onderdeel
Tekst... Electrical energy
Activity 2, page 20. The energy consumption of an appliance is determined by two quantities: the power of the appliance and how long the appliance is in use. An appliance with a power of one watt converts one joule every second. An appliance with a power of 150 W uses per minute 60 × 150 J = 9 000 J. Conversely you can calculate the power of an appliance from the energy used and the time.
Calculate the electrical energy consumption of an appliance of 2 500 W that is used for 2 hours. Fill in the formula E = P × t: E = 2,5 kW × 2 h = 5,0 kWh The appliance has therefore converted 5 kWh.
Example calculation 1
The units kilowatt-hour and joule can be converted using a fixed conversion factor. One kilowatt-hour is equal to 3,6 million joules. One kilowatt-hour of electrical energy costs approximately € 0,20.
Calculate the power of an appliance that has used 60 000 joules (60 kJ) in 20 minutes.
Energy consumption per year
The power is the amount of energy used by the appliance in 1 s. 20 minutes is 1 200 s. 60 000 J = 5 J/s = 5 W 1 200 s
Formula for energy
You can also use an formula for calculations with energy. To calculate the energy E in joules, multiply the power P in watts by the time t in seconds: energy = power × time This formula can also be expressed in symbols: E=P×t Where E is the energy converted (J), P is the power (W) and t is the time (s).
Calculating with kWh
The electricity company reads the power meter once a year. The meter measures energy in kilowatt-hours. Note that this is not kilowatts per hour: kilowatt-hour means kilowatts multiplied by hours. To calculate the energy in kilowatt-hours you use the same formula: energy = power × time. Now fill in the time as the number of hours that the appliance is in use, and the power as the number of kilowatts. You can see that the units are correct on both sides of the formula: kilowatt, hours and kilowatt-hours.
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Example calculation 2
The energy label of appliances gives how many kWh an appliance uses per year. This assumes average use of the appliance per year. There are considerable differences between appliances. The most economical fridge-freezer uses 125 kWh per year and the least economical uses 475 kWh per year. The difference of 350 kWh costs € 70 annually. An average person uses about 1 500 kWh of electricity per year. Some appliances use a large amount of energy. A tumble dryer uses more than twice the amount of electricity than a washing machine does. If you want to save money on your power bill, look at these energy-hungry appliances first. You can also calculate the annual consumption of an appliance yourself. To do this you have to estimate the number of hours that you use the appliance per year. The power is given on the appliance. You can then calculate the energy consumption using the formula for energy. Another method is to measure the energy consumption during one week using an energy meter. Then multiply the amount of energy you have measured by 52.
Example calculation 3 How much does the electricity used during an evening on the computer cost? You sit at your computer for six hours one evening. You calculate how much your 200 W (= 0,200 kW) computer uses: E=P×t E = 0,200 kW x 6 hours = 1,2 kWh 1,2 × € 0,20 = € 0,24
1.1 0.0
Electrical energy Titel paragraaf
Example calculation 4 An iPod has a power of 0,25 W. The rechargeable batteries can provide 3,5 kJ of energy. How long can you listen to your favourite music with fully charged batteries? E=P×t 3 500 J = 0,25 W × t gives t = 14 000 s Converting this to hours and minutes gives 3 hours 53 minutes.
Large and small numbers
With very large or very small numbers prefixes such as kilo and milli are used for convenience. Table 4 gives an overview of these prefixes. Prefix
Symbol Factor
micro milli kilo mega giga
μ 0,000 001 m 0,001 k 1 000 M 1 000 000 G 1 000 000 000
Comprehending Onderdeel
Figure 10 shows that the total electricity consumption of the Netherlands (domestic, industrial, transport etc.) has increased considerably since 1960, even though the efficiency of appliances has continued to rise. Around 1960 people had little more than a radio, a record player, a few lights and sometimes a black and white TV at home.
Efficiency Activity 3, page 21. Figure 11 shows what happens to the electrical energy in an incandescent light bulb. Of each 100 J just 5 J is converted into useful energy. The remainder is converted into heat. The efficiency of the light bulb is therefore 5%.
light 5 joules electric energy 100 joules
heat 95 joules
TABLE 4
3 μA is therefore 3 × 0,000 001 A = 0,000 003 A 2,5 MW = 2,5 × 1 000 000 W = 2 500 000 W
More and more electricity kWh 8000
FIG 11 A 100 W incandescent light bulb only gives 5 W of light.
You can use a ratio table to calculate the efficiency. The energy input is always 100 %. Einput Euseful
100 J 5 J
100%
: 100 and × 5
7000 6000 5000 4000 3000
Example calculation 5 The efficiency of a microwave oven is 42 %. 350 000 J of energy is required to heat up a meal. How much electrical energy does the microwave oven use? Use a ratio table.
2000
Einput 100% 100 × Euseful 42% 350 000 J 42
1000
Answer: 833 333 J, rounded off: 833 kJ.
0 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2008 FIG 10 Electrical energy consumption per inhabitant per year (source: CBS)
Ch1 Electrical appliances
15
Comprehending
Questions: Comprehending 13 For each of the following statements answer true or false. If the statement is false, change it to make a statement which is true. a The SI or ‘official’ unit for energy is the kilowatt-hour. b A kilowatt-hour means a kilowatt per hour. c To calculate the energy consumption you have to multiply the power by the time. d 100 W is the same as 6 kilojoules per minute. e A 100 W appliance uses approximately 500 kW in 5 hours.
14 Convert the following: a 2 500 mA =…A b 0,25 MW =…W c 0,039 GJ = … MJ d 380 kV =…V e 25 W = … kW f 400 000 kWh = … MWh 15 A chip pan with a power of 750 W melts the solid frying fat in 5 minutes. a How many joules of electrical energy does the appliance use in this time? b How many kilowatt-hours is this? c How much does this cost if one kWh costs € 0,20?
16 The batteries in a pocket torch can provide 8 000 J of
electrical energy. The light bulb has a power of 0,35 W. a Calculate for how many seconds you can have the torch switched on. b How many hours is this?
17 A clothes iron has a power of 800 W. It takes ten
minutes to iron a pair of trousers. If you have the trousers ironed professionally it costs € 2,00. a How many kilowatt-hours of electricity do you need to iron the trousers? b Show with a calculation that ironing the trousers yourself is cheaper than having them ironed. Once the iron is hot enough the thermostat switches the heater off. c Explain why the actual energy costs are lower than the calculated value.
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18 At Peter’s house there is a computer which is used by all family members. The power of the computer and monitor together is 250 W. Peter estimates that the computer is switched on for about ten hours a day. a Calculate how many kilowatt-hours of energy the computer uses per day according to this estimate. The computer is plugged in via an energy meter. In one week the computer uses 26 kWh. b Is this more or less than the estimated value? c Calculate the annual consumption of the computer using this measured value. A laptop has a power of 45 W. d Calculate how much money Peter’s family can save per year by using a laptop.
19 A certain refrigerator has a power of 250 W. This is the power of the pump which cools the interior. a Calculate how many joules the pump uses in one minute. b Calculate how many joules the appliance uses if the refrigerator pump runs for sixty minutes. c Convert the answer for question b to kWh. d Show how you could have calculated the answer in kWh much more quickly. In practice the refrigerator uses about 1 kWh per day. This is because the pump only operates for some of the time. e Calculate how many hours per day the refrigerator pump is running.
20 A decoder for a digital TV consumes 8,1 W of power when in use. When on stand-by the consumption is 6,7 W. The appliance is used for six hours a day. The rest of the time the appliance is on stand-by. a Calculate how many kWh the appliance consumes daily. b Calculate the annual energy consumption costs if one kWh costs 20 cents. c Explain why the energy consumption is not really affected by the number of hours that the decoder is switched on. With a new type of decoder the stand-by consumption has been reduced to 0,7 W. d Show that the energy costs are now less than half of what they were with the old type of decoder. Look at figure 10 again. e Explain why energy consumption continues to rise, even though most appliances have become increasingly more efficient.
1.1
Appliance
Electrical energy
Power (W)
Time for 1 kWh (hours)
light bulb 40 25 tumble dryer 3 000 computer 150 vacuum cleaner 650 music centre 50 television 250 TABLE 5
21 Table 5 gives the power of several appliances. a A 40 W light bulb can be used for 25 hours on 1 kWh. Check this with a calculation. b For each appliance calculate how long it will operate on 1 kWh of electrical energy. Write this in the last column.
Comprehending
24 Mopeds increasingly have an electric drive mechanism. In these mopeds energy is stored in an accumulator. The advertisement for such a moped includes the following paragraph: ‘An electric moped is much cheaper to run than a petrol moped. It costs about 40 eurocents to fully charge the accumulator. A charged accumulator contains 2 kWh of energy, which is sufficient to travel a distance of 130 km.’ a What is the cost of 1 kWh of electricity according to this advertisement? A petrol moped travels a distance of 35 km with 1 litre of petrol on average. b How much does it cost to travel a distance of 130 km with such a moped? Ask your teacher for the price of petrol.
22 There is a wind turbine with a power of 2,0 MW in Flevoland. This means that when there is enough wind the turbine supplies 2,0 million joules of electrical energy to the electricity network every second. a Calculate the amount of electrical energy that the turbine can supply in one year in kWh. In practice the turbine supplies 4 050 000 kWh in one year. b Explain the difference compared to your answer to question a. The electricity supplier pays the owner of the turbine 8 cents per kWh. c Calculate how much the owner of the turbine earns in one year.
FIG 12
25 Figure 12 shows a patio heater. A brochure for patio 23 A vacuum cleaner uses 0,8 kWh energy in half an hour. a Calculate the power of the vacuum cleaner. b Calculate how long the vacuum cleaner can operate
on one kilowatt-hour. In one year the vacuum cleaner uses 50 kWh. c Calculate how many hours the vacuum cleaner is used in one year. T he efficiency of the vacuum cleaner is 35%. d Calculate how many joules of heat the vacuum cleaner produces in half an hour.
heaters contains the following information: ‘For a householder the cost of sitting on a heated patio for 100 hours per year is € 34,00 per year, using a 2 000 W heater. For a cafe owner the kilowatt-hour costs are proportionally lower, because this extra energy consumption falls under a lower scale for energy tax purposes and he purchases a much greater amount of energy.’
a Calculate the price used for 1 kWh of electrical
b Explain that the efficiency of a patio heater is very
c Why is a patio heater nonetheless not very
energy in this brochure. high, from a scientific point of view.
efficient?
d What is your opinion regarding the use of patio heaters?
Ch1 Electrical appliances
17
Knowing more Onderdeel
Tekst... using smart technology Saving
Electrical appliances can be made even more efficient using smart technology. Modern televisions are much more efficient than the TVs used a few years ago. Incandescent light bulbs have been replaced by light emitting diode (LED) lamps. Refrigerators and freezers are increasingly more efficient. Smart appliances are often more expensive than the old appliances. For this reason you should first look at the energyhungry appliances, as these will provide the greatest saving. A tumble dryer is an appliance that uses a lot of energy. A smart tumble dryer has a heat pump which uses the residual heat produced. Such a tumble dryer costs more to buy, but this soon pays for itself due to the lower energy costs.
Extra-economical light bulbs The efficiency of a filament light bulb can be made much higher than 5%. In 2003 the first halogen light bulbs with HIR technology (Halogen InfraRed) became available. These are low-voltage light bulbs with an in-built reflector. The reflector reflects the infrared heat radiation back to the filament which helps maintain the high temperature of the filament. The most recent types of light bulbs with HIR technology have an efficiency of about 30%. This is almost the same as the efficiency of an energy-saving light bulb.
Formula for efficiency
The formula to calculate the efficiency is: η=
Euseful Einput
× 100%
Where Euseful is the desired energy and Einput is the electrical energy supplied. The same unit must be used for both types of energy, for example joules. η (pronounced ieta) is the efficiency. η is between 0% and 100%. If the efficiency is 100% then all of the energy supplied is converted into useful energy.
FIG 13 H alogen light bulbs with HIR technology have an efficiency of 30%.
Example calculation 1 Calculate the efficiency of a light bulb with an electrical power of 11 W that emits 5 W of light energy. Per second the input electrical energy is 11 J and the useful energy is 5 J. η =
5J × 100% = 45% 11 J
The efficiency is 45%.
Example calculation 2 You replace the incandescent light bulbs in the living room with LED lights. The old bulbs had an efficiency of 5% and used 720 kWh per year. The new LED lights have an efficiency of 40% and emit the same amount of light. Calculate how many kWh you save per year. The light energy from the old bulbs was 5% of 720 kWh. Therefore Euseful = 36 kWh light. Therefore for the new LED lights: 40% =
36 kWh × 100% Einput = 90 kWh Einput
The saving is 720 kWh – 90 kWh = 630 kWh.
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1.1 0.0
Electrical energy Titel paragraaf
Questions: Knowing more 26 For each of the following statements answer true or false. If the statement is false, change it to make a statement which is true. a The efficiency of an appliance gives the percentage of energy used for the purpose for which the appliance is designed. b An incandescent light bulb has an efficiency of about 95%. c Appliances with an A rating energy label have an efficiency of 100%. d When using the formula for efficiency the energy must be given in kWh.
27 A halogen light bulb with HIR technology has an
a What happens to the infrared radiation in a HIR light
efficiency of 30%.
b Is the temperature of the filament in a HIR light bulb
bulb?
higher or lower than that in a normal incandescent light bulb? c Explain how the HIR technology provides a higher efficiency. The HIR light bulb in figure 13 has a power of 75 W and an efficiency of 30%. d Calculate what the power of an incandescent light bulb must be in order to provide the same amount of light. Assume that the efficiency of an ordinary light bulb is 5%.
Knowing more Onderdeel
29 Scientists at the ECN (Energy research Centre of Netherlands) think that they can increase the efficiency of wind turbines by about 1,5%. The blade of a wind turbine is fitted with an extending edge or rib, thus increasing the efficiency of the turbine. This rib significantly alters the air flow around the blade. For a 2 MW turbine over a lifetime of about 10 years the increase in efficiency of 1,5% amounts to an extra production of 800 000 kWh. In economic terms this amounts to an extra income of € 55 000. Fitting the ribs costs about €2 500 per wind turbine, therefore the investment is earned back within six months. a Calculate the price of 1 kWh of electrical energy produced by the turbine. For a 2 MW turbine the increase in efficiency is 1,5%. b Calculate the extra income if the turbine runs for one year. c Show by calculation that the turbine takes ten years to reach the extra production of 800 000 kWh.
28 The following question is posted on an Internet forum:
‘ I am not sure if I should get a “normal” condenser tumble dryer or a heat pump tumble dryer. I went to the shop today to ask about this and was told that a heat pump dryer is more economical to use (1,7 kWh to dry a load of laundry compared to 3,5 kWh), but more expensive to purchase (€ 650 compared to € 350). There are four loads of laundry a week at our house. Which tumble dryer should I buy?’
a Calculate how many kWh a heat pump dryer saves
b How many years does it take for the extra cost to be
c Explain why the time to earn back the extra cost will
FIG 14 A researcher shows a model of a wind turbine with the newly developed rib.
per year. earned back? Assume a price of € 0,20 per kWh. actually be slightly shorter. d Write a reply to the post on the forum with an explanation.
Ch1 Electrical appliances
19
Experimenting Onderdeel
Tekst... Activity
1
Electrical appliances, energy and power
What to do
Every electrical appliance uses energy, but how do you know if a particular appliance uses a large or small amount of electricity?
• • •
• •
Which appliance do you think consumes the most electricity? Inspect each appliance to see if you can find the power rating (in watts) marked on it. Note down these values. Measure the power of each appliance using the energy meter. Note down if this is different from the value given on the appliance. Measure how many kWh the appliance uses in 6 minutes. Estimate how many hours the appliance used in an average family per week.
Questions
FIG 15
You will need • •
different electrical appliances such a curling/ straightening tongs, shaver, radio, coffee machine, etc. energy meter
• List the appliances in order, with the highest power appliance at the top. • Note down the energy consumption in 6 minutes next to each appliance. • Calculate the energy consumption of each appliance in 60 minutes. What do you notice? • Calculate the energy consumption of each appliance per week. • List the appliances in order, with the appliance which uses the most energy per week at the top.
Activity 2 Taking measurements at home using an energy meter You can measure the power of an appliance with an energy meter. To measure the consumption of electrical energy the appliance must be connected to the energy meter for a time. You can carry out one of the following experiments using an energy meter: 1 How much can you save by switching off the TV decoder instead of leaving it on stand-by? 2 Does the energy usage of your refrigerator agree with the value given by the manufacturer? 3 How much energy does a computer use per year? 4 How much does the energy used to wash clothes (washing and drying) cost per year? (Make sure that the energy meter is suitable for the power of the washing machine).
20
FIG 16
What to do • • •
First write a plan for your investigation describing how you will carry out the measurements. Find out how the energy meter works. Carry out the experiment.
You will need
Question
• •
Write a short report describing the results of your experiment.
energy meter electrical appliance with variable power
1.1 0.0
Electrical energy Titel paragraaf
Experimenting Onderdeel
Activity 3 Incandescent light bulbs and energy-saving bulbs
You will need
Energy-saving light bulbs are more economical than ordinary light bulbs, but is the difference really as big as is claimed? According to the manufacturer an energy-saving bulb produces four to five times more light than an ordinary light bulb of the same power. Research question: Does an energy-saving bulb actually produce four to five times more light than an incandescent light bulb of the same power?
• • • • • •
energy meter light fitting with suitable (safe) lead assorted light bulbs with various power ratings assorted energy-saving bulbs with various power ratings lux meter or light sensor with computer or smartphone app (e.g. Megaman) lab stand
FIG 18 Lux meter app on a smartphone
What to do •
Measure the power of each incandescent light bulb and energy-saving light bulb using the energy meter. • Fix the light sensor in a lab stand. • Measure the light intensity at the same distance from each light bulb. Choose the distance such that each bulb gives a high reading. • Draw a table for your results, and for each bulb fill in: - the power given by the manufacturer; - the power measured by the energy meter; - the light intensity measured by the lux meter. • Plot a graph of the light intensity measured (vertical axis) with the power measured. Draw smooth curves through the points for incandescent bulbs and the points for energy-saving bulbs.
Question Write a report which includes: • the aim of the experiment • the method used, perhaps including a photo or sketch • the table of results • the graph • your conclusion
FIG 17
Ch1 Electrical appliances
21
Discovering
Batteries If your torch no longer works, the batteries are ‘dead’ and you replace them. Does a ‘dead’ battery contain no energy at all? In fact many discarded batteries still contain some energy. Perhaps the battery will still work in a clock. You probably don’t have a device to measure the energy in a battery at home.
Do Activity 4. Give a few examples of applications where this simple device is sufficient. 4 For which application is a better device necessary? The battery tester you have made cannot give the amount of energy in the battery. 5 What is the difference between a measurement and the test which you have carried out? 6 Why does weighing a battery not provide any useful information? 7 What do you think happens to the voltage of a battery when it is dead? 8 Explain if a battery which is nearly dead can provide a high current.
FIG 19 Many discarded batteries still contain some energy.
In many homes there are discarded batteries lying around in odd places: in drawers, old toys, or holiday gear for example. These batteries are often dead, but sometimes they can be used.
Investigate! Look for old batteries in your home. Bring at least two to school. 1 How can you make a device to test how full a battery is? When you test a battery you want to find out: • if a rechargeable battery needs charging • if an non-rechargeable battery (found somewhere at home) is new or has been used. 2 Think of a third possible use for the battery tester.
Do Activity 5. 9 Which of the three rows of the table in Activity 5 gives the clearest indication of how much energy is in a battery? Rechargeable batteries work differently than disposable batteries. Can you test rechargeable batteries with your battery tester?
Repeat Activity 5 for rechargeable batteries. 10 Look at your measurements. What differences do you see between disposable batteries and rechargeable batteries?
11 Does the method for testing batteries work better, worse or just as well for rechargeable batteries?
End result •
The battery tester must be: • easy to use by non-scientific people; • easy to make.
• •
3 Think of two other requirements that the battery tester should fulfil.
22
Collect together the information from the batteries which you and your classmates brought from home. Estimate the percentage of batteries which are lying around and that can in fact be used. Now design a device that people can use at home to test how full a battery is. Make sure that it is easy to use. Ask your teacher if you can make this device.
1.2
Voltage and current
Discovering
Activity 4 A simple battery tester
You will need
You are going to test some batteries. You will do this by attaching a small light bulb to a battery.
• •
The following terms will be used to describe the charge in a battery: • A battery is said to be full when it works as well as a new battery. So it does not need to be completely full, as it is when just purchased. • A battery is said to be half-full when the light bulb clearly gives less light than with a new battery. • A battery is said to be dead when the light bulb hardly lights up or does not light up at all.
a number of 1,5 V batteries, full, half-full or dead pocket torch light bulb that shines brightly at a voltage of 1,5 V
What to do • • •
Number the batteries. Test the batteries. Start with a battery that you know is completely full. Note your results in a table.
Activity 5 Voltage and current from a battery A battery is always marked with the voltage that it provides. Look at the batteries that you have collected. You can also measure the voltage of a battery. This gives you information about how full the battery is.
What to do • • • • • • • • • •
Number the batteries using a felt-tip pen. Draw a table with five rows and as many columns as you have batteries (see Table 6). Draw a cross through the battery number you are testing in the top row. Write down the voltage marked on the battery in the second row. Insert two leads into the two contacts of the voltmeter. Measure the voltage across the battery as shown in figure 20. Write this voltage down in row 3. Connect the light bulb to the battery and measure the voltage of the battery again. Write this voltage down in row 4. Write in row 5 whether the battery is full, half-full or as good as dead. Battery number
FIG 20
You will need • • • •
a number of batteries, full, half-full or dead pocket torch light bulb four leads voltmeter (or multimeter)
1 2 3 4 5
Voltage given on battery (V) Voltage measured (V) Voltage measured when light bulb is connected (V) How full is the battery? TABLE 6
Ch1 Electrical appliances
23
Understanding Onderdeel
Electrical appliances such as a television and a music Tekst... player use electrical energy. How does this energy get there? What is the difference between voltage and electric current?
Key terms
Power supply, circuit, electrons, current, amp, ammeter, voltage, volt, voltmeter.
Closed circuit
For an electrical appliance to work a suitable power supply and a closed circuit are necessary. The power supply provides the energy. The electric current carries the energy from the source to the appliance. In figure 21 this is a light bulb. The light bulb converts the electrical energy into light and heat. After transferring the energy to the light bulb the current flows back through the output lead back to the power supply.
Current is measured using an ammeter. The ammeter must be connected in the input or the output lead, so that the current passes through the meter (see figure 22). Low currents are given in milliamp (mA) and high currents are given in kiloamp (kA). 1 mA = 0,001 A 1 kA = 1 000 A
Connecting an ammeter Figure 22 shows how an ammeter should be connected. Proceed as follows: A C onnect the positive terminal of the ammeter to the positive terminal of the power supply. B Connect the negative terminal of the ammeter to the appliance.
A
A
+ -
B
A
+ -
FIG 21 The circuit must be closed.
The electric current consists of tiny particles called electrons. After the electrons have ‘passed on’ their energy they return to the power supply. The power supply then provides the electrons with more energy. The electric current flows around: this is a called an electric circuit. Since no electrons are lost in the circuit, the current entering and leaving the appliance is the same. The electric current tells you how many electrons pass through the light bulb per second.
Electric current
The unit of electric current is the amp (A). The symbol for current is I.
24
FIG 22 An ammeter is connected in the electric circuit.
Power sources Activity 6, page 34. There are many types of power supply. This includes hundreds of types of batteries, varying from small button cells for hearing aids to huge accumulators for lighthouses. Batteries and accumulators are direct current sources: they have a positive and negative terminal.
1.2 0.0
Voltage current Titeland paragraaf
The mains socket may appear to be the most important power source in the home, but in fact the power source is the generator at the power station. All power sources have one thing in common: they allow an electric current to circulate. You can imagine a power source as a pump which pumps the electrons around. The positive and the negative poles exert a force on the electrons, which are then set in motion.
Understanding Onderdeel
Circuit diagram
A circuit diagram shows clearly how a circuit is connected using symbols for electrical components and straight lines for the electrical wires. The most important symbols are shown in figure 24.
light bulb
Dynamos and generators produce alternating current. The positive and negative poles continually swap over rapidly, so the electric current keeps changing direction. With a battery, an accumulator and a solar cell the positive and negative poles do not swap over: they are direct current sources. These energy sources therefore produce direct current. With direct current the current always travels in the same direction.
wire
+
-
power source switch (open) switch (closed)
Unit of voltage: the volt Activity 7, page 34.
resistor
V
alternating current power supply FIG 24
Voltage and energy
A
+ -
FIG 23 How to connect a voltmeter to a light bulb.
The unit of voltage is the volt (V). An AA battery has a voltage of 1,5 V, a bicycle dynamo can produce 6 V and a mains socket has a voltage of 230 V. The symbol for electric voltage is U. The voltage is measured using a voltmeter. This enables you to measure the voltage of a power source or the voltage across an appliance. With a power source you connect the voltmeter to the positive and negative terminals; with an appliance you connect the voltmeter across the two connectors of the appliance. A voltmeter is not connected in the circuit; it is outside the circuit (see figure 23).
A higher voltage gives a higher current, as the force on the electrons is greater. However, the voltage is also related to the energy which the electric current can supply. The higher the voltage, the more energy each electron can pass on. This is why appliances that use a large amount of electricity work on a high voltage. For a clock a voltage of 1,5 V is enough; for household appliances 230 V is needed; the trains of the Dutch rail network run on a voltage of 1 500 V.
Where does the energy come from?
Power sources generate electrical energy from a different type of energy. A water turbine generates electrical energy from kinetic energy. The water wheel turns a large dynamo called a generator. A generator converts kinetic energy into electrical energy. Large generators turn in both power stations and windmills thus producing electricity. In power stations generators are driven by steam turbines. Heat is needed to make the steam. This heat usually comes from the combustion of fossil fuels, but there are also power stations which obtain heat from solar radiation or from the energy released by nuclear fission.
Ch1 Electrical appliances
25
Understanding Onderdeel
A battery generates electricity using a chemical reaction Tekst... between two substances, so it converts chemical energy. When one of the substances cannot react any more, the battery is said to be empty, flat or ‘dead’. Solar cells generate electricity directly from light from the Sun. The source is therefore solar energy.
+ -
A
B
+ -
A
A
+ -
C
D
+ -
V
Summary
• A n electric current consists of moving electrons that gain energy at the power source and release energy at the appliance. • The power source ensures that the electrons are pumped around. The higher the voltage the higher the current and the more energy is supplied to the appliance. • Voltage is given in volts (V). Current is given in amps (A). • A voltmeter is connected across the two connectors of the power source or appliance. A voltmeter is outside the circuit. • An ammeter is connected inside the circuit: either in the input or the output lead. • The current supplied to an appliance is the same as the current leaving the appliance. • There are power sources which generate electrical energy from chemical energy or solar energy. • A circuit diagram shows clearly how a circuit is connected.
Questions: Understanding 30 For each of the following statements answer true or false.
If the statement is false, change it to make a statement which is true. a Electrical energy is transferred by moving electrons. b The electric current moves from the + terminal to the - terminal of the power source. c A voltmeter is connected in the circuit. d Electric current is the force exerted on the electrons. e The unit of electrical energy is the amp. f The electrical energy depends on the current only, not on the voltage.
31 Figure 25 shows four circuit diagrams. In which circuit(s) is the meter connected correctly?
26
A FIG 25
32 Correct the errors in the following paragraph: A hair-dryer has a current across it. The lead has 2,5 V through it. If you touch the live wire, you get a shock of 230 V electric current. Then 15 mV passes through your body.
33 Match the following appliances with the correct voltage. 1. Hair-dryer 2. High speed train 3. Analogue clock 4. Car headlight 5. Local train 6. Pocket torch
A 1,5 V B 4,5 V C 12 V D 230 V E 1 500 V F 25 000 V
34 A moped has a dynamo, a front light and a rear light. The engine is running and the dynamo is turning, but the lights are not connected to the dynamo. a Does the dynamo provide a voltage, current, both of these, or neither of these? The lights are now connected to the dynamo. b Explain in which of the following situations the dynamo provides the highest current: • The front light only shines. • The rear light only shines. • Both the headlight and the rear light shine. Both lights are connected to the dynamo. You rev up the engine so that the lights shine brighter. c Explain what has changed: • Only the current has increased. • Only the voltage of the dynamo has increased. • Both the voltage and the current have increased. There is only one wire from the dynamo to the front light. d What is the path of the current flowing back to the dynamo?
1.2 0.0
Voltage current Titeland paragraaf
Understanding Onderdeel
35 Complete the following sentences. a The unit of voltage is the ... b The unit of current is the ... c Electrical energy is transferred by ... d An ammeter is connected in the ... e When the current has passed through the appliance it flows back to the ...
f A dynamo supplies ... g A solar cell supplies ...
36 A 4,5 V battery is connected to a light bulb. The current
from the battery to the light bulb is 0,5 A. a What is the current from the light bulb back to the battery? The light bulb is now connected to a voltage of 6 V. b Give two reasons why the light bulb shines more brightly at 6 V than at 4,5 V.
-
+ 0
1
2
3
4
FIG 26
39 In a conventional power station the heat from the fuel is first converted into a different type of energy and then into electrical energy. a Which type of energy is the heat first converted into? b What is the name of the electrical device that produces electrical energy? There are also power stations that do not use combustible fuels. c Name at least one other type of energy that is used to generate heat in a power station.
40 On some roofs you see solar panels which are connected to the mains electricity via an electronic control switch. a Which two changes are necessary to ensure that the electricity from the solar panel can be used in the home without any problems? There are also isolated holiday homes that are entirely dependent upon energy from solar panels. During the day the panels charge an accumulator, which can then provide electricity during the evening. The accumulator is sufficiently large that having a few days without much sunshine is not a problem. All of the appliances in this type of house work on 12 V. b In which form is the energy from the Sun stored? To see how well the solar panels are working there is sometimes an ammeter that shows how much current the panels are generating. c Draw a circuit diagram for this type of holiday home. The appliances are just two lights. Put switches in all places where they are needed. Draw both the accumulator and the solar panels as the power source and label these accumulator and solar panels. The ammeter should also be included in the circuit.
37 The circuit shown in figure 26 shows a power supply,
a light bulb and a meter. Explain if the meter is an ammeter or a voltmeter.
38 You compare three types of power source: dynamos, solar cells and batteries. a Which power sources supply direct current? b Which power source is found in most power stations? A hydrogen fuel cell uses a reaction between hydrogen and oxygen. c Explain if a hydrogen fuel cell is comparable to a dynamo, a solar cell or a battery.
Ch1 Electrical appliances
27
Comprehending Onderdeel
Tekst... Modelling electricity as a flow of water
Activity 8, page 35. An electric current can be compared to a flow of water through water pipes. Water particles move in the flow of water; an electric current is made up of moving electrons. You can therefore compare the size of the current to the amount of water flowing out of the tap per second at a particular place. The water flows due to the pressure of the water supply. There is a force acting that causes the water to move. Electric voltage is a measure of the amount of force pushing the current. If the pressure on the water pipes is increased, the water flows faster. A higher voltage therefore means that there is a higher force on the electrons. The energy supplied by an electric current depends upon the voltage and the current. A greater force means more energy; a higher current can carry more energy. A high pressure nozzle (figure 27) uses little water, but the pressure is very high. This high pressure means that the water has very high energy. This can be compared to a power source that uses a high voltage with a small current to deliver a high power. There is another similarity between water pipes and electricity. If you need a large flow of water, you use a large diameter water pipe. For a large electric current you need a thick cable. A cable that is too thin will get hot. There is however one big difference: the water does not have to go back to the pump; whereas electricity will only flow when there is a closed circuit.
Power, voltage and current Activities 9 and 10, page 35. The power of an appliance is the amount of energy that the appliance converts per second. A higher voltage means that there is a greater force pumping the electrons around, therefore more energy. The energy is transferred by the current. A higher current therefore means more energy. When a power source must provide a large amount of energy it is best to use both a high voltage and a high current. Appliances that are plugged into the mains supply all work at the mains voltage of 230 V. An appliance that uses a large amount of energy has a high current. The pocket torch shown in figure 28 works on a voltage of 4,5 V. The current is just a few tens of milliamps. The battery of an electric car has a high voltage of about 200 V, and can therefore provide a current of up to 300 A.
FIG 28
Relationship between power, voltage and current
The power of an appliance depends upon the voltage and the current. Expressed as a formula: power = voltage Ă— current In symbols this is written: P=UĂ—I
FIG 27 A high-pressure cleaner uses little water. The high pressure means that the water has very high energy.
28
where P is the power in watts (W) U is the voltage in volts (V) I is the current in amps (A)
1.2 0.0
Voltage current Titeland paragraaf
Comprehending Onderdeel
Questions: Comprehending
Example calculation 1 A car battery has a voltage of 12 V. A current of about 250 A is needed to start a car. What (electrical) power is needed to start the car?
41 For each of the following statements answer true or false.
The ‘start-power’ is:
P = U × I = 12 V × 250 A = 3 000 W = 3 kW
Combining formulas
The energy used by an appliance depends upon the power and the time. The power depends upon the voltage and the current. The energy consumption can be calculated using E = P × t, and the power is calculated using P = U × I. Sometimes you need both formulas to calculate the energy consumption.
Example calculation 2 It takes 2 s to start a car. The battery supplies a voltage of 12 V and the current during starting is 250 A. How much energy is needed to start the car?
If the statement is false, change it to make a statement which is true. a amps x volts = kilowatt-hours b The power supplied by a power source depends upon the voltage and the current. c For household appliances that are plugged into the main supply the power depends upon the current only. P d The formula P = U × I is the same as I = U
42 Figure 29 shows five types of batteries. The two thin batteries supply 1,5 V and the rectangular battery supplies 9 V. a With which battery will a light bulb shine most brightly? b With which battery is the force pumping the electrons around the greatest? The four cylindrical batteries all supply 1,5 V. c Apart from the diameter, what is the difference between a large and a small cylindrical battery?
You calculate the energy using E = P × t, but first you have to calculate the power P: P = U × I = 12 V × 250 A = 3 000 W E = P × t = 3 000 W × 2,0 s = 6 000 J = 6 kJ
Example calculation 3 A vacuum cleaner uses 0,10 kWh energy. The current in the mains lead is 4,35 A. How many minutes has the vacuum cleaner been in use?
FIG 29
43 A 25 W incandescent light bulb gives the same amount
P = U × I = 230 V × 4,35 A = 1 000 W = 1 kW E=P×t 0,10 kWh = 1 kW × t
0,10 kWh = 0,10 hour = 6 minutes 1kW
t =
of light as a 25 W car light bulb. A car light bulb works on 12 V. a What voltage does a light bulb use at home? b Explain without doing any calculations which light bulb has the higher current through it. c Calculate the current through each light bulb. A car light bulb has a thicker filament than a normal light bulb. d Explain why this is necessary.
Ch1 Electrical appliances
29
Comprehending
44 Many vacuum cleaners have a variable power control. With a vacuum cleaner like this you can adjust the power from a minimum of 400 W to a maximum power of 1 200 W. a Explain why the voltage on the vacuum cleaner does not change when you use a higher power setting. The power regulator in the vacuum cleaner lowers the voltage across the vacuum cleaner motor. b Calculate the current through the electromotor. Hasna buys a vacuum cleaner in the Netherlands and takes it with him to Morocco where the mains voltage is 220 V. c Explain if the maximum suction power of the vacuum cleaner is different in Morocco compared to in the Netherlands.
45 A 230 V toaster has a current of 2,5 A passing through it. a Calculate the power of the toaster. A vacuum cleaner has a power of 1 200 W and is connected to the mains electricity supply (230 V). b Calculate the current through the vacuum cleaner. A car vacuum cleaner that works on the 12 V voltage of the car battery also has a power of 1 200 W. c Calculate the current through the car vacuum cleaner.
The car needs a power of 11 kW to travel at a speed of 80 km/h. c Calculate the distance that the car can travel at this speed, starting with a fully charged battery.
47 To understand the difference between voltage and current you can compare an electric circuit with a fountain spraying out water. a Is the pressure of the water pump comparable with the voltage or the current? b What is pumped around in an electric circuit? c What is equivalent to the current in the fountain? d What happens to the fountain if you increase the ‘current’?
48 A laptop consumes 129 kJ of energy in 20 minutes. a Calculate the average power consumption of the laptop. b Why can you only calculate the average power consumption here? The average current through the laptop during this 20 minute period is 5,0 A. c Calculate the voltage of the laptop battery pack.
49 The lights on your bicycle are broken. You find two light bulbs in the shed. One light bulb is marked (6 V, 0,5 A) and the other (6 V, 0,6 W). a Explain which bulb is for the front light. b Calculate the power that the bicycle dynamo must generate so that both light bulbs shine normally. Assume that the current through the light bulbs is proportional to the voltage of the dynamo. When you cycle slowly, the voltage of the dynamo is also lower. c Calculate the current through each light bulb when the voltage across the dynamo is 4,0 V. Use a ratio table if necessary. d Is the power of the light bulbs also proportional to the voltage? Explain, or show your calculation. A wire runs from the dynamo to each light bulb. Your father says, “The dynamo produces alternating current. The current travels to and fro through that wire to the light bulb and back. That is why there is no need for a second wire.” e Is this explanation completely wrong or is it partly true? Correct the mistakes.
FIG 30 An electric car at a charging station
46 The battery of the electric car shown in figure 30 has a voltage of 360 V. The maximum power of this car is 90 kW. a Calculate the current when the battery supplies maximum power. When the battery is fully charged it can supply 24 kWh of electrical energy. The battery can be charged in half an hour. b Calculate the current during charging.
30
1.2
Voltage and current
50 You use two 1,2 V rechargeable batteries to run your
pocket torch. The light bulb is marked ‘2,4 V, 0,8 A’. The batteries are marked with a capacity of 4 400 mAh. This means that the batteries can supply a current of 4 400 mA for 1 hour (h). Then the battery is ‘empty’. If the battery supplies a current that is half of this value, it will work for twice as long. With a higher current the battery will be discharged faster. a Calculate the voltage across the light bulb. b Calculate the power of the light bulb. c Calculate how long the light bulb can operate using these batteries. d Calculate how many joules of energy the batteries will then have supplied in total.
TYPE AJ3 220-240V 50/60Hz 1850-2200 W CAPACITY 1,5 LITERS
Mx
N”Q/96 252
P
52 A power station supplies a power of 2,5 GW. The
electrical energy is transferred via high tension cables. The voltage on the high tension cables is 350 kV. a Calculate the current through the cables. High tension cables are very thick, about 4 cm in diameter. An electrical wire at home is about 1,7 mm thick. b Explain why a high tension cables must be very much thicker.
53 Nick has two different light bulbs. Both light bulbs are made for mains electricity, but the power of bulb 1 is three times greater than that of bulb 2. a Which light bulb has the higher current through it? b How many times greater is the current? With two other light bulbs the power of the two bulbs is the same. The voltage that bulb 1 works at is three times greater than the voltage required by bulb 2. c Which bulb has the higher current through it? d How many times greater is the current compared to the other light bulb?
54 During a storm a voltage of 500 000 V can arise
Made in France
4784959C F1115116 423
Comprehending
T
FIG 31
51 Baz and Rolf have bought an electric kettle. The kettle has a plastic base and a separate jug with a heating element. The type plate of the kettle is shown in figure 31. Baz and Rolf wonder why there are two power ratings on the type plate. They each suggest a possible reason for this. Baz says, “The power of the kettle is not known exactly, because it depends upon the mains voltage.” Rolf says, “The power of the heating element is not known exactly, because it depends upon how much water you put into the kettle.” a Which of these statements is/are correct? The power is 2 200 W with a voltage of 240 V. b Calculate the current at this voltage. c Explain why the current is lower at 220 V.
between clouds. A lightning strike has a duration of 1 ms. During the lightning strike the current is 80 kA. a Calculate the power of the lightning strike in watts. b Calculate the energy released during the lightning strike in J. c Also calculate the energy in kWh. The annual energy consumption of an average Dutch family is 2 000 kWh. d How long would the energy of one lightning strike last for an average Dutch family?
55 Designers want to develop a new type of kettle that can boil a litre of water within half a minute. About 400 kJ of energy is needed to boil a litre of water. a Calculate the minimum current necessary through the kettle. b Explain why it is not possible to boil a litre of water in half a minute using a kettle. The maximum current at home is 16 A. c Calculate how fast a litre of water can boil if the current through the kettle is 16 A. d Explain why most kettles have a power rating whereby the kettle draws a current which is less than the maximum current.
Ch1 Electrical appliances
31
Knowing more Onderdeel
Tekst...currents High
A car battery has a voltage of 12 V. The current that this battery must supply to operate an appliance component is much higher than at home. The headlights of a car have a power of 55 W, and they need a current of 4,6 A. The current through a 55 W lamp at home is just 0,24 A. The higher current is necessary because the voltage is so low. A small force means less energy. The current must therefore be higher in order to supply enough energy. You can also see the relationship between voltage and current in the formula P = U Ă— I. If P has to be the same and U becomes smaller, then I must become greater. The value of U Ă— I must remain constant. With components in a car that use a lot of electrical energy the current can get very high. The current for the air conditioning is about 60 A, for example, and the start motor needs a current of about 250 A.
FIG 32
Figure 32 shows a welder joining two pipes together. The current through the welding electrode is about 100 A. The welding equipment does not need a very high power, since the voltage is just a few tens of volts.
32
High voltages
Power stations generate vast amounts of electrical energy. A large power station may generate as much as several gigawatts. To transfer this amount of energy both a high voltage and a high current are necessary. The main high tension network in the Netherlands operates at 380 kV. The current in these cables can reach a couple of thousand amps. The high current could mean that the cables get hot. (The high tension is less important here; it is the current that is relevant.) In order to prevent this high tension cables are always very thick. Of course, there is a limit to this, because very heavy cables could break under their own weight. High tension is dangerous, but for the purposes of transferring electrical energy it is important that the voltage is very high. If, for example, the voltage was chosen to be ten times lower, then ten times more cables would have to hang from a high tension electricity pylon in order to transfer the same amount of energy.
FIG 33 An electric eel produces a high voltage.
There are also animals which make use of high tension. An adult electric eel, a freshwater fish from the Amazon, can produce a voltage of up to 600 V at a current of 2 A. The electric eel uses electric pulses to navigate through murky water as well as to stun or kill prey.
1.2 0.0
Voltage current Titeland paragraaf
Questions: Knowing more 56 An adult electric eel produces a voltage of up to 600 V at a current of 2 A. The electric eel produces a pulse every 20 s. The pulse is about 200 Âľs in duration. a Calculate the power of this electric eel. b Explain that the eel produces a high power, but not very much energy. c Calculate how much electrical energy the electric eel produces per minute. If the electric eel had a ten times lower voltage and a ten times greater current the energy it produces would be the same. d Explain why it is not useful for the electric eel to use a lower current. For an electric ray that lives in the sea the maximum voltage is 60 V. For scanning its surroundings the current is the most important factor; the current produced by an electric ray is several amps. e Why does an electric ray need a much lower voltage to navigate?
57 The starter motor of a car supplies a current of 250 A for 3,5 s. The voltage of the battery is 12 V. a Calculate how many joules of energy is used to start the car. When the car battery is empty the engine can be started using the car battery of another car. The other battery is attached using thick jump leads. b Explain why the jump leads must be thick. The air conditioning of the car is used for 45 minutes. The current is 60 A. c Calculate how many kWh the air-co uses.
Knowing more Onderdeel
59 An arc welding tool has a current of 140 A at a voltage of 45 V. a Calculate the power of the welding tool. The welding tool is connected to the mains electricity supply and has an efficiency of 90%. b Calculate the current required by the welding tool. The voltage is reduced from 230 V to 45 V inside the welding tool. One of the reasons for this is that at a high voltage the current is also much higher than necessary. c Name another reason why the voltage is reduced.
FIG 34
58 Electric cars and hybrid cars use much higher voltage
than normal cars. The batteries of a certain electric car supply a voltage of 500 V. The electromotor that powers the car has a maximum power of 50 kW. a Calculate the current through the car when the car is running at full power. b Explain why electric cars are not powered by engines that work at 12 V or 24 V. The battery of an electric car can store 72 MJ. c Calculate how long the car can be driven at full power.
60 Figure 34 shows an improvised arc welding tool which is used, in an emergency, for welding in places where no mains electricity is available. Three batteries are needed for this. Why is one battery not sufficient?
61 A 3,0 km long underground high tension cable lies between the centre of Rotterdam and the Ommoord housing estate. The cable must supply a maximum of 400 MW of electrical power at a voltage of 150 kV. The cable consists of a bundle of copper wires, each of which is designed for a maximum current of 200 A. a Calculate the minimum number of copper wires that make up the high tension cable. Protest groups are demanding that the voltage is reduced to 50 kV for safety reasons. b What would happen if the same power was sent through these cables at a voltage of 50 kV? c How would the cable have to be changed so that it could be used to transfer the same power at 50 kV?
Ch1 Electrical appliances
33
Experimenting Onderdeel
Tekst... Activity
6
Activity 7
Power sources
Measuring voltage and current
When you hear the term power source you might think of batteries or mains electricity, but in fact there are many other different types of power source. In this experiment you will compare the different types of power sources.
In this experiment you will measure the voltage and the current in a circuit.
+
-
A
V 0
FIG 36
You will need FIG 35
You will need • • • • • • • •
solar cell light source coil magnet bicycle dynamo (with bicycle wheel) and/or mechanical dynamo LED light and bicycle light voltmeter electromotor
• • • •
What to do • • • • •
What to do • • • •
Form groups. Each group will investigate one or more power sources. Connect the voltmeter to the power supply. Investigate if you can change the voltage, and if so, how. Investigate if you can use your power source to run an LED light, a bicycle light or an electromotor.
variable power supply front and rear bicycle lights in casing ammeter and voltmeter connecting leads
• • •
Adjust the power supply to 6 V. Measure the voltage of the power supply with the voltmeter. Connect up one of the bicycle lights and measure the voltage across the casing. Unscrew the light bulb from the casing and again measure the voltage across the casing. Change the circuit so that you can also measure the current through the light bulb (figure 36). Unscrew the light bulb and measure the current again. Replace the light bulb by the other light bulb and measure the current again. Finally reduce the voltage of the power supply and observe what happens to the current.
Questions Questions • How can you change the voltage produced by the power source? • Which power source(s) can you use to run the bicycle light, the LED light or the electromotor? • Does the power source supply power when no appliance is connected to it?
34
• Why can you still measure a voltage when the light bulb is unscrewed? • Why does the current change when you connect a different light bulb to the same voltage? • Copy and complete the following sentence: The voltage is determined by the ... ; the current depends upon the ... and the ....
1.2 0.0
Voltage current Titeland paragraaf
Experimenting Onderdeel
Activity 8 Current and energy simulation This simulation enables you to see how an electric current supplies energy in the form of light and heat. You will also investigate how the heat produced changes when you increase the current.
You will need
current and energy simulation program
What to do • •
Open the required simulation program at www.impact-online.nl or via the school network. Investigate what happens when you change the voltage using the slide bar.
•
Investigate what happens when you change the thickness of the light bulb filament using the other slide bar.
Questions • •
Explain what happens when you increase the voltage. Describe what happens when the filament is made thinner. The electrons move faster in the thin part of the wire than in the thick part of the wire. • Explain if the current in the thin part is smaller than, greater than or the same as in the thick part. If the wire is made thinner, the current becomes smaller in the thick part as well. • Why does the current in the thick part also get smaller?
Activity 9 Voltage, power and current In this Activity you will compare the properties of electrical appliances.
• • •
You will need • •
v arious electrical appliances, both battery and mains appliances ammeter, three leads for measuring, crocodile clips
What to do •
Put the appliances in order of energy consumption. For each battery-operated appliance measure the current, or read the current from the appliance. Ask your teacher how to connect the ammeter between the batteries and the appliance.
Questions • •
Compare all of the appliances that work on mains electricity. Is the current proportional to the power? Using the information given on the appliance and the formula for voltage, can you determine the current and power?
• • •
The unit is mAh. Connect the light bulb to the battery. Connect an ammeter in the circuit. Measure the current every 10 minutes and write this down. When the current is half of the current at the start, stop the measurement. Calculate how many mAh the battery has supplied.
ote down the power and voltage of each appliance N (type plate/sticker)
Activity 10 The capacity of a battery One battery lasts longer than another. In this experiment you will measure how much energy a battery can supply.
You will need • • •
fully charged 1,3 V rechargeable battery, pocket torch light bulb stopwatch, clock, watch or mobile phone to measure the time ammeter
What to do •
Look to see if the capacity is given on the battery.
• •
Questions • Do the results of your measurement agree with what is given on the battery? • Explain any differences.
Ch1 Electrical appliances
35
Discovering
Liar! Patsy has been caught. She walked out of a shop with an expensive lipstick in her hand without paying for it. Patsy says that she really did intend to pay for the lipstick, but the owner of the shop doesn’t believe her. She must pay for the lipstick and cannot go in the shop again. Patsy wants to find a way to convince the shop owner of her innocence and finds a description of a lie-detector on Internet. How does a lie-detector actually work?
5 Think of an experiment to simulate the changes in your skin. Ask your teacher if you can carry out this experiment using the lie-detector you have made.
Do Activity 12. 6 Discuss with two classmates which question they have each chosen, and decide if this ‘guilty knowledge’ is something that the real thief will respond differently to than someone who did not do anything.
7 Compare the objects that you have each chosen as well. Together decide which object is most suitable to show to the suspect.
FIG 37
Investigate!
FIG 38 P sychologists at the University of Maastricht are investigating the reliability of the lie-detector.
Do Activity 11.
Read Source 2.
1 Does the current increase or decrease when the test subject is lying?
8 For each of the six characteristics detected by the polygraph note down whether the value will increase or decrease during questioning.
2 Does this simple lie-detector help to detect lies?
9 Which of the six characteristics could you measure 3 Think of a reason why the current increases. What happens to your skin when you lie? a Your skin becomes ... b Your skin becomes ... c Your skin becomes ... 4 Which of these responses results in a higher current? A lie detector responds to changes in the skin, but it is not yet clear which changes these are. In order to investigate this you need to simulate the changes that occur in your skin when you lie in another way.
36
with the equipment available at school?
10 Which quantity in Physics (related to electricity) is measured during measurement of the heart rate (ECG) and making an electroencephalogram (EEG)?
End result Patsy wants to be able to go back to the shop. Write an e-mail to the shop on Patsy’s behalf. Explain that Patsy really did not intend to steal the lipstick and explain to the shop keeper how to perform a lie-detector test. Also explain how a lie-detector works.
1.3
Resistance
Discovering
Activity 11
Activity 12
A simple lie-detector
Guilty knowledge
This lie-detector consists of two strips of aluminium foil which must be in good contact with two fingers of one hand. A battery is used to allow a small and therefore harmless current to pass through your hand. You measure this current.
A lie-detector can also give a false positive. This happens when a test subject becomes highly stressed by a question. This does not necessarily mean that the person is lying, of course. With direct questions such as, “Did you kill him?” almost everyone becomes stressed. For this reason the questioner must ask about things which the guilty person will react to. The police make use of guilty knowledge for this. This is information that the guilty person connects with the crime, and that an innocent person does not.
You will need • • •
9 V battery 2 strips of aluminium foil about 1 cm wide and 5 cm long, with leads attached using sticky tape sensitive ammeter (microammeter)
+
+
A
-
-
Source 2 contacts
FIG 39
What to do •
Say that someone has stolen a mobile phone from a shop. • Think of a question that would cause the real criminal to become stressed, and an innocent person to remain non-stressed. • Think of an object that you could show to the crimi nal that would cause the real criminal to become stressed, and an innocent person to remain unstressed.
Tape the aluminium strips around two fingers of one hand. • Attach both leads to a 9 V battery and connect an ammeter in the circuit; see figure 39. To test the lie-detector the ‘questioner’ asks a few questions which the test subject knows the answer to, but the questioner does not, for example, “What is your mother’s first name?” The test subject can choose to lie, or not, with each question. The questioner must guess if he/she is lying. • Think of five questions where the test subject knows the answer but the questioner does not. • Ask the test subject these questions, one at a time. • Look at the ammeter with each answer. • For each answer guess if the test subject is lying.
The polygraph
An actual lie-detector works in a slightly more complicated way. The device is called a polygraph. It measures other physical characteristics as well as how well the skin conducts electricity. The polygraph usually measures the following characteristics: • heart rate (ECG) • blood volume (plethysmogram) • blood pressure • skin conductivity • skin temperature • breath volume Sometimes a polygraph can also be measure responses of the central nervous system such as the electroencephalogram (EEG) and electromyogram (EMG). The polygraph is used by the police for interrogation in particular in the USA. There has been discussion about the reliability of the lie-detector for a number of years. One advantage is that suspects confess more readily. The judge can however attach no significance to the results of the device. There are also companies that use lie-detection during job interviews.
Ch1 Electrical appliances
37
Understanding Onderdeel
Domestic electrical appliances all work on the same Tekst... voltage, but the current through each appliance is different. The current is higher through appliances that use a lot of energy and smaller through appliances that use little energy. Why is the current through one appliance higher than the current through another appliance?
Resistance is given in ohm (Ω). The symbol for resistance is R. If an appliance has a resistance of 500 ohm, this is written R = 500 Ω. With large resistances units of kilo-ohm or mega-ohm are used. 1 kΩ = 1 000 Ω and 1 MΩ = 1 000 000 Ω.
Key terms
Resistance, ohm, Ohm’s law
Resistance
In section 1.2 an electric current is compared to a flow of water. Figure 40 shows narrow streams of water flowing from a shower head. If the holes are made smaller, less water flows out of the shower head. The shower head is a resistance to the flow of water.
FIG 41 A heating element for a washing machine (left) is thicker than the filament in a light bulb (right). The resistance is lower, so the current and the power are higher.
Resistance of an appliance
Every electrical appliance has resistance. Table 7 shows the resistance of various appliances. Appliance
Resistance Appliance
Washing machine 15 Ω Hair-dryer 53 Ω 100 W light bulb 530 Ω
40 W light bulb 1 W LED light iPod 2
Resistance 1,3 kΩ 53 kΩ 500 Ω
TABLE 7
FIG 40 A shower head acts as a resistance to the flow of water.
Similarly, a filament in a light bulb is a restriction that hinders the flow of electricity. It takes effort to push the electricity through a thin wire. The thinner the filament, the lower the current. In electric circuits the term resistance is used. The resistance is how much the current is ‘obstructed’ by a wire. With a high resistance the current is low; conversely with a low resistance the current is high. A thicker wire therefore has a lower resistance than a thinner wire. The resistance also depends upon the length of the wire. A longer wire has a higher resistance.
38
You can see that the resistance of an appliance decreases as more current is required for that appliance. In your day to day life you don’t really have to be concerned with the resistance of an appliance. However the designer of an electrical appliance must make sure that the appliance has the correct resistance.
Resistance, materials and temperature
The resistance of an appliance is not only related to the thickness and length of the wires inside the appliance. The temperature and the type of material are also relevant. The resistance of metals increases at higher temperature, because the electrons must move between the atoms in the metal. When the temperature rises the atoms of the metal vibrate more, so it becomes more difficult for the electrons to move between the atoms. The resistance therefore increases.
1.3 0.0
Resistance Titel paragraaf
Understanding Onderdeel
Resistors and electronics
Summary • • •
Electrons are obstructed as they move through a wire. This obstruction to motion is called the resistance of the wire or appliance. The symbol for resistance is R and the unit is ohm (Ω). Thick wires have a lower resistance and therefore allow the current through more easily. • Longer wires have a higher resistance. • The resistance also depends upon the temperature. When a metal wire gets hotter the resistance increases.
A resistor is a component in electronics as shown in figure 42. You can see many different resistors on a printed circuit board. These resistors determine the current in the circuit. The resistors are often just a thin layer of carbon in a plastic cylinder. A resistor is marked with a code of coloured bands. This code gives the resistance of the resistor, that is, the number of ohms.
Questions: Understanding 62 For each of the following statements answer true or false.
If the statement is false, change it to make a statement which is true. a The higher the resistance of a wire, the lower the current. b The unit of resistance is the ohm. c A thick wire has a higher resistance than a thin wire. d At a higher temperature the electrons move more easily through a metal wire. e Good conductors have a low resistance. f A long wire has a higher resistance than a short wire.
63 A light bulb is connected to a mains socket. The lead is black
0
green
5
brown
1
blue
6
red
2
purple
7
orange 3
grey
8
yellow
white
9
4
band 1 = number band 2 = number band 3 = number of zeros
damaged and the two wires are in contact. a What happens to the resistance in the circuit when the wires touch each other? b Explain why there is a very high current with a short circuit. c Explain why there is a risk of fire with a short circuit.
64 You can sometimes make a broken light bulb work by FIG 42 T he resistor illustrated has a resistance of 4 500 Ω. The last band is gold-coloured and gives the tolerance: 5%.
Resistance of a light bulb A lit-up, hot light bulb has a higher resistance than a cold light bulb. When you switch a light bulb on, there is therefore a much higher current for a very short time. This high current flows for just a few milliseconds. The resistance of a ‘hot’ light bulb can be more than ten times greater than that of a ‘cold’ light bulb. Fortunately most electrical appliances do not get as hot as a light bulb.
tapping it. The broken ends of the filament wire then melt together. The light bulb shines slightly more brightly than before. a Is the ‘repaired’ filament longer, shorter, thicker or thinner than before? b How has this changed the resistance? c Explain why the light bulb now shines more brightly.
Ch1 Electrical appliances
39
Understanding Onderdeel
Tekst... 65 Figure 43 shows various car light bulbs. All the bulbs work on a voltage of 12 V, but the headlight shines much brighter than the rear lights and the indicator lights. a Which has a higher power: the headlight or the indicator light? b Which has a higher current through it: the headlight or the indicator light? The filament wires in the light bulbs are the same length. c Explain which contains the thinner wire: the headlight or the rear light bulb. The car headlight has a power of 55 W. This is approximately the same as an ordinary filament light bulb. The headlight also emits about the same amount of light as a filament light bulb. The car headlight works on 12 V and a filament light bulb works on 240 V. d Explain why the two light bulbs can shine with the same brightness. e Which light bulb has the greater resistance: the car headlight or the filament light bulb?
FIG 44
67 The filament of a light bulb is made from a long, thin wire FIG 43
66 The resistance of a light bulb is measured using a digital multimeter (figure 44) during a practical lesson. The dial indicates which quantity is being measured as well as the maximum value that can be measured with that setting. a Look at the position of the dial. What is being measured: voltage, current or resistance? b What is the maximum value that can be measured with this setting? c What is the resistance of the light bulb? A 2,6 A B 2,6 V C 2,6 k立 D 2,6 立
40
formed into a spiral. The resistance of a wire depends upon the length, the thickness and the type of metal. At the instant that the light bulb is switched on the current is very briefly much higher than normal. a Explain why this is. You replace the filament with a wire made from a metal that does not conduct as well. The length and the thickness of the wire are the same. b Explain if the light bulb now shines more brightly or less brightly.
68 You compare a television (189 W) and a clothes iron
a Which appliance has the greater power rating? b Explain which appliance uses more electrical energy
(1 800 W).
c Explain which appliance has a higher resistance. d Explain how many times higher this resistance is.
in one year.
1.3 0.0
Resistance Titel paragraaf
69 Compare a garden hose connected to a household tap with a fire hose attached to a fire hydrant. a Which hose has more water flowing from it per second? b How can you see from the flow of water that the speed of the water from the fire hose is much greater? You now attach the garden hose to the fire hydrant. c Is the amount of water leaving the hose per second more, less or the same as with the household tap? The water flow depends upon the thickness of the hose. Water flows less easily through a thinner hose. The resistance of the hose holds the water back. d Explain which hose has the higher resistance.
Understanding Onderdeel
Compare the magnitude and the speed of flow of the current in the thick and the thin wire in figure 45. d Is the current in the thick wire higher, lower or the same as in the thin wire? e Is the speed of flow of current in the thick wire higher, lower or the same as in the thin wire?
71 Look at the information in Table 7. Compare the resistance of a 1 W LED light bulb with that of a 100 W incandescent light bulb. a How many times greater is the resistance of the LED bulb? b Explain why the resistance must be so high.
c What is the power of the hair-dryer in the table? d What is the resistance of a 25 W incandescent light bulb?
72 A car headlight works at a voltage which is 20 times
FIG 45 Cross-section of an incandescent light bulb
lower than the voltage in the home. However, the power of a car headlight is the same as that of a domestic incandescent light bulb. a Is the resistance of the headlight greater or less than the resistance of the light bulb? b Explain why the current through the headlight must be twenty times higher than the current through the light bulb. So for the headlight the voltage is twenty times smaller and the current is twenty times greater. c Which statement about the resistance of the headlight compared to the light bulb is true? Explain your answer. A The resistance is the same. B The resistance is twenty times smaller. C The resistance is forty times smaller. D The resistance is four hundred times smaller.
70 Look at the simulation from Activity 8 of the last section
again. The electrons in an electric current are subject to resistance when they travel through a wire. a Explain if the thin or the thick wire has the greater resistance. Look at the filament in a light bulb (figure 45). b Which wire is thicker: the input and output wires or the filament wire? c Explain why the resistance of the input and output wires must be much lower than the resistance of the filament wire.
Ch1 Electrical appliances
41
Comprehending Onderdeel
Tekst... law Ohm’s
Activities 13, 14 and 15, page 48. The voltage is the source of the electric current. The current through an appliance depends upon the resistance and the voltage. A higher resistance gives a lower current. At a constant voltage the current is inversely proportional to the resistance. This is Ohm’s law.
Ohm’s law
Ohm’s law can be written as an formula: voltage = current × resistance This formula can also be written using symbols: U=I×R where U is the voltage in volts (V) I is the current in amps (A) R is the resistance in ohms (Ω) When you increase the voltage across a metal wire during an experiment the current also increases. At a constant resistance the voltage is proportional to the current. Doubling the voltage gives a doubling of the current. The ratio of voltage to current is then constant. This relationship can also be written as a fraction.
For a car headlight the current is high and the voltage is low. The fraction UI is then small hence the resistance is low. With an LED light at home the current is very small at a high voltage. The fraction UI is then large therefore the resistance is high. Ohm’s law shows that an appliance with a high resistance needs a high voltage to achieve the same current. You can also say that a greater force is needed to push the same current through a thinner wire. Domestic appliances all work at the same voltage of 230 V. From this formula you can deduce that the current is smaller through an appliance with a high resistance: the value of current x resistance must remain constant (230 V). This situation can also be compared to a flow of water. A high-pressure cleaner operates using high pressure and a small water flow. The resistance of the appliance must therefore be high. This is certainly true: the water has to pass through a narrow opening. The resistance of an appliance is not always the same. A hair-dryer has different settings. The setting that produces the highest power has the lowest resistance.
Example calculation 1 A car battery has a voltage of 12 V. When you switch the car lights on a current of 3,0 A flows. The resistance of the car lights is calculated using U=I×R 12 V = 3 A × R
Calculating resistance
The resistance is the same as the ratio of voltage to current. The formula for resistance can be written: R=
R=
12 V = 4,0 Ω 3A
U I
This is the formula U = I × R rearranged.
FIG 46
42
1.3 0.0
Resistance Titel paragraaf
Comprehending Onderdeel
Questions: Comprehending
Example calculation 2 An electrician accidentally drops a ring spanner onto the terminals of a car battery. The resistance of the spanner is very low at about 0,01 Ω. A high current flows: U=I×R 12 V = I × 0,01 Ω 12 V I= = 1 200 A 0,01 Ω
73 For each of the following statements answer true or false.
If the statement is false, change it to make a statement which is true. a In units, ohm = volt × amp. b For a constant resistance the current is proportional to the voltage. c If you connect a resistance of 24 Ω to 12 V a current of 2 A flows. d The formula U = I × R is the same as R = U × I.
74 An electric drill works on a voltage of 230 V. The current The power is then P = U × I = 12 V ×1 200 A = 14 400 W = 14,4 kW
This is sufficient to melt the metal of the ring spanner!
The resistance of a metal wire Activity 16, page 49.
through the drill is 2,3 A. a Calculate the resistance of the drill. A loudspeaker is connected to an amplifier. The current is 0,4 A. The loudspeaker has a resistance of 4,0 Ω. b Calculate the voltage across the loudspeaker. c Calculate the current through the loudspeaker when the voltage is 1,0 V.
75 a Calculate the current through a 230 V, 40 W light bulb.
b Repeat this for a 230 V, 100 W light bulb. c Using a calculation show which of the two light bulbs has the greater resistance.
The resistance of a metal wire depends upon the length, the thickness and the temperature. With a toaster element the resistance increases as the element gets hotter.
76 Two light bulbs both have a power of 55 W. One light
The resistance of a wire also depends upon the length of that wire: the longer the wire, the greater the resistance. Compare this to pushing water through a very long garden hose: this needs more effort than with a short hose. The resistance is therefore higher than with a short hose. When a metal wire is doubled in length the resistance is also doubled. The resistance is therefore proportional to the length. The opposite is true for the thickness. If the ‘thickness’ is doubled the resistance is halved. The resistance is inversely proportional to the thickness. The thickness of the wire actually means the size of the gap through which the electric current passes. The ‘thickness’ is therefore the cross-sectional area of the wire.
bulb is for the home; the other is a car headlight and works at 12 V. a Explain in your own words which light bulb has the greater resistance. b Calculate how many times greater this resistance is. c Give an explanation for the large difference in resistance. Use the fact that the voltage with the headlight is almost twenty times lower than with the filament light bulb.
77 There is a short circuit somewhere in the house because two copper electrical wires are in contact. a Explain why the current is then very high. Use the term resistance in your explanation. The wires making the short circuit are 20 m long in total. The resistance of these wires is 0,15 Ω. b Calculate the current during this short circuit. c Calculate the power during this short circuit. d What happens to the energy during a short circuit? e Explain which danger arises during a short circuit.
Ch1 Electrical appliances
43
Comprehending Onderdeel
The wire is cut in half. Carl repeats the measurements with a half length piece of wire. d Is the new graph still a straight line through the origin? e Describe the differences compared to the first graph. Carl now places the two wires next to each other, to form a wire with double the cross-sectional area of the original wire. f Deduce what the resistance of the double wire will be.
I (A)
Tekst... 0,5 0,4 0,3 0,2 0,1 0
0
1
2
3
4
5
6
U (V)
FIG 47
78 Lisa and Chad measure the current through a light bulb
I (A)
at different voltage values. The voltage must not be higher than 6 V, otherwise the light bulb will break. Figure 47 shows the graph of their results. a Where in the graph is the power greatest? b Calculate the resistance of the light bulb at that point. c How can you see from the graph that the relationship between voltage and current is not constant? d Calculate the resistance when the current is 0,10 A. e What is the reason for the change in the resistance? 0,4 0,3 0,2 0,1 0
0
2
4
6
8
10
12
14 U (V)
FIG 48
79 Carl measures the current through a wire at various voltages. He plots his results in the graph shown in figure 48. a How can you see from the graph that the voltage is directly proportional to the current? U b Use two examples to show that the ratio is I constant. c Calculate the resistance of the wire at 6 V and at 12 V.
44
80 A vacuum cleaner has a power of 1,8 kW. It uses
mains electricity (230 V). a Calculate the current through the vacuum cleaner. b Calculate the resistance of the vacuum cleaner. The power is variable. At the lowest setting the power is 0,25 kW. c Which quantities (voltage, current and resistance) stay the same? d Calculate what the resistance of the vacuum cleaner is now.
81 To prevent water pipes from freezing during the winter farmers and factories wrap a heating cable around them. This is connected to the mains socket. A power of 150 W to 200 W is needed to heat 12 m of water pipe. a Calculate the minimum and maximum resistance of the heating cable. A farmer has two cables of different length and the same thickness. b Explain which of these two cables has the greater heating power. There are also cables that work on a 12 V battery. These cables are much more expensive per metre than the cables that work on 230 V. c Explain why these cables are more expensive. d How many times larger is the cross-sectional area of these cables if they supply the same power for the same length?
1.3 0.0
Resistance Titel paragraaf
Resistivity
Knowing more Onderdeel
The resistivity is the multiplier in the formula. Table 8 contains the resistivities of several commonly used metals.
Activity 17, page 49.
Resistivity (in Ω.mm2/m)
Metal
In order to investigate which metal conducts best different metals are compared under identical conditions (see figure 50): • The wire is 1 m in length. • The cross-sectional area of the wire is 1 mm2. • The wire is at a temperature of 20 °C.
gold 0,022 silver 0,016 copper 0,017 iron 0,105 aluminium 0,027 TABLE 8
This gives the electrical resistivity of the metal. Every substance has its own resistivity. For example, copper has a lower resistivity than iron. The resistivity also enables you to calculate the resistance of a longer and/or thicker wire.
1mm2
1m
FIG 49
FIG 50 H igh tension cables are made from a core of aluminium or copper surrounded by an insulating coating to provide strength.
The resistance of a metal wire is directly proportional to the length of the wire. For example, if the wire is five times longer the resistance is five times greater. The resistance of a metal wire is inversely proportional to the cross-sectional area. If the area is ten times larger, the resistance is ten times smaller. These relationships are evident in the formula for the resistance of a wire.
Resistance of a wire
The resistance of a metal wire can be calculated using: R=
ρ×I A
Example calculation 1 A 20 km long high tension cable is made of aluminium. The cross-sectional area is 1 000 mm2. What is the resistance? First look up the resistivity of aluminium in Table 8: 0,027 Ω•mm2/m. Put the values into the formula for resistance. R=
0,027 Ω•mm2/m × 20 000 m = 0,54 Ω 1 000 mm2
where R is the resistance in ohm (Ω) ρ is the resistivity (Ω•mm2/m ) l is the length in metres (m) A is the cross-sectional area (mm2)
Ch1 Electrical appliances
45
Knowing more
Superconductors and semiconductors
Example calculation 2
Metals do not all conduct electricity equally well. Copper is a better conductor than iron; silver and gold conduct better still. There are also substances called superconductors that conduct better than metals. Below a certain temperature their resistance can be as low as zero. If there is no resistance, the wire does not get hot. It is then possible to pass currents of thousands of amps through a wire. Superconductors are used to make very powerful electromagnets.
A wire of an unknown metal has a length of 50 cm. The cross-sectional area is 0,028 mm2. When the wire is connected to a voltage of 6 V there is a current of 0,75 A. What is the resistivity? First calculate the resistance: R=
U 6,0 V = = 8,0 Ω l 0,75 A
Filling in R =
8,0 Ω =
ρ×l gives: A
ρ × 0,50 m 0,028 mm2
ρ × 0,50 m = 8,0 Ω × 0,028 mm2 ρ = 0,45 Ω•mm2/m FIG 51 A n MRI scanner with superconducting magnets
200 mobile phones contain enough gold for one wedding ring What happens to your discarded laptop, fax machine or mobile phone? These are taken apart and the raw materials are shipped to China and India. At the Sims recycling facility in Eindhoven there are rows of twometre tall crates. They are stuffed full of laptops as thick as telephone books, display screens as big as a housemoving box, fax machines and other old appliances. According to director Johan Zwart, “We see the history of technology pass through here.” In 2007 Sims Recycling Solutions made a profit of € 5,5 million with a turnover of € 26,8 million and 100 employees. Recycling is a lucrative business. Recycling ICT appliances is even more lucrative. This is because the materials in the devices, such as aluminium, nickel and copper, are increasingly expensive on the world market. Upcoming markets such as China and India are very keen to have these raw materials. Last year the company recycled 60 million kilograms of electronic devices. Zwart says, “Recycling is above-ground mining.” NRC
Semiconducting materials such as silicon have a resistance that is a billion times greater than that of metals. This might seem problematic, but by adding a small amount of another substance the conductivity can be adjusted as required. All modern electronics is based on semiconductors. Microchips are built up of layers of semiconductors that are alternately insulating and conducting.
Questions: Knowing more 82 For each of the following statements answer true or
false. If the statement is false, change it to make a statement which is true. a Copper conducts electricity better than iron. b A thick, short copper wire has a lower resistance than a long, thin copper wire of the same weight. c The resistivity of a substance is the resistance of one kilogram of that substance. d Good contact points are often made from gold. e Gold is the best electrical conductor.
83 The resistivity of copper is 0,017 Ω•mm2/m. a Explain the meaning of the term resistivity by completing the following sentence: The resistance of a copper wire with a length of ... and a cross-section of ... is ...
46
1.3
Resistance
Knowing more
I (mA)
The wires of the domestic electrical wiring are made of copper. A domestic wiring wire 20 m long has a resistance of 0,14 Ω. b Calculate the resistance of one metre of electrical wire. c How can you tell that the cross-section of this wire is greater than 1 mm2? d Calculate the cross-sectional area of the wire in the domestic electrical wiring circuit.
86 You can buy loudspeaker cable in various qualities. The cheapest cable costs € 0,25 per metre and has a crosssectional area of 0,4 mm2. The most expensive cable costs € 2,50 per metre and has a cross-sectional area of 4,0 mm2. a Explain why the thicker cable is more expensive. Loudspeakers usually have a resistance of 8 Ω. b Explain why it could be useful to buy extra thick cables if there is a large distance between the amplifier and the loudspeaker. c Explain why a 20 m long cable has an electric circuit that is 40 m long. d Calculate the resistance of a 40 m long cheap cable and of a 40 long expensive cable.
1000 800
87 An electric current can flow indefinitely through a ring
600 400 200 0
0
1
2
3
4
5
6 U (V)
FIG 52
84 You measure the current through a light bulb at various
of superconducting material. a Explain why no voltage is necessary to keep the electric current flowing. b Explain why the wire does not get hot due to the flow of current. c How could you stop the current without breaking the ring? Microchips for computers and other electronic devices are built up of many thin layers. Some layers are conducting while other layers are insulating. d Which type of material is used for the manufacture of computer chips? e Describe how this material can be used to make both insulating and conducting layers.
voltages and plot a graph of your results. This graph is given in figure 52. a How can you see from the graph that the resistance of the light bulb is lower at 2 V than at 6 V? The resistance of a cold filament is 2,5 Ω. b How can you see from the graph that the resistance is 2,5 Ω? The filament is made of tungsten with a resistivity of 0,055 Ω•mm2/m. The filament is 20 cm long. c Calculate the cross-sectional area of the wire. d Tungsten has a very high melting point of almost 3 500 °C. Explain why this is important with an incandescent light bulb.
85 According to a headline in an article in the NRC newspaper there is enough gold in 200 mobile phones to make one wedding ring. A wedding ring contains on average about 10 g of gold. What is the value of the gold in a mobile phone in euro? Use a gold price of € 45 per gram.
88 In general you do not take the resistance of connecting leads into consideration. Is this resistance actually negligible? A 50 cm long lab lead is made from copper. The cross-sectional area is 0,5 mm2. a Calculate the resistance of this lead. b Can you generally neglect this resistance? At a campsite a 50 W car light is connected to a 12 V battery. The copper connecting cable is 50 m long and the same thickness as the lab lead. c Calculate the resistance of the light and of the connecting cable. d Explain why the resistance now cannot be neglected.
Ch1 Electrical appliances
47
Experimenting
Activity 13
Activity 14
Resistance of a light bulb
Resistor colour codes The coloured bands on a resistor enable you to read off the value of the resistance.
You will need several resistors, multimeter
What to do • •
Note down the resistance of each resistor using the colour code. Measure the resistance of each resistor with the multimeter.
Question The last band is gold or silver coloured. With a gold band the actual resistance deviates no more than 5% from the value according to the rings. With a silver band this tolerance is 10%. Check if all of the measured values agree with this condition.
Activity 15 FIG 53
In this Activity you will compare the resistance of a cold light bulb with the resistance of a light bulb that is already shining brightly.
You will need • •
230 V light bulb, not in a fitting ohmmeter
Current and voltage in a wire You will investigate how the current changes as you change the voltage across a wire.
You will need • • •
metal wire mounted on a board power supply, voltmeter, ammeter connecting leads and light bulb
What to do What to do • •
48
Read the power of the light bulb from the bulb. Measure the resistance of the cold light bulb using the ohmmeter.
•
•
Measure the current through the metal wire for five different voltages. Note down your measurements in a table. Repeat the measurements with the light bulb.
Questions
Questions
• Calculate the current through the light bulb when it is shining brightly, using this information. • Calculate the resistance of the light bulb when it is shining brightly. • How many times greater is the resistance of the filament when the bulb is operating at the correct voltage? • What is the cause of this?
• Plot a graph of both sets of measurements. Plot the current I (vertical) against the voltage U (horizontal). • Look at the graph and the table. Does the current increase proportionally with the voltage with the metal wire? What about the light bulb? U • Is the ratio I constant for each set of measurements? • Calculate the resistance of the metal wire. • Why is the resistance of the light bulb not constant?
1.3
Resistance
Experimenting
Activity 16 The resistance of wires
You will need
In this experiment you will investigate the relationship between the current and resistance in a wire at a fixed voltage. Several metal wires of different thickness and/or length are mounted on a board. The metal that the wires are made from is given on the board.
• • • • •
several metal wires mounted on a board power supply voltmeter ammeter connecting leads
What to do
+
• •
-
•
A
•
V 1
2
4
3
5
6
8
7
9
10
FIG 54
Connect the circuit shown in figure 54. Set the power supply to a fixed voltage. This voltage is not changed during this experiment. Measure the value of the current through each wire at this voltage. Note down your measurements in a table. U Calculate for all measurements and fill this in your I table.
Questions • Look at the measurements in your table. Which wire has the highest current? Did you expect this? • Find out if the current is inversely proportional to the resistance. • Which wire has the highest resistance? • Compare the resistance of each wire with the length and/or thickness. Find out how the resistance changes as the wire gets longer or thicker.
Activity 17 Not all metals are equally good conductors
What to do
You can calculate the resistance of a metal wire or appliance using the formula U = I × R. The resistance of a metal wire depends upon the thickness (the cross-sectional area), the length and the type of metal. Different types of metal wires are mounted on a board. Each wire is labelled with the type of metal, the length and the cross-sectional area.
• • •
Questions •
In order to compare the metals fairly you will determine the resistance of a wire of length 1 m with a cross-sectional area of 1 mm2. This is the resistivity of the metal.
Measure the resistance of each lead. Ask your teacher what the cross-sectional area of the wires is. Note down the type of metal, the measured resistance, the length and the cross-sectional area in a table.
•
Use the measurements to calculate the resistance of a wire of length 1 m and cross-sectional area 1 mm2 for each type of metal. Arrange the metals in order of resistivity.
You will need • • •
board with wires of different metals digital ohmmeter connecting leads
Ch1 Electrical appliances
49
Discovering
Bumper cars You are watching the bumper cars at the funfair. Only two cars are being used. Your little brother, who is still at primary school, asks you all sorts of questions, “Why is the floor made of steel? Will you die if you touch the net on the ceiling? And if there are ten cars moving, do they not go as fast?” Well, try answering that lot! Then you also hear the son of the man running the ride say, “I want to soup-up one of the cars. I’m going to put in a more powerful engine.” Your brother asks, “Can he do that? Will the other bumper cars still work properly then?” You want to answer all of his questions.
There is an electric voltage of between 70 V and 110 V on the net above the bumper cars. It is dangerous to touch this net at the same time as you touch a supporting pole or the floor. If you jump up to the net and only touch the net then you would not get a shock. 4 Explain this. 5 E xplain why the birds in figure 56 can happily sit on the wires.
FIG 56 ‘Online’ birds
FIG 55 How do bumper cars actually work?
Investigate! The bumper cars have a long rod that makes contact with a metal net above the floor of the ride. Sometimes there is a spark at the net. This shows you that the cars work on electricity. You look up information about vehicles that work using electricity. Maybe this will help you understand how the bumper cars work.
Read Source 3. 1 E xplain which part of the ‘train circuit’ can be compared to the metal net suspended over the bumper cars.
Do Activity 18. 6 Which part of Activity 18 can be compared to using a souped-up bumper car? 7 Explain why using a souped-up bumper car does not affect the operation of the other bumper cars.
Read Source 4. 8 E xplain in which way the floor of the bumper car ride can be better compared to train rails than to the street under a moving trolley bus.
End result •
2 Explain which part of the ‘train circuit’ can be compared to the metal floor with the bumper car ride. 3 Explain why the bumper cars run just as well when more of them are in use. Use a comparison with the example of the trains in Source 3 for this.
50
•
Write a one page report with short paragraphs and illustrations. Describe the similarities and differences between bumper cars, trains and trolley buses. Explain why it is possible for several vehicles to run using the same power supply. The number of bumper cars is not infinite, and there cannot be too many trains pulling away from a station at the same time. Find out why this is.
1.4
Source 3
Electric circuits
Discovering
Trains
Source 4
Figure 57 shows a schematic illustration of a piece of train rail with trains. Each train is shown by an E for ‘engine’. The wires of the overhead cable are the positive terminal (1 500 V) and the rails are the negative terminal. A train engine works on a voltage of 1 500 V. On the left of the illustration is a point where a voltage of 1 500 V is put on the cables. There is a connection between the positive terminal (overhead cable) and the negative terminal (rails) drawn in three places. These three places are where the trains are. Currents flow from the overhead cable via the overhead train contacts to the engines (E) and through the rails.
Trolley buses
Trolley buses run in Arnhem. These work slightly differently to trains. Trolley buses have two overhead cables. One is the positive terminal; the other is the negative terminal. The road cannot be the negative terminal of the circuit, because the road and the bus tyres are not conductors.
overhead cable
+ -
E
E
E rails
FIG 58 T rolley buses have two pantographs (connectors) and two overhead cables.
FIG 57 Diagram of a circuit with three trains
Each train is directly connected to the voltage of 1 500 V. Each train therefore has its own electric circuit. When more than one train travels on the same piece of rail, the power supply provides a higher current. The voltage stays the same. Each train therefore operates using a voltage of 1 500 V.
Trolley buses are quieter and cleaner than normal city buses. The disadvantages are that the overhead cables are expensive and clutter the view of the street. In addition the buses cannot travel through other streets.
Activity 18 Modelling with aluminium foil
What to do
You will build a model of the bumper cars circuit. You will use a light bulb to represent a car.
•
You will need
• •
• •
•
• • •
4,5 V battery 2 pieces of aluminium foil approximately 2 cm wide and 15 cm long, stuck 10 cm apart onto a sheet of paper bicycle lights with connecting leads ammeter voltmeter
• • •
Connect the two strips of aluminium foil to the battery via the ammeter. Connect the voltmeter to the battery. Measure the voltage and the current when one light shines. Connect up two lights and see if the first light is as bright as before. Measure the voltage and the current again. Now do this again with three lights. Finally take one light with a higher power and see what effect this has on the operation of the other lights and on the current.
Ch1 Electrical appliances
51
Understanding Onderdeel
All electrical appliances in the home work on 230 V. Tekst... This is the voltage of mains electricity. There is just one power supply with many appliances. How are these appliances connected to the power supply? This section is about different types of circuits.
Key terms
Voltage and current in a parallel circuit
Series circuit, parallel circuit
Activity 19, page 64.
Two lights with one power supply
You want to connect two light bulbs to one 4,5 V battery. Both lights are suitable for 4,5 V, but what is the correct way to connect them to the battery? Figure 59 shows two possibilities. In figure 59A the lights are connected one after the other, so that the current flows first through one light and then through the other light. This is called a series circuit: the light bulbs are said to be connected in series. The lights can also be connected alongside each other as shown in figure 59B. The current splits so that part of the current passes through one light and part passes through the other light. This is a parallel circuit: the lights are said to be connected in parallel.
-
+
-
4,5 V
A
With a parallel circuit each light bulb is connected directly to the terminals of the battery. The electrons in the light bulb therefore feel the force of the battery. With a series circuit this force has to push the electrons through two light bulbs one after the other. A higher voltage source is then required so that the lights shine brightly.
At home all appliances are connected to the mains supply. These appliances are designed for the mains voltage of 230 V. Each appliance must be connected directly to 230 V. The domestic mains circuit is therefore a parallel circuit.
Train departures: don’t all set off at the same time A large amount of energy is required to set a train in motion. A 1 000 A current passes through the electromotors of a train. This can lead to problems. Trains are connected in parallel, just like the different appliances in the home. When several trains leave at the same time, the total current increases. If three trains were to leave at the same time a current of 3 000 A would be needed. However, there is a limit to the current that can be supplied and therefore also the number of trains that can leave simultaneously. It is therefore sensible to have the trains leave shortly after one another.
+
4,5 V
B
FIG 59 L ight bulbs can be connected to a battery in two ways.
The light bulbs in the parallel circuit in figure 59B shine brightly. In the series circuit they produce much less light. In order to have two 4,5 V lights shining brightly in a series circuit a voltage of 9 V is needed. The voltage gives the amount of force ‘pushing’ the electrons into motion. This force is generated by the positive and negative terminals of the battery.
52
FIG 60 W hen a train leaves the station the overhead cables must supply a thousand amps.
1.4 0.0
Electric Titelcircuits paragraaf
Understanding Onderdeel
The appliances at home do not affect one another. When one appliance is switched off or breaks, the other appliances carry on working, as they are still connected to 230 V and have their own circuits. When you switch an appliance on or off, the total current to all of the appliances together increases or decreases. You can calculate the total mains current by adding up all the individual currents.
If instead of 23 bulbs the manufacturer places fewer identical bulbs in the lead, the resistance in the circuit will be lower. The current will then be higher, and each bulb will have a larger proportion of the voltage across it. Both changes mean that the lights will shine brighter and will therefore break sooner.
Current and voltage in a series circuit
The batteries in a pocket torch are almost always connected head to tail: a positive terminal touches a negative terminal. These batteries are therefore also connected in series. The voltages of the individual batteries are then added together. In a pocket torch with four 1,5 V batteries there is a voltage of 6 V across the light bulb.
Activity 20, page 64. A series circuit with light bulbs is not very useful, because if one of the light bulbs breaks all the lights stop working. Series circuits are used with Christmas tree lights. You cannot switch the lights on and off individually. One advantage of a series circuit is that the voltage across each light bulb is lower. A cable with small light bulbs can then be connected to the mains supply. The manufacturer chooses light bulbs that work on 10 V, for example, and 1 connects 23 in series. There is then 23 of 230 V = 10 V across each light bulb. With a series circuit the current through all the light bulbs is the same. This is because there is just one current, and this flows through all of the light bulbs.
Power sources and circuits
Batteries are only connected in parallel in situations where a large current is required. Three batteries side by side can provide a higher current than one battery. In practice a larger battery is usually used, as this will also provide more current.
Electric eels Electric eels also use a series circuit to generate a high voltage. Their body contains electric organs that work like small batteries. There are more than one hundred of these organs one after the other between the head and tail. The electric eel can produce electric pulses with a voltage of 600 V. This is enough to paralyse or even kill prey or an attacker.
FIG 61 T he greater the number of light bulbs connected in series, the lower the voltage across each light bulb.
Resistance in a series circuit
In a series circuit with 23 light bulbs the current must pass through a filament 23 times before it returns to the mains socket. This can be considered as a single wire that is 23 times longer. The current therefore experiences a resistance 23 times greater than if it passed through one filament only. FIG 62 Electric eels can produce a voltage of up to 600 V.
Ch1 Electrical appliances
53
Understanding Onderdeel
Questions: Understanding
Tekst... Series and parallel circuits in electric racing cars With electric toy racing cars each player has a speed control. A press-button switches extra resistances in or out. The more the button is pressed in, the lower the resistance.
89 For each of the following statements answer true or
FIG 63 The speed control in this car racing circuit is also a resistor.
The speed control is connected in series with the motor of the car. A lower resistance means that a higher current passes through the speed control and the car, so the car moves faster. Each car has its own track. The tracks are connected in parallel to the power supply so that the cars move independently of each other.
false. If the statement is false, change it to make a statement which is true. a In a parallel circuit there is a single circuit of electric current. b In a parallel circuit the voltage across each appliance is the same. c When batteries are connected in parallel the voltage increases. d The total resistance decreases as more appliances are connected in parallel. e The current decreases when more appliances are connected in series.
A
-
+
Summary • • •
There are two ways to connect electrical appliances: in parallel or in series. Domestic electrical appliances are connected in parallel. With a parallel circuit: - the voltage across each appliance is the voltage of the power supply; - each appliance can work independently, including when the other appliances are switched off or broken; - the power source supplies the total current of all appliances together. • With a series circuit: - the appliances all work at the same time (or not at all); - the voltage supplied by the power source is divided among the appliances; - the current through each appliance is the same; - the current is reduced when another appliance is added to the circuit; - the total resistance is higher than that of the individual appliances. • Batteries are often connected in series. They then provide a higher voltage equal to the sum of the voltages of the individual batteries.
54
uit
aan
B
-
FIG 64
Nask_LB_13vwo_01_04_07a
+
4,5 V
1.4 0.0
Electric Titelcircuits paragraaf
Understanding Onderdeel
90 Figure 64 shows a series circuit and a parallel circuit. Draw the circuit diagram for both circuits.
A
91 You connect a light bulb and a radio to a multiway
Q
socket. The light bulb shines and the radio plays. a Why does the light bulb continue to shine when you switch off the radio? b How can you calculate the total current from the mains socket? c Explain how the appliances at home are connected to the mains electricity supply.
P off
on R
-
S
+
4,5 V
92 Figure 65 shows a power supply, two resistors and an ammeter. a Explain if this is a series or a parallel circuit. b Draw the circuit diagram for this circuit. c Explain if the ammeter is connected correctly. The resistors are not the same. However, the current through the resistors is the same. d Explain this.
2,25 V
2,25 V
B
-
+
4,5 V
-
+
FIG 66
20 Ω
93 Look at the circuit in figure 66A. a Why doesn’t the light bulb shine? 0
60 Ω
1
2
3
4
The switch is closed.
b What is the voltage across the light bulb now? Q P c Is the current through the switch now higher, lower
off on or the same as that through the light bulb? I n figure 66B two identical light bulbs are in R connected S series. + d Why is the voltage across each light bulb 2,25 V? e Explain if the current through each light bulb is the same now. f Why do you not know4,5if Vthe light bulbs shine normally, weakly or very brightly?
30 60
FIG 65
Ch1 Electrical appliances
55
Understanding
94 Figure 67 shows two circuits. a Is figure A a series or parallel circuit? b Is figure B, with the switches open, a parallel or
spring to hold batteries in contact
series circuit? c What type of circuit is figure B when both switches are closed? In figure B, if one switch is open and the other is closed only one light bulb shines. d Explain why, using a diagram.
A
metal casing slide switch insulated metal reflector
+ + FIG 68
96 A pocket torch uses two 1,5 V batteries (figure 68). a The base of the light bulb makes contact with the
B
+ + -
FIG 67
95 The circuit for the electric toy racing cars in figure 63 has a power supply, two cars and two controls. Draw the electric circuit.
56
+ terminal of the battery, but there is no wire to the − terminal. How does the current flow? b What is the voltage across the light bulb?
97 Look at figure 61. Six identical lamps are connected to a voltage of 24 volts. This voltage is provided by two 12 volt accumulators. The lamps shine brightly. a Explain if the accumulators are connected in series or in parallel. b Draw the circuit diagram for this circuit. c What is the voltage across each lamp? Explain why. You remove one lamp and close the circuit again. d Calculate the voltage across each of the remaining lamps. e Deduce how the current through each lamp has changed. f Explain if the lamps now shine more or less brightly. You could also connect the lamps in parallel with each other to one of the accumulators. g What would the voltage across each lamp be? h Why is this circuit not a very good idea?
1.4
Electric circuits
Comprehending
Calculating current in a parallel circuit
In a parallel circuit the voltage across each appliance is the same as the voltage of the power supply, since each appliance is connected directly to the supply voltage. To calculate the total current you add up the currents in each parallel branch of the circuit.
Itotal
I1
I2
I3
+ FIG 69 Three lamps connected in parallel
Calculations for a series circuit Activities 21 and 22, page 65. In a series circuit there is just one single circuit of electricity flowing. This is why the current through each appliance is the same. In a series circuit the voltage of the power supply is divided among the different appliances. If a resistor is added, the total resistance in the circuit increases. The total resistance is calculated by adding the individual resistances together. Ohm’s law, U = I x R, can be used for each appliance individually and for the whole circuit. The current can be calculated from the power supply voltage and the total resistance. Then you can use U = I x R to calculate the voltage across each appliance individually.
+
For a circuit with appliances connected in parallel: • The voltage U is the same for each appliance. • The current that the power supply must provide is the sum of the currents through the appliances. For example, three appliances are connected in parallel (see figure 69).Then: Itotal = I1 + I2 + I3 Where I1, I2 and I3 are the currents through the individual appliances. Itotal is the total current that must be supplied by the power supply.
Example calculation 1 An electric oven (R = 100 Ω) and a kettle (I = 0,9 A) are connected in parallel to a mains socket. What is the current from the mains socket? First calculate the current through the oven using U = I × R: 230 V = I1 × 100 Ω 230 V I1 = = 2,3 A 100 Ω
R3 = 250 Ω
-
R2 = 550 Ω
R1 = 1 k Ω
FIG 70 A series circuit with three appliances
For a circuit with appliances connected in series: • The current through each appliance is the same. • The total resistance is the sum of the resistances of the appliances. • The voltage of the power supply is divided across the appliances. If the appliances are identical, each appliance has an equal proportion of the voltage. Consider a series circuit of, for example, three appliances. Then: I = I1 = I2 = I3 Rtotal = R1 + R2 + R3 Utotal = U1 + U2 + U3
The total current from the mains socket is then Itotal = I1 + I2 = 2,3 A + 0,9 A = 3,2 A
Ch1 Electrical appliances
57
Comprehending
Questions: Comprehending
The current through the series circuit and the voltage across appliance 1, for example, can then be calculated as follows:
98 For each of the following statements answer true or false. U I = total Rtotal U1 = I × R1
Example calculation 2 Two resistors of 200 Ω and 300 Ω are connected in series to a 10 V power supply (figure 71). Calculate the voltage across the 300 Ω resistor.
-
+
If the statement is false, change it to make a statement which is true. a In a series circuit the total resistance is equal to the sum of the resistances. b In a parallel circuit the total current is equal to the sum of the currents. c Ohm’s law is also valid for the whole circuit. d When more lamps are connected in series, a higher voltage is needed so that they all shine brightly. e In a parallel circuit the current is the same throughout the circuit; in a series circuit the voltage is the same throughout the circuit.
99 Three lamps are connected in parallel to a 12 V battery. 10 V
R2 = 300 Ω
R1 = 200 Ω
FIG 71
The resistances of the lamps are R1 = 4,0 Ω, R2 = 3,0 Ω and R3 = 2,0 Ω. a Draw the circuit. b Calculate the current through each of the three lamps. c Calculate the current supplied by the power source.
100 Three resistors are connected to a 12 V supply. First calculate the total resistance: Rtotal = R1 + R2 = 200 Ω + 300 Ω = 500 Ω Then calculate the current using: Usupply = I × Rtotal 10 V = I × 500 Ω
R1 = 5,0 Ω, R2 = 3,0 Ω en R3 = 2,0 Ω. a Draw the circuit. b Calculate the total resistance in this circuit. c Calculate the current supplied by the power source. d Calculate the voltage U1 across resistor R1. e Explain why resistor R1 has half of the voltage. f Also calculate the voltage across resistors R2 and R3.
101 Three resistors are connected in parallel to a 6,0 V 10 V I= = 0,02 A = 20 mA 500 Ω Now calculate the voltage across the 300 Ω resistor: U2 = I × R2 = 0,02 A × 300 Ω = 6 V
58
power supply. R1 = 20 Ω, R2 = 75 Ω and R3 is unknown. a Draw the circuit diagram. b Calculate the current I1 through R1. c Calculate the current I2 through R2. The total current from the power supply is 0,44 A. d Calculate the unknown resistance.
1.4
Electric circuits
Comprehending
102 Three appliances are connected to the mains supply at
home: a 180 W computer, a 50 W printer and a scanner with a resistance of 460 Ω. a Calculate the current through the computer. b Calculate the current through the printer. c Calculate the current through the scanner. d Calculate the total current.
103 A set of Christmas tree lights has 30 light bulbs. Each bulb is marked 7,5 V; 0,4 A. a Calculate the resistance of a single light bulb. b Calculate the resistance of the 30 light bulbs in series. c Calculate the current through the Christmas tree lights when the bulbs shine normally. d Explain why the set of bulbs is not intended to be connected to 7,5 V. e Find out if these light bulbs shine brightly if you connect the lead to the mains socket. f What is the voltage across each light bulb then?
105 Ben looks for a 10 Ω resistor in his electronics hobby kit, but he can’t find one. His sister tells him that he can use a combination of 18 Ω and 22 Ω to achieve the same effect in a circuit. Ben investigates if this is true. He connects the two resistors in parallel to a 6,0 V power supply. a Draw the circuit diagram. b Calculate the current through the 18 Ω resistor. c Calculate the current through the 22 Ω resistor. d Calculate the total current through the power supply. Ben actually wanted a 10 Ω resistor. e Calculate the current through a 10 Ω resistor when it is connected to 6,0 V.
106 The electromotors in a train are connected to 750 V.
R1 = 2 Ω
I1
R2 = 6 Ω
I2
To travel slower the electromotors must have a lower voltage. In older trains the driver would switch an extra resistor into the circuit in series with the electromotor. a Draw the circuit with the electromotor connected directly to a 750 V power supply. b Draw the circuit again, but now with a 2 Ω resistor in series with the electromotor. c Explain why the train travels fastest without the resistor in the circuit. With the 2 Ω resistor in series the electromotor gets a voltage of 500 V. d What is the voltage across the 2 Ω resistor? e Calculate the current through the resistor. f Explain what the current through the electromotor is now.
107 At home different appliances are switched on at the
+
-
FIG 72
104 Look at the circuit diagram in figure 72. a Explain which current is higher: I1 or I2. b Explain why the higher current is three times greater
same time: a vacuum cleaner (800 W), a toaster (900 W), a television (80 W) and several lights (total 250 W). You want to calculate the total current. a Calculate the current through each appliance individually. b Use this to calculate the total current. Your mother says, “You’d be better off adding up the power of each, then calculating the current.” c Use a calculation to find out if this is true.
than the lower current. The current through the 6 Ω resistor is 1,5 A. c Calculate the current through R1. d Calculate the voltage across R1. e Explain what the voltage of the battery is. f Calculate the current from the battery. g Calculate the total resistance of the circuit.
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Comprehending
A
B
C
D
E
111 A bicycle front light contains three LEDs. Each LED is
FIG 73
108 Harry has connected a number of 1,5 V batteries in series as shown in figure 73. a Between which two points is a battery connected the wrong way round? b What is the voltage between points A and C? c What is the voltage between points C and E? d Explain why the total voltage supplied by these batteries is 3 V, not 6 V. e Draw a circuit with these four batteries where the voltage is as high as possible.
109 Three resistors R1, R2 and R3 are connected in parallel
In a series circuit the voltage is divided between the resistors. Each resistor does not get an equal voltage. d Is the voltage across each resistor proportional to the size of the resistor? Show this using a calculation.
to a power supply. The current from a power supply is 1,0 A. R1 = 60 Ω, R2 = 20 Ω and R3 is unknown. Calculate the resistance of the unknown resistor. R3 = 250 Ω
R2 = 550 Ω
R1 = 1 k Ω
designed to work at a voltage of 4,5 V. This voltage is supplied by three batteries each supplying a voltage of 1,5 V. a Draw a circuit diagram of this bicycle front light showing the three batteries and three LEDs. Draw in a switch so that the LEDs can be switched on and off at the same time. When the three LEDs are shining the power supply provides a current of 0,028 A at a voltage of 4,5 V. b Explain that a current of 0,028 A flows from each battery. c Calculate the resistance of one LED. d Show by calculation that the batteries together provide 126 mJ electrical energy per second. With full batteries the power supply can provide 50 kJ electrical energy. e Calculate how many hours the bicycle front light can be used for.
112 An incandescent light bulb is connected in series with
10,5 V
+
-
FIG 74
110 Three resistors are connected in series to a power
60
supply. R1 = 250 Ω, R2 = 550 Ω and R3 is 1,0 kΩ. The voltage U1 across R1 is 10,5 V. See figure 74. a Calculate the current through R1. b Calculate the voltages U2 and U3. c What is the voltage of the power supply?
a variable resistor to a power supply.
a Explain why you can use this circuit as a dimmer. b How does the voltage across the light bulb change when the variable resistance is lowered? Explain. If the variable resistance is 0 Ω the light bulb shines at 100% of the power. c At what percentage of full power does the light bulb shine when the variable resistance is the same as the resistance of the light bulb? Assume that the resistance of the light bulb is constant.
113 Underfloor heating is increasingly popular. One method is to install an electric heating film under the flooring. This heating film is a roll of very thin plastic 0,2 mm thick. The film contains two copper strips along the length of the roll. These two copper strips are connected by a large number of carbon strips across the width of the roll. The copper strips are connected to the two terminals of a power supply. See figure 75.
1.4
Electric circuits
Comprehending
The underfloor heating film is available in different lengths. Table 9 is given in the Instruction leaflet provided. Type
Power in watts
FH160 160 FH200 200 FH240 240 FH280 280 FH320 320
Current in amps 0,7 0,9 1,1 1,3 1,5
copper
Length in centimetres 200 250 300 350 400
TABLE 9
Henry and Annie use a 2,0 m long strip for their caravan. a Calculate the resistance of a 2,0 m long piece of underfloor heating film from the information given.
b Are the carbon strips connected in series or in
The carbon strips each have a resistance of 13 kΊ.
c Calculate the current through one carbon strip. d Calculate how many carbon strips are attached
parallel?
carbon plastic film
FIG 75 Electric underfloor heating film
Henry and Annie want to know how much the extra energy for the underfloor heating costs. They estimate that the heating film is switched on for 9 hours a day on average. 1 kWh costs â‚Ź 0,22. e Calculate the extra energy costs due to the underfloor heating if they go camping for 14 days. The types FH240, FH280 and FH320 actually have a higher power than that given in the table. f Give an explanation for this. Include a calculation in your answer.
to the 2,0 m long film.
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Knowing more
+
-
+
-
2
1
1 2
3 A
3
B
FIG 76 Three lamps in a combined circuit.
Combined circuits
•
Activity 23, page 65. • The circuits shown in figure 76 cannot be described as series or parallel circuits: they are combinations of both. Part of the circuit has the characteristics of a series circuit and another part has the characteristics of a parallel circuit. The lamps are all identical. For the circuit in figure 76A: • Lamps 2 and 3 are connected in parallel; together they are connected in series with lamp 1. • T he total current passes through lamp 1, but a branched current passes through lamps 2 and 3. Since the lamps are identical, lamps 2 and 3 each get half of the total current (Itotal = I2 + I3). • There is the same voltage across lamps 2 and 3. • Lamp 1 shines brighter because the current through lamp 1 is greater than that through the other two lamps. For the circuit in figure 76B: • Lamps 2 and 3 are connected in series; together they are connected in parallel with lamp 1. • Lamp 1 is connected directly to the power supply. The voltage across lamps 2 and 3 together is the same as the power source, but they each only get part of this (Usupply = U2 + U3). • There is the same current through lamps 2 and 3. The current through lamp 1 is greater because lamp 1 is connected directly to the power supply.
62
The resistance of lamps 2 and 3 together is greater than that of lamp 1. This is why a lower current flows through lamps 2 and 3. Lamp 1 shines brighter.
Scientific notation
With very large and very small numbers it is useful to work using orders of magnitude. With this notation you use powers of 10. Three examples: 30 000 = 3 × 10 × 10 × 10 × 10 = 3•104 2,5 MW = 2,5 × 1 000 000 W = 2,5•106 W 380 kV = 380 × 1000 V = 3,8•105 V Small numbers are given by negative powers of 10. 1 Hence 10−3 is the same as 1 000 and 10−6 is the same as 1 0001 000 . Three examples: 34 0,000 34 = = 3,4•10−4 100 000
5,8 mA = 6,78 μW =
58 1 000 6,78 1 000 000
= 5,8•10−3 A = 6,78•10−6 W
The advantage of this notation is that you can type this into your calculator directly, so you don’t have to type in a large number of zeros. Use the Exp or EE button.
1.4
Electric circuits
Knowing more
I (A)
Questions: Knowing more
117 A copper atom in a piece of copper occupies a space of 2,5·10−10 m. A copper plate is 0,2 mm thick. Calculate how many layers of copper atoms are stacked up in the plate.
0,5 0,4
118 A large power station has a maximum power of
0,3 0,2 0,1 0
0
1
2
3
4
5
6
U (V)
FIG 77
114 In the circuit shown in figure 76A the lamps are identical. They shine normally at 6,0 V and 0,48 A (see figure 77). The voltage is chosen so that lamp 1 shines normally. a Explain why lamps 2 and 3 are less bright than lamp 1. b Explain if the voltage of the battery is greater than, less than or equal to 6 V. c Deduce the value of the current through lamp 2. d Read the value of the current through lamp 2 from figure 77. e What is the voltage of the power supply? Lamp 2 is unscrewed. f Explain what then happens to lamps 1 and 3.
115 In the circuit shown in figure 76B the lamps are also identical. They shine normally at 6,0 V and 0,48 A (see figure 77). The voltage is chosen so that lamp 1 shines normally. a Explain why lamps 2 and 3 are less bright than lamp 1. b Explain if the voltage of the battery is greater than, less than or equal to 6 V. c Use figure 77 to determine the current through lamps 2 and 3. d What is the current supplied by the battery? Lamp 2 is unscrewed. e Explain what then happens to lamps 1 and 3.
870 MW and supplies 5,0·109 kWh electrical energy per year. a Calculate how many hours the power station must operate at full capacity to generate 5,0·109 kWh. b Assume that the power station operates all year round. Calculate at what percentage of the maximum capacity the power station operates on average. A household uses on average 4 000 kWh per year. c Calculate how many households this power station can supply with energy.
119 A 230 V energy-saving light bulb has a resistance of 7,5·103 Ω. In the city of Apeldoorn there are 8,0·104 families, each of which uses five such energysaving lights for four hours per day. a Calculate the total power of all of these lights in MW. b Calculate how many kWh all of these lights combined use in one year.
120 In a copper wire of diameter 0,2 mm the electrons move at a speed of 1 mm/s. One copper atom has a mass of 1,05·10−25 kg. The density of copper is 8,6·103 kg/m3. a Show that the cross-sectional area of the wire is approximately 8·10−9 m2. b Calculate the number of copper atoms in 1 mm of copper wire. For each copper atom present one electron can move freely through the wire. c Explain how many electrons pass through the wire per second. A current of 1 A means that 6,25·1018 electrons pass through per second. d What is the current through the wire?
116 Carry out the following calculations using the Exp or EE button on your calculator. Give the answer in scientific notation. a 1,25·108 × 4,3 •106 = b 2,5·105 × 3,1•1012 =
1,50 •1011 c 3,0 •108
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Experimenting
Activity 19 Light bulbs connected in parallel
What to do
In this experiment you will investigate how to connect up light bulbs in parallel.
• Connect one light bulb to the variable power supply. • Adjust the voltage so that the light bulb shines normally. • Measure the current supplied by the power supply. • Leave the voltage unchanged and connect the second identical light bulb in parallel with the first. • Measure the current in the circuit. • Now connect the third light bulb, also in parallel, and measure the current again.
Questions
FIG 78
• Why do two light bulbs shine just as brightly at the same voltage as with one light bulb? • Why does the current change when you connect up the second light bulb? You have measured the total current through the circuit three times. • Now calculate the current through the second and the third light bulbs.
You will need • • • •
three light bulbs, two identical and one other variable power supply connecting leads ammeter
Activity 20 Light bulbs connected in series In this experiment you will investigate how to connect up light bulbs in series.
You will need • • • •
two identical light bulbs variable power supply connecting leads ammeter
What to do • • • •
64
Connect one light bulb to the power supply. Include the ammeter in the circuit. Adjust the voltage so that the light bulb shines normally and note down this voltage. Note down the current. Leave the voltage unchanged.
• • •
Place the second light bulb in the circuit in series with the first. Again note down the current. Now adjust the voltage until both light bulbs shine normally again.
Questions • Why does the current become smaller when you use two light bulbs? • Explain why you need to increase the voltage so that the light bulbs shine normally. • Explain why the voltage must be higher for two light bulbs connected in series, but not the current. • Predict what will happen with three light bulbs connected in series. What will the voltage need to be then? • What will the current be?
1.4
Electric circuits
Experimenting
Activity 21 Measurements in a series circuit 1 In this Activity you will investigate the voltage in a circuit with components connected in series.
You will need • • •
three different resistors variable power supply voltmeter and connecting leads
What to do • • • • • •
Connect the three resistors in series with the power supply. Set the voltage to 9 V. Measure the voltage U1 across the first resistor. Measure the voltage U2 across the second resistor. Measure the voltage U3 across the third resistor. Measure the voltage across the three resistors together.
FIG 79
Question Which rule have you demonstrated here?
Activity 22 Measurements in a series circuit 2
•
In this lab test you will investigate the current and the resistance in a series circuit.
•
You will need
.
• •
Questions
three different resistors, connecting leads variable power supply, ammeter
What to do • •
Connect the largest resistor to the power supply. Set the power supply to a voltage of 6 V and measure the current.
Place a second resistor in series with the first. Do not change the voltage and measure the current again. Place the third resistor in series with the first two. Do not change the voltage and measure the current.
• Calculate the total resistance for each measurement using Usource = Itotal × Rtotal • Which rule about the resistance in a series circuit can you deduce from your measurements?
Activity 23 Measuring resistance
What to do
In this lab test you will predict and check the total resistance of different circuits.
• •
You will need
•
• •
•
three different resistors digital multimeter and connecting leads
•
Measure the resistance of each resistor. Choose a circuit with two or three resistors, in series or in parallel, or a combination. Predict (individually) what the resistance of the entire circuit is. Connect up the circuit and check by measurement who was correct or who had the nearest answer. Repeat the Activity with different circuits.
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Discovering
Plunged into darkness Rubin leaves the bathroom in his dressing gown and bare feet after having a shower. It is dark outside so his bedroom light is on. His computer is also switched on. Rubin wants to put some loose papers into his bag quickly, but one sheet slips behind the cupboard it was on. He decides that he can just about reach it. He puts his hand into the narrow gap between the cupboard and the wall. Rubin gets a massive electric shock. The light and the computer turn off. Rubin has to recover for a few moments, but he is not injured.
Investigate! Something caused all the electricity to cut out when Rubin made contact with the 230 V voltage. This saved Rubin’s life. He wants to know what type of safety measure this is. First he thinks, “Perhaps the fuse blew.” Fuses blow or break at a current of 16 A.
Read Source 5. 1 S how using a calculation that the current flowing through Rubin could not possibly have been greater than 16 A. Use the information given in Source 5. 2 For each of the circumstances in Table 10 decide if this led to a more or less powerful shock, or if it made no difference. Copy Table 10 and fill it in. worse shock / no difference / less powerful Rubin had bare feet Rubin had just showered The light was on TABLE 10
If the current flows for a time, then a current of more than a few tenths of an amp is fatal. 3 Explain why Rubin would certainly not have survived this electric shock if there had not been a safety measure in place. Rubin searches the web and finds an American website.
Read Source 6. 4 In Britain a different term is used. What is this term? FIG 80 What saved his life?
Later the cause of the shock was identified. The cupboard was placed in the corner years ago. In the corner there was a plug in a plug socket that was squashed when the cupboard was pushed into place. A loose metal pin from the broken plug was still in the plug socket.
66
The number of deaths in the USA fell drastically as a result of the introduction of this safety feature, but it was not reduced to zero. 5 Name one reason why there are still fatalities. This can be deduced from Source 6.
1.5
Electricity and safety
If your family buys an old house in France, or if you live in an old house in the Netherlands, you must check that all the necessary up-to-date safety measures have been installed. 6 At which location in the house can you check this? 7 W hat is the profession of someone who is qualified to carry out this work?
End result Search on internet for information about an accident involving electricity. Describe in two lines either which safety feature ensured that the accident was not as bad as it could have been, or, if there was no such safety measure in place, which measure would have helped prevent the accident ending so badly. Accidents have happened involving domestic wiring and high-speed train lines, for example. Use these search terms to find information, or search ‘electrical accident consequences’ to see a short film about accidents in the home.
Discovering
Source 5
Resistance of the skin
When a current flows through the skin of your fingers, then through your body, and finally through the skin on your feet, the resistance of your skin determines the amount of current that flows. When your skin is dry the total resistance is about 100 kΩ. When your skin is wet then this can fall to as low as 1 % of this value, that is, about 1 kΩ. Shoes and gloves form an extra resistance, so when you are wearing both of these the current must pass through gloves, skin, body, skin and shoe soles. So exposure to an electric voltage is less dangerous when you have gloves and/or shoes on. However, the same is true for shoes and gloves as for skin: if they are wet, their resistance falls drastically.
Source 6
Ground Fault Circuit Interrupters
Before the introduction of GFCIs in newly constructed homes, 700 people died from household electrocutions in the USA per year. As of 2001, that number has decreased to about 400 deaths each year, according to the Electrical Safety Foundation.
FIG 81 In old houses the electrical installation is often not protected by suitable safety measures.
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Understanding
From time to time you hear in the news that someone has been killed by an electric shock or that a fire has started due to a short circuit. How can you prevent accidents involving electricity?
Key terms
Circuit overload, fuse, short circuit, group, earth leakage circuit breaker.
Wires
Always unroll the cable If you want to connect an appliance at some distance from a mains socket you use a cable reel. The current through the cable also generates heat. When the cable is rolled up the wires are close together and the heat cannot escape easily. The temperature can get high enough for the plastic insulation to melt. This leads to a short circuit or even a fire. Therefore you must always unroll the cable completely. Some cable reels have a built-in safety switch which switches off the power if the current gets too high.
Activity 24, page 78.
FIG 82 T he electrical wires in a Dutch domestic wiring installation are contained inside plastic installation pipes.
At home the electric current passes through copper wires to the mains sockets. A high tension cable that runs from the power station to cities has much thicker wires. Electrical wires always have a small amount of resistance. The wires get warm due to the electric current passing through them. Energy is therefore lost during the transfer of electrical energy in the form of heat. This loss is greater at higher currents. The high currents through a high tension network therefore require thick wires with low resistance. The thickness of the wires in a domestic installation is chosen so that they do not get too hot with a 25 A current. For safety a maximum current of 16 A is often used.
68
FIG 83 A rolled-up reel of electrical cable can become dangerously hot.
Circuit overload and short circuit
At home the current can get too high when too many appliances are used at the same time. This is called circuit overload. When there is a circuit overload a fuse switches off the current. This prevents the wires getting too hot and possibly starting a fire. The fuses at home switch off as soon as a current of 16 A is exceeded. The fuse also protects against a short circuit when two wires are in contact and the current becomes very high.
1.5
Electricity and safety
Understanding
Consequences of an electric shock 0,5 - 5 mA 5 - 20 mA 20 - 40 mA
Harmless; produces a slight tingling sensation. Muscle cramps in the hands and arms. From 20 mA it is impossible to let go. Breathing becomes laboured. Nerve centres can become paralysed. 50 - 200 mA The heart stops beating and circulation stops. 200 - 1 000 mA Burn wounds in tissues, nerves and muscles. > 1 000 mA Kidneys become poisoned. Instant death. TABLE 11
FIG 84 L eft: an old fashioned bottle fuse (a stop); right: a modern automatic switch fuse.
The maximum of 16 A is nowhere near enough current for the many appliances in the home. The domestic wiring is therefore divided into groups. Each group has separate wires from the consumer unit to a particular part of the house, with a separate 16 A fuse. A short circuit or circuit overload therefore only switches off the electricity for that group, and not the power throughout the house.
Electric shock
As well as the fire risk, working with electricity also means that there is a risk of getting an electric shock. A high current through the body is fatal. Table 11 shows the effect of various currents through the body. The current flowing through the body depends on the way in which the wires are touched and the resistance of your body. If you touch two wires with two hands, at the same time, the current passes through your heart. This is very dangerous because your heartbeat can then become deregulated. If you have damp hands it is even more dangerous. The resistance of damp skin is lower, so the current is higher. The most important safety measure to prevent an electric shock is plastic insulation. Appliances with a plastic outer casing and a moulded plug are very safe. These appliances are said to be double insulated. High tension is more dangerous because the high voltage can always lead to a high current, even when the resistance is high. The very brief shocks from an electric fence around farm land are not dangerous, however, because these are very short in duration.
FIG 85 T wo earth leakage circuit breakers at left, alongside two automatic switch fuses.
Earth leakage circuit breaker
An earth leakage circuit breaker is included in the domestic electrical wiring to protect against an electric shock. The device compares the current entering the house with the returning current. If there is a difference, electricity must be ‘leaking’ away somewhere. A leakage current arises when there is a loose wire inside an appliance or a current is passing through your body. The earth leakage circuit breaker switches off the power immediately. An earth leakage circuit breaker at home switches off the power at a leakage current of 30 mA. They are also known as residual circuit devices (RCDs).
Activity 25, page 78.
Earth connection
Many electrical appliances have a metal casing, for example washing machines and refrigerators. If the insulation around a wire gets damaged a wire could come into contact with the metal casing. Then there is a voltage across the appliance. To prevent this, appliances with a metal casing are earthed. The lead of such an appliance has an extra wire which is connected to the metal casing. This wire is connected to an outside earth wire via the earth connection in the mains socket.
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Understanding
Then there cannot be a voltage on the outside casing of the appliance, because any current will flow to earth. In addition the earth leakage circuit breaker will switch the current off. In older houses not all mains sockets are earthed. If you plug an earthed plug into a mains socket without an earth connection, the appliance is not protected by earthing. An earth connection can also provide protection for sensitive electronics. Computers almost always have a metal casing to protect the electronics inside from electricity from outside. For example, static electricity from your hand flows away through the outer casing and the earth connection.
.
Questions: Understanding 121 For each of the following statements answer true or false. If the statement is false, change it to make a statement which is true. a Circuit overload is when the electrical wires at home get too hot. b High tension cables do not get hot because they have almost no resistance. c A fuse is a safety feature which prevents a high current flowing through your body. d An appliance with a plastic casing must always be earthed. e An earth leakage circuit breaker prevents a high current flowing through your body. f The plug of a double insulated appliance does not have an earth. You must not plug this into an earthed mains socket.
122 An accident results in 230 V on your body.
a Calculate the current through your body. b Look up the effect of this current in Table 11. c Explain why a fuse would not protect you in this
The resistance of your body is 6,0 kΩ.
d Explain how you could limit the current through your
e Does an earth leakage circuit breaker offer protection
f Explain briefly how an earth leakage circuit breaker
case. body, using the term resistance. FIG 86 M ains socket with earth connection and earthed plug. The lead therefore has three wires.
Summary • • •
A current-carrying wire always gets slightly warm. This results in energy loss during the transfer of electrical energy. If a wire gets too hot this may cause a fire. A fuse protects the domestic circuit from a very high current due to circuit overload or short circuit. • The earth leakage circuit breaker protects you from an electric shock. This switches off the power immediately when current leaks away somewhere in the house. • The earth connection prevents the possibility of a voltage across the metal casing of an electrical appliance.
70
from an electric current through your body? works.
123 A cable reel must always be completely unrolled before use. a Which danger is present when the cable is rolled up? The reel is marked ‘rolled max 3A, unrolled max 9A’. b Explain the difference. During a party in the garden several cable reels are used for the lighting at the back of the garden. The cables are connected together in a line. c Explain why the lighting then shines less brightly than normal.
1.5
Electricity and safety
Understanding
124 A fuse protects from a current that is too high. a What is the danger of a current that is too high flowing through electrical wires? There is a power cut just as you switch the kettle on. b Explain if there is circuit overload or a short circuit. You plug the vacuum cleaner into the mains socket. The vacuum cleaner is not switched on but the power cuts off. When you plug the vacuum cleaner into the mains socket of a different group the power also cuts off there. c What is happening here? d How can you solve this problem?
125 An earth leakage circuit breaker is also known as a residual current device (RCD). a Explain how an earth leakage circuit breaker works. b Do you think that ‘residual current device’ is a good term? c What current difference does an earth leakage circuit breaker switch at? d Why is this low current chosen? e Explain why a fuse does not prevent a current flowing through your body. A short circuit occurs when two electrical leads are in contact. f Explain why a fuse then cuts off the power, but an earth leakage circuit breaker does not.
126 Some appliances are earthed and others are not. a How can you tell from the plug if an appliance is not earthed?
b Explain why appliances with a metal casing are
c What happens as soon as a current flows through
earthed. the earth wire?
FIG 87 You can check if a plug socket is live using a mains tester.
The current through a mains tester is a maximum of 0,01 mA. d Explain why this is a maximum current value. Use the term ‘series’ in your answer. e Calculate the resistance of the mains tester.
128 An earthed mains socket has three connection points: the live wire, the neutral wire and the earth wire. There is a voltage on the live wire, but there is almost no voltage on the neutral wire. a Explain why the fuse breaks if the live wire touches the earth or neutral, but not if the neutral touches the earth. b If the neutral touches the earth the power will usually be cut off. What has happened? An old extension lead doesn’t work very well. However a mains tester indicates that there is a voltage at one of the connecting points. c How can there be a voltage at a connecting point when no current flows? Plugging the extension lead in with the plug the other way round enables you to investigate which wire in the lead is broken. d Describe how to do this.
127 A mains tester (figure 87) enables you to check if there is a voltage across a plug socket. If the light shines when you place your finger at the end of the tester there is a voltage at that socket hole. a Why do you have to place your finger at the end of the mains tester? b Explain why it is not dangerous that a current flows through your body. The mains tester contains a neon light and a large resistor. c Explain whether the neon light and the resistor are connected in series or in parallel.
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Comprehending
Calculations with circuit overload
You want to make plenty of tea and coffee during a party, so you want to plug three kettles into one plug socket. The maximum current in this group is 16 A. How can you calculate if the three kettles will use too much current? The current is not given on most appliances, but the power is almost always given on a label. You can calculate the current if the power is known using the formula P = U × I. The voltage at home is always 230 V. There will be circuit overload if the total current is higher than 16 A. You can also add up the power of each kettle first. Using the same formula you can calculate that you can use a maximum of 3 680 W (16 A × 230 V). Certain appliances, for example a large electric oven, use so much power that they are connected to a separate electrical group with a maximum current of 25 A.
Example calculation 1 Two kettles have a power of 2,2 kW and a third kettle has a power of 2,0 kW. The group is protected by a 16 A fuse. Can you connect all three appliances at the same time? Method 1 Calculate the current through a 2,2 kW kettle using P = U × I: I=
P 2 200 W = = 9,6 A U 230 V
Calculate the current for each kettle. The total current is then: I = 9,6 A + 9,6 A + 8,7 A = 27,9 A This is much greater than 16 A. Even using just two of these kettles will not work! Method 2 The maximum power of the group is: P = U × I = 230 V × 16 A = 3 680 W The three kettles have a total power of 6 400 W. This is much too high to connect to one group.
FIG 88 Is it possible to plug three kettles into one socket?
Calculations with electrical energy transfer
High voltage is used for the transfer of electrical energy across long distances. High tension cables are suitable for currents of up to 500 A. There is no point using even thicker cables, as these would be too heavy and too expensive. A high voltage and a high current are needed to transfer large amounts of electrical energy. The formula P = U × I is valid for the transfer of electrical power.
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At a voltage of 230 V the current from a power station would be much too high (also see Example Calculation 2). For a lower current, the voltage must be high. The high tension network in the Netherlands uses voltages of up to 380 kV. There is always some energy loss in high tension cables. The resistance of the cables is very low, but not zero, so a small amount of voltage is lost. Part of the voltage is used to push the current through the cables, so the voltage at the end of the cable is slightly lower than at the power station. In fact the cables are connected in series to the substation at the end of the network, therefore the voltage is shared between the cables and the substation.
1.5
Electricity and safety
Comprehending
A great danger with high tension is electrocution – a fatal electric current through the body. If you touch a high tension cable, it will kill you instantly. If a crane of other metal object makes a connection between a high tension cable and the ground, there is also a potentially deadly flow of current.
Fatal kite-flying High tension is dangerous. Energy supplier Essent has launched an information campaign in newspapers and on internet to warn people flying kites, sailing and fishing to keep themselves and their equipment well away from high tension cables and high tension masts.
FIG 89 H igh tension cables transfer electrical energy across long distances.
The voltage loss in the cables depends on the resistance and the current. Ohm’s law applies: Uloss = I × R You can then calculate the power loss using the formula Ploss = Uloss × I
Example calculation 2 A power station supplies a power of 690 MW. Why is high tension necessary? Suppose that the power station works with 230 V. The current would be: I=
P 690 000 000 W = = 3 000 000 A U 230 V
Even a cable as thick as your wrist would melt at this current. At a high voltage of 380 000 V the current is much lower: I=
P 690 000 000 W = = 1 816 A U 380 000 V
This current can be used for electricity transfer across long distances. Four parallel cables, each suitable for 500 A, are sufficient.
FIG 90
A high tension cable can have a voltage of 380 000 V across it. You don’t even have to touch the cable to get electrocuted. Just getting close with a kite or fishing rod can result in a fatal outcome. Essent are not doing this just to keep you alive. If there is a fault somewhere in the distribution network, this also has consequences for the power supply. If your kite hits a cable, it’s not just your light that goes out. www.energiezekerheid.blogspot.com
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Comprehending
The transformer Activity 26, page 79. The high tension used for the transfer of electrical energy has to be converted into 230 V for the consumer. For various appliances in the home the voltage must then be lowered again, for example to 12 V. This conversion takes place in a transformer. A transformer converts the voltage to a higher or lower value. Transformers always use a small amount of energy, but with good transformers this is negligible compared to the energy that is being converted. The electrical power output is almost the same as the power input to the transformer. This means that when the transformer converts to a lower voltage the current increases, because in the formula P = U × I the power P stays the same. Conversely if the voltage is increased, then the current decreases.
Questions: Comprehending 129 For each of the following statements answer true or false. If the statement is false, change it to make a statement which is true. a With a 16 A fuse the maximum power is 16 kWh. b The maximum power of a group can be calculated using the formula P = U × I. c At a higher voltage the power loss in the input leads is greater. d At a higher transfer voltage the current in the input and output cables is higher. e A transformer is always used to convert a high voltage into a low voltage. f A transformer works using direct current and supplies alternating current.
130 Each group of the domestic wiring circuit is protected by
With a transformer the voltage and the current change by the same factor. If the voltage increases by 20 times, the current is 20 times smaller. The current is inversely proportional to the voltage.
a 16 A fuse. a What is the term used when too many appliances are connected to one group? b Calculate the maximum number of 60 W lights that can be connected to one group.
131 In the kitchen there is a washing machine with a power of 1 800 W and a tumble dryer with a power of 1 700 W connected to a double socket and to the same group. a Show using a calculation whether the 16 A fuse for this group will work. The kitchen also has a fridge (500 W), a microwavecombination oven (3,2 kW) and several mains sockets. b What is the minimum number of groups needed for this kitchen? c For one of the groups in the kitchen a maximum of 25 A is available. Calculate the maximum power for that group.
FIG 91 High voltage is converted to 230 V in an electricity substation.
In populated areas the transformers are located in substations (see figure 91). At home there are small transformers in the adapters of various types of appliances. A transformer only works with alternating current. This is why the electricity grid uses alternating current. However, many electronic devices in the home work on direct current. The adapter of these appliances contains both a transformer and a rectifier which converts alternating current into direct current.
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132 Mikey, Wayne, Piers and Samantha are in a band. They play at a school party. Mikey’s guitar amp has a power rating of 2 200 W, Wayne’s bass amp is rated 3 000 W and Samantha’s vocal amp is rated 1 800 W. They also use six sets of lights of 600 W each. The school has a number of groups, each protected by a 16 A fuse. a How much power does the band need in total? b Calculate the minimum number of electrical groups necessary for the performance. The band also wants to use a smoke machine, but there are no groups left to use. c Calculate what the maximum power of the smoke machine can be.
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Electricity and safety
133 Thick cables lie on the floor of the North Sea. They carry electricity from the Netherlands to England or viceversa. Read the news report. AMSTERDAM (Dow Jones) – A joint venture between the Dutch power network TenneT and Britain’s National Grid plc began on Tuesday with the laying of a cable under the North Sea to connect the high tension networks of the Netherlands and the United Kingdom. “This joint venture contributes to a more open Europe and will increase the reliability of electricity supplies in the UK and the Netherlands,” according to the director of the joint venture. The cable has a capacity of 1 000 MW and a length of 260 km.
134 High tension cables transfer electrical energy from a
The power loss in the high tension cables is 0,7 MW. d Show using a calculation that the voltage loss in the high tension cables is 10 kV. e Calculate the current at the power station. f Calculate the factor by which the transformer increases the voltage from the power station.
135 A transformer converts a voltage of 230 V to 18,2 volts with no energy loss. The transformer is connected to a light with a power of 46 W. a Calculate the current through the light. b By what factor has the voltage decreased after conversion? c Calculate the current in the input and output wires of the transformer.
a What is the maximum power that can be transported
through this cable? b Explain why the cable has two strands. The maximum current through the cables is 820 A. c Calculate the voltage used for the transfer of electrical energy. The documentation says that the temperature in the cable can reach 50 °C. d Explain why the cable gets so hot.
Comprehending
power station to a town. See figure 92. The households in the town use a total power of 7,9 MW at a voltage of 230 V. The high tension cables have a voltage of 132 kV at the town. The power station supplies a voltage of 20 kV. Assume that there is energy loss in the high tension cables only. a Explain why two transformers are necessary; one at the power station and one at the town. b Calculate the current in the town. c Calculate the current in the high tension cables.
136 A rolled up cable reel can get dangerously hot. A cable reel is marked ‘rolled max 3A, unrolled max 9A’. The wires in the cable have a total resistance of 0,5 Ω. a Explain why some of the voltage is lost in the wires of the cable reel. A 2,0 kW electric heater is connected to the rolled up cable reel. b Calculate the current through the cable reel. c Calculate the voltage loss in the cable reel. d Calculate the power loss in the cable reel. e Also calculate the voltage and power loss with currents of 3 A and 16 A.
A
D
B
transformer
E
F
H
G
transformer
power station
C town
T1
T2
FIG 92 Transfer of electrical energy
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Knowing more
Static electricity Activity 27, page 79. You can sometimes get a shock when you touch a door handle or tap. Friction between your shoes and the carpet has caused you to become electrically charged. As soon as you touch a metal object the charge flows away, and you can feel it. When you take off your jumper on a dry winter’s day you may hear a crackling sound. You can also feel your hair stand on end. Friction between your jumper and the layer of clothing underneath has caused your jumper to become electrically charged. This is called static electricity. The shock that you experience is the discharge. In the dark you can also see tiny sparks. Lightning is also a discharge of static electricity. Lightning is therefore a flow of electricity through the air.
Positive and negative charges attract each other. Positive charges are mutually repellent, as are negative charges. In the electroscope shown in figure 96 the two metal leaves always have the same charge, so they repel each other.
Static discharge
An electrostatic generator can generate substantial sparks using friction. The charge on two metal spheres is gradually increased, thus increasing the voltage between them. When the voltage is high enough there is a discharge: an electric current flows through the air across the gap. The electrons from the negatively charged sphere jump across to the positively charged sphere, and you see sparks. These sparks are in fact very small electric currents at very high voltage. The current is low and of very short duration.
All materials contain electrons. When exactly the right number of electrons is present the material is electrically neutral or ‘discharged’. If there are too many or too few electrons there is a static charge. When there are too many electrons the material is said to be ‘negatively charged’ and when there are too few electrons the material is said to be ‘positively charged’. If you rub an inflated balloon on your clothes static electricity is produced. During rubbing electrons jump from your clothes to the balloon. The balloon is then negatively charged and your clothes are positively charged.
FIG 94 A n electrostatic generator generates a very high voltage between positive and negative charges, causing a spark to pass through the air.
Dust explosions
Static charge itself is not dangerous, but a spark can lead to an explosion. This can happen in a dusty environment. Movement of the dust causes it to become statically charged. A large number of grain silos have exploded due to a tiny spark. Nowadays there are strict safety guidelines to prevent sparks in dusty environments where powders are processed.
Lightning FIG 93 T his little girl has become electrically charged due to friction between the plastic hammock and the girl’s clothes.
With a metal an excess of electrons can flow away easily, but this is not the case with plastic or wool. A statically charged balloon therefore remains statically charged for a time.
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Strong air currents move up and down alongside each other in tall cumulus clouds. As a result of this friction the top of the cloud becomes highly positively charged and the bottom of the cloud becomes highly negatively charged. When this negative charge becomes sufficiently high a spark flows from the bottom of the cloud to the ground below, through hundreds of metres of air. This discharge, the lightning, consists of electrons.
1.5
Electricity and safety
Knowing more
The current in a lightning strike is tens of thousands of amps, but the discharge is of very short duration. After the lightning you hear a clap of thunder. This is the result of the sudden heating and expansion of the air at the lightning strike.
FIG 95 E lectrical discharges through the gas in a fluorescent tube light and a plasma ball light cause light to be emitted.
Current through a gas
Just as a current passes through the air during lightning, an electric current passes through a gas in fluorescent tube lights and energy-saving light bulbs. You can see this if you hold a fluorescent tube next to a plasma ball light. Just as with lightning, the current through the fluorescent tube causes a purple-blue light. The white layer on the inside of the glass tube converts this into white light.
Questions: Knowing more
FIG 96 The electroscope
139 When you take off your jumper you hear it crackle. If you
137 For each of the following statements answer true or false.
If the statement is false, change it to make a statement which is true. a When an electric charge passes through air you see a spark. b If you rub a balloon on your jumper, both the jumper and the balloon become negatively charged. c A spark heats up the air. d A high voltage is needed for a discharge to take place. e In a lightning strike electrons move from the ground to the cloud.
138 You can find out if an object is electrically charged with
an electroscope (figure 96). If you hold a charged object next to the ball terminal, the two leaves of silver paper move apart. a Why do the two leaves of silver paper move apart? b Why does an electroscope not enable you to find out if the charge is positive or negative?
do this in the dark you will see small sparks of light. a What is the cause of the sparks of light? b What is the cause of the crackling sound? A jumper only crackles when the air is very dry. c What effect does water vapour have on the electric charge?
140 You can sense static electricity with the hairs on your arm. a Explain why you can sense static electricity with the hairs on your arm.
b Are the hairs electrical conductors or insulators? c Why are metals generally not electrically charged? d Why is an electroscope mostly made of metal? e How is the charge in an electroscope stopped from flowing away?
141 You can also see an electric current passing through air in a plasma ball light. a Why is the light in the plasma ball the same colour as lightning? If you hold an energy-saving light next to the plasma ball, the energy-saving light glows. See figure 95. b Explain this phenomenon. c Why is the light from the energy-saving light white whereas the light in the plasma ball is purple-blue?
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Experimenting
Activity 24 Heat in a metal wire
What to do
In this experiment you will investigate the heating effect of an electric current.
• • • • • • •
Fix the metal wire between the stands as shown in figure 97. Hang the mass on the wire. Place a folded piece of tissue paper over the wire. Connect the wire to the power supply, but don’t switch the power supply on yet. Predict what will happen to the mass and the paper when the power is switched on. Switch the power supply on, and carefully increase the voltage gradually. Note down all your observations.
Questions FIG 97
You will need • • • • • •
thin metal wire two stands with insulators mass piece of tissue paper variable power supply connecting leads
• Were your predictions correct? At home the electric current also passes through metal wires. • What happens to those wires when the currents get very high? • What can happen at home if you connect up too many appliances at the same time? There are various regulations regarding the domestic wiring circuitry. This includes how thick the wires must be, for example. • Explain why electric wires in the home must have a minimum thickness.
Activity 25 Using a mains tester
•
In this Activity you will investigate how a mains tester works. •
You will need • •
mains tester ‘screwdriver’, with properly insulated shaft multiple plug socket or earthed extension lead
• •
Connect the multiple plug socket to the mains socket, then place the multiple plug socket on the bench. Use the mains tester to test which connection points have a voltage. Remove the plug from the plug socket, rotate it half a turn, and put it back in the socket. Again test which connection points have a voltage.
Questions
FIG 98
What to do •
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aution: working with mains electricity can be C dangerous. Follow the instructions carefully and accurately.
• In the last case was there a voltage at one of the earth connections? • Explain how a current can flow if there is a voltage at only one pin of a plug. • Explain why you don’t feel a current through your body when using the mains tester. • Explain why you don’t detect a voltage at an earth connection.
1.5
Electricity and safety
Experimenting
Activity 26 Transfer of electrical energy
•
In this Activity you will investigate energy loss during energy transfer.
• •
You will need • • • • • •
lamp (must work at low voltage with high current) long, thin electrical wire variable power supply voltmeter ammeter connecting leads
What to do •
•
Draw the circuit diagram for the circuit where you can measure the voltage across the light and the current through the light. Connect up this circuit using normal connecting leads.
• • •
Adjust the power supply to the voltage appropriate for the lamp, so that it shines normally. Note down the current and voltage. Now connect the lamp to the power supply using the long wire. Measure the voltage and current again. Now increase the voltage from the power supply until the lamp functions normally. Again measure the voltage and the current.
Questions • Why does the lamp no longer shine normally when it is connected with a long wire? • How much extra voltage was needed for the lamp to function normally again? • Explain why this extra voltage was needed.
Activity 27 Static electricity You will investigate the force between electrically charged objects.
-
+ +
-
FIG 99
You will need
Questions
• • •
• Can you rotate the holder with the two glass rods? • Do the two charged glass rods attract or repel each other? • What about the PVC tubes? • Explain whether the PVC tube and the glass rod have the same charge. The glass rod becomes positively charged when rubbed. • What charge does the silk cloth now have? • Does the PVC tube become positively or negatively charged? • Do electrons move to the PVC tube when it is rubbed, or are they removed?
two glass rods and a silk cloth two PVC tubes and a wool cloth rotating holder
What to do • • •
Rub one glass tube firmly with the silk cloth and lay this rod in the rotating holder (see figure 99). Rub the second glass rod as well and hold one end next to the first rod. Repeat this experiment with the two PVC rods and the wool cloth.
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Concluding Concluding tasks 1
Do you understand everything already?
a Look through the chapter page by page and look in particular at the Key terms and Summary paragraphs. b Note down all the terms or sections that you still don’t quite understand. c First read the accompanying explanation for each term that you don’t understand. d Cross out the terms that you now understand. e Ask your teacher or a fellow pupil to explain the remaining terms.
Application assignments The knowledge from this Chapter is used in various reallife situations in these assignments. The assignments are available on www.impact-online.nl or the school network. A Electric art B Adjustable lights C Charging a phone with a 12 V battery D Growing cannabis: a dangerous business
Test yourself 1 An electricity company charges € 0,21 for one kWh.
2 Key terms test Take the Key terms test on www.impact-online.nl or on your school network.
3 Old and new a For each section write down what you already knew and what was new. Do this as follows: I already knew this This was new to me ...................................... …………………………... …………………............. …………………………... b Compare your list with that of a fellow student. Check that you understand everything in a.
4 Formulas with electricity a Write down all the formulas from this Chapter. Write down the units with each formula. b For each formula name a situation where you can apply this formula (you don’t have to do a calculation). For example: E = P × t units: J = W × s of kWh = kW × h. An 1 800 W kettle is switched on for 6 minutes. Calculate the energy consumption in J and in kWh.
5 Electrical appliances a Choose an electrical appliance that you use often
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and find out as much information as possible about the appliance that is related to electricity: the voltage, power, alternating or direct current, earthed or not. b Use this information to calculate the other electrical data such as the current and resistance. c Estimate how many joules or kWh the appliance uses in one year. d Compare the energy costs with the purchase price. How many years does it take before the energy costs are equal to the purchase price?
An average household uses 3 500 kWh annually. a What does the energy consumption per day cost? b What does 1 MJ electrical energy cost? An old fridge uses 10 kWh per week, whereas a modern fridge uses just 3 kWh per week. c What is the annual saving in euros with a modern fridge?
2 A large plasma screen uses 420 kWh (€ 90) of electricity
annually, whereas a low-energy LED screen can manage with 160 kWh of electricity. The screen is switched on for four hours a day. a Calculate the power of the plasma screen. b What is the cost of watching TV for one hour with the plasma screen? c What is the annual saving in euros with an LED TV? d Calculate the resistance of the LED TV.
3 The current through a 230 V incandescent light bulb is 0,175 A. a Calculate the power of the light bulb. b A 12 V halogen light uses the same amount of energy per second. Calculate the current through the halogen light. c A second incandescent light bulb of 885 Ω is connected in parallel with the first light bulb. Calculate the total current.
4 Draw the circuit diagram for the following circuit: Three identical lamps are connected in parallel to a 12 V direct current. Lamps 2 and 3 can be switched on and off at the same time using a switch. Four meters are included in the circuit. These measure the following: the current through lamp 2, the current from the source, the voltage across lamp 3 and the voltage of the source.
Concluding
a U = I × R b R =
I U
c
E = R × t
d P =
E t
I (A)
5 Write down if each of the following formulas is correct: e U = P × I f
I=
0,5 line A
0,4
P U
0,3
6 Moshi finds a box containing a collection of old
0,2
incandescent light bulbs. He reads the information on the bulbs. Lamp 1: 230 V, 40 W Lamp 2: 25 W, 2,5 A Lamp 3: 230 V, 400 mA Lamp 4: 6 V, 2,4 W a Which lamp has the highest current? b Which lamp has the highest resistance? c Why can’t you say, “The lamp with the highest resistance has the thinnest filament”?
10 Figure 100 shows two lines. Line A is for an incandes-
7 a With an incandescent light bulb the same current
flows through the input and output wires as flows through the filament. Explain why the input and output wires do not get very warm, whereas the filament gets very hot. b A combined electric screwdriver-drill has a resistance of 1,35 kΩ. The current is 17 mA. Calculate the power of the screwdriver-drill. c A hair-dryer has a much higher power rating than a fridge. However a fridge uses much more energy per year. Why is this?
8 Note down which words are correct in the following paragraph. All household appliances are designed to work at the same voltage / current / resistance. For this reason all household appliances are connected in parallel / series. The more appliances are connected, the greater the voltage / current / resistance. If too many appliances are connected at the same time this is called short circuit / circuit overload / earthing. Then the earth leakage circuit breaker / live wire / fuse will switch off the power.
line B
0,1 0
c Calculate how many 25 W lamps can be connected
electric shock?
2
3
4
5
6
U (V)
cent light bulb, line B is for a metal wire. a How can you see that line A is for an incandescent light bulb? b With which line is the current proportional to the voltage? c With which line is there a linear relationship between U and I? d How can you tell from the graph that the resistance of the lamp is lower than the resistance of the metal wire? The lamp and the wire are connected in parallel to 5,0 V. e What is the total current?
The lamp and the metal wire are connected in series. The current is 300 mA. f What is the total voltage?
11 A high tension connection consists of six cables alongside each other. Each cable can carry a maximum current of 0,7 kA. The voltage is 380 kV. a Calculate the maximum power of the connection. Each cable has a resistance of 17 Ω. b Calculate the voltage loss in one cable. c Calculate the power loss in one cable. d Calculate what percentage of the power is lost in the cable.
from fire or from an electric shock?
b Does a fuse give protection from fire or from an
1
FIG 100
9 a Does an earth leakage circuit breaker give protection
0
You can find a Summary test and a Chapter test on www.impact-online.nl
to a group protected by a 16 A fuse.
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