Handbook p9 electricity

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Electricity Handbook P9

“Do not fear mistakes. You will know failure.

Continue to reach out.�

-Benjamin Franklin (1706-1790)

1

Handbook P9 ELECTRICITY OVERVIEW OF HANDBOOK ABL

CONCEPT

ABL1 ABL2 ABL 3 ABL 4 ABL 5 ABL 6 ABL 7

Introduction to electricity Static electricity Current electricity Measuring current and voltage Ohm’s law Series and parallel connections Effective resistance in series and parallel connections

NO. OF ACTIVITIES 1 3 3 2 3 3 4

ABLs WITH REFERENCE TO STANDARD S.No. 1 2 3 4 5 6 7

STANDARD 6 and 7 9 6, 7 and 10 6, 7 and 10 9 and 10 9 and 10 9 and 10

PREFERRED ABL ABL 1 ABL 2 ABL 3 ABL 4 ABL 5 ABL 6 ABL 7

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TIME (min) 20 45 45 55 60 60 65

PAGE NO.

2

LIST OF FIGURES, CHARTS AND WORKSHEETS S. No Fig 1 Fig 2 Fig 3 Fig 4 Fig 5 Fig 6 Fig 7 Fig 8 Fig 9 Fig 10 Fig 11 Fig 12 Fig 13 Fig 15 Fig 16 Fig 17 Fig 18 Fig 19 Fig 20 Fig 21 Fig 22 Chart 1 Chart 2 Chart 3 Chart 4 Chart 5 Chart 6 Chart 7 Chart 8 Chart 9 Chart 10 Chart 11 Chart 12 Chart 13 Worksheet 1 Worksheet 2 Worksheet 3

Name Lightning Testing Facility Static Electricity Behavior of Electric Charges Simple Circuit Direction of current in circuit Ammeter, Voltmeter, Rheostat Examples of incorrect circuits Open Circuit Agastya electricity boards no.4 model Circuit diagram for ABL 5.1 Ideal graph for Ohm’s Law Electricity Kit Circuit Diagram for ABL 5.2 Circuit Diagram for ABL 5.3 Circuit Diagram for ABL 6.1 Solar Energy kit Bulbs in series and parallel Parallel connection of light bulbs Circuit Diagram for ABL 7.1 Circuit Diagram for ABL 7.2 Circuit Diagram for ABL 7.3 Activity Game Chart Accumulation of electric charge on balloon Lightning and Thunder Battery Symbols to denote electrical devices Different types of cells Parts of a cell Lithium ion battery Circuit diagram for ABL 4.1 Diagram of Photovoltaic cell Photovoltaic cell, modules, panels and arrays Serial sets Bulbs in series and parallel Observations for Ohm’s Law Observation sheet for relation between Resistance and Length of a conductor Observation sheet for relation between Resistance and cross sectional area of a conductor

Page No 6 14 18 29 29 46 47 51 57 58 59 62 63 65 71 75 78 81 85 87 91 6 15 21 30 30 35 36 38 47 75 76 79 80 58 63 65

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3 Worksheet 4 Worksheet 5 Worksheet 6 Worksheet 7

Observation sheet effective resistanceresistors in series Observation sheet effective resistanceresistors in parallel Observation for calculation of Power Electricity bill

85 87 91 99

Note to Instructor: All the figures in this handbook are for the Instructor’s reference only. The Charts need to be printed and shown to the learners during the course of the activity. Worksheets need to be printed out in advance for the learners. The number of worksheets required is mentioned in the Material List.

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4

ABL 1 Introduction to electricity Activity

Learning objective

1.1

What is electricity?

Key messages  

 



Electricity is the form of energy most commonly used by people.

It is generated by negatively charged elementary sub-atomic particles, called electrons.

There are two kinds of electricity, static and current electricity.

Static electricity is caused by extra electric charges remaining bound to the surface of a body, creating an electric field outside it.

Current electricity is generated when electrons carrying negative charges flow through circuit devices such as conductors.

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Time (min) 20

5

ABL 1.1

Time: 20 min

LEARNING OBJECTIVE – What is electricity?

Note to Instructor – These activities illustrate the role of electricity in students’ everyday lives.

ADVANCE PREPARATION Material List

1 2 3 4

Material Activity Game Chart Torch light Candle Matchbox

Required quantity 1 set per class 1 per class 1 per class 1 per class

Things to do Print out the following flip charts.

Safety Precautions Not Applicable

SESSION Link to known information/previous activity Not Applicable Procedure Show students the flip charts thoroughly to make them understand the important role played by electrical energy in our day-to-day life.

Pass out the torchlight and the matchbox and candle, as a demonstration of the way electric energy can make our day-to-day lives easier.

UNDERSTANDING THE ACTIVITY Leading questions 1. What makes the things shown in the flip charts work?

2. What is the form of energy used?

3. What is the source of this energy?

4. What is current (electricity)?

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6

Discussion and Explanation Discussion Item 1: Electrons and atoms  We need energy to support every activity in our lives. Electricity is the form of energy most commonly used by humans.  Electricity is generated by elementary particles called electrons.  Electrons are particles present in atoms, which are the building blocks of all materials in this world. There is no matter in this world without atoms and electrons inside it. Atoms also contain other particles, such as protons.  The charge on an electron is negative; the charge on a proton is opposed to it and therefore positive.  The negative charge carried by electrons enables the conversion of other forms of energy into electrical energy.

Discussion Item 2: Static and current electricity  There are two forms of electricity, namely static electricity and current electricity.  Static electricity exists when the electric charges remain stored on the surface of a body and generate a force (called an electric field) in the surrounding space.  Current electricity exists when the negative electric charges (electrons) flow freely in a metal like copper or aluminum to produce a useful form of energy necessary for the operation of devices and machines like electric bulbs, heaters, air conditioners, and motors.

KEY MESSAGES     

Electricity is the form of energy most commonly used by people.

It is generated by negatively charged elementary sub-atomic particles, called electrons.

There are two kinds of electricity, static and current electricity.

Static electricity is caused by extra electric charges remaining bound to the surface of a body, creating an electric field outside it.

Current electricity is generated when electrons carrying negative charges flow through circuit devices such as conductors.

LEARNING CHECK Give an example of something you use at home that runs on electricity. Give an example of something you use at school. Give an example of something you use in your free time.

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Activity Game Chart Chart 1

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TRY IT YOURSELF When you go home tonight, try to notice each object that you use that utilizes electricity – a light, a phone, a television, a refrigerator, or whatever else you might have at home. Make sure to turn it off as soon as you are done using it to conserve energy!

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12

INTERESTING INFORMATION Additional information on electrostatics A body is called electrically charged when it loses or gains electrons. Electrons can be exchanged between materials in contact; materials with weakly bound electrons tend to lose them, while materials with sparsely filled outer shells (outer orbits of electrons around the central nucleus) tend to gain them. A balloon rubbed against a nylon cloth becomes negatively charged; when near a wall, the charged balloon is attracted to positively charged particles in the wall, and can "cling" to it, appearing to be suspended against gravity. As you walk across a woolen carpet, electrons move from the rug to you. Now you have extra electrons and a negative static charge. Touch a doorknob that is a conductor. The electrons jump from you to the knob, and you feel the static shock. We usually only notice static electricity in winter when the air is very dry. During summer, the air is more humid. The water in the air helps electrons move off you more quickly, so you do not build up a big static charge on your body.

Practical applications of the electrostatic force 1. Pollution control The electrostatic force is used to remove dust and ash particles from the exhaust gases flowing out of the chimneys of thermal power plants and other factories. If a static charge is applied to solid particles in the exhaust gases, the particles are attracted and collected by plates of the opposite electrical charge placed in the chimney. Such devices are called electrostatic precipitators.

2. Spray-painting car parts When paint is sprayed from a paint gun, the painter normally needs to use a fair amount of skill to ensure that the paint goes on evenly. By connecting the spray nozzle to a negative electrode, it is possible to charge each droplet of paint. If the car part is then given the opposite charge, the paint droplets will be attracted to the car part, and stick and dry on it evenly.

3. photocopying of documents (Xeroxing) A photocopier (also known as a copier or copy machine) is a machine that makes paper copies of documents and other visual images quickly and cheaply. Most current photocopiers use a technology called xerography, a dry process that uses electrostatic charges on a light sensitive photoreceptor to first attract and then transfer toner particles (a powder) onto paper in the form of an image. Heat, pressure or a combination of both is then used to fuse the toner onto the paper. (Copiers can also use other technologies such as ink jet, but xerography is standard for office copying.)

Lightning test facility (appropriate for advanced classes) You may wonder whether airplanes flying in clouds can be affected by lightning or thunderstorms.

Modern aircraft are built using a great deal of strong, lightweight, non-metallic material such as Carbon Fiber Composite (CFC) and Fiber Reinforced Plastic (FRP), in addition to aluminum alloys.

Composite materials are not good conductors of electricity and this could cause problems for an aircraft if it were to be hit by lightning, because they provide limited shielding against the electromagnetic fields in lightning, so the electronics inside the aircraft would be exposed to these

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13 higher strength fields. This would damage the electronic equipment. Aircraft must therefore be designed appropriately to withstand the effects of lightning.

Lightning protection schemes require thorough testing to qualify an aircraft against the effects of lightning. In the Lightning Test Facility (LTF) established at HAL airport in Bangalore, a complete aircraft and all of its equipment can be tested for the ability to withstand lightning strikes. To simulate the direct effects of lightning, voltages up to 4,000 kilovolts (kV) and currents up to 2,00,000 amperes (A) are generated and are applied to the aircraft. 1

Fig 1: Lightning Test Facility

WEB RESOURCES http://www.ndt-ed.org/EducationResources/HighSchool/Electricity/electricityintro.htm (a brief introduction to electricity; if you click through to the next few pages, you will find a good explanation of the chemistry that is relevant to the study of electricity) http://www.georgiapower.com/in-your-community/electric-safety/ (a very comprehensive resource on electric safety, with information about power lines, around-the-house power use, power tools, and more)

VOCABULARY 1) 2) 3) 4) 5)

Electricity – A form of energy generated by electrons Electron – An elementary particle with a negative charge; electrons generate electricity Atoms – The building blocks of all matter in the world, containing electrons and protons Proton – An elementary particle with a positive charge Static electricity – Electricity in which electrons remain stored on the surface of a body and generate an electric field 6) Current electricity – Electricity in which electrons flow freely in a metal to produce a useful form of energy for the operation of devices 1http://alpha.drdo.res.in/sites/default/files/

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14

ABL 2 Static electricity Activity

Learning objective

2.1

1. What is static electricity?

Key messages  

2.2

2. What is a charged body?

Understanding the forces between electric charges

  

2.3

Understanding lightning



Static electricity is the study of charges at rest.

It is a body which can exert a force of attraction on any light material brought close to it.

Electric charges are either positive or negative.

Like (similar) electric charges repel each other.

Unlike (opposite) electric charges attract.

Lightning is a giant discharge of static electric charge after it has built up in moisture-bearing clouds.

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Time (min) 15

15

15

15

ABL 2.1

Time: 15 min

LEARNING OBJECTIVE – What is static electricity and what is a charged body?

Note to Instructor –These activities show the differing effects of static electricity on different materials.

ADVANCE PREPARATION Material List

1 2

Materials Balloons Dry cloth/nylon cloth

Required quantity 2 per group 1 per group

Things to do Not Applicable Safety Precautions Not Applicable

SESSION Link to known information/previous activity In the previous activity, we were introduced to the concept of electricity. Now we will investigate a type of electricity, called static electricity.

Procedure Divide students into groups with 5 to 6 learners in each group. Distribute the materials to each group before starting the following group activity.

Inflate two balloons and rub them with the dry cloth or paper. Bring them close together; what happens? Bring one balloon close to your classmate’s hair; what happens? Now bring the other balloon close to the wall; what happens?

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UNDERSTANDING THE ACTIVITY Leading questions 1. What happened to the balloons when they were brought close to each other after being rubbed with the dry cloth?

2. What happened when one of the balloons was brought close to your classmate’s hair?

3. Why did the balloon get attached to the wall?

4. Can you guess the reason behind this?

5. Can you relate this to the previous activity?

6. Can you find similar things happening around you?

Discussion and Explanation Discussion Item 1 – Where static electricity comes from  When a balloon is rubbed with a dry cloth, electric charges are transferred to the surface of the balloon.

 

Chart 2: Accumulation of electric charge on balloon These electric charges have the property of exerting a force on any other electric charges in their neighborhood.

If two similarly charged balloons are brought near each other, each balloon exerts a force on the other one, trying to push it away. A charged balloon also exerts a force on the charges on a student’s hair or the charges on the wall nearby.

Discussion Item 2 – Properties of static electricity  You can experimentally observe this force even when the two charged bodies are several centimeters apart. Thus the force between electric charges acts remotely. Such a force is called a non-contact force.  The amount of force experienced by the charges depends on the distance between them.

The magnitude of the force increases very much when they are brought closer; similarly, the force decreases when they are taken far apart. When the charged bodies are taken very far apart, for example about one meter apart, the force becomes so weak that we might not be able to notice its effect.

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17

Discussion Item 3 – How static electricity acts on various common materials  The force between two charged bodies may be an attractive force or a repulsive force, depending on the materials the bodies are made of. Balloons are made of latex rubber; pens are made of phenolic material; straws are made of thin plastic; hair is an organic tissue. Each of these materials has different properties of static electricity.  A student’s hair stands up when the balloon is brought close to the hair. The hair attaches to the balloon because the negative charge of the balloon and the positive charge of the hair attract each other.  When a balloon is rubbed with a dry cloth and brought close to the wall then it is attracted towards the wall. It has a negative charge on its surface, and the charged balloon induces the opposite charge on the surface of the wall, so the balloon is attracted towards the wall.  Similarly, when you move close to a TV or computer screen currently turned on, the hair on your hand stands up. This is because the hair is positively charged and a TV screen is negatively charged, due to the electrons in the beam inside the TV hitting the phosphorcoated screen.  (To be explained to higher classes only) This won’t happen if you touch the TV screen when it is switched off. Such accumulation of charge also does not happen on the LED screens that exist on laptops, because there are no electrons hitting the screen to give it a negative charge.

KEY MESSAGES  

Static electricity is the study of charges at rest.

It is a body which can exert a force of attraction on any light material brought close to it.

LEARNING CHECK How can a balloon make your hair move without touching your hair?

Can you explain why static electricity causes the shock that you sometimes experience when you touch a doorknob?

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18 Time: 15 min

ABL 2.2

LEARNING OBJECTIVE – Understanding the forces between electric charges Note to Instructor – These activities show how the effect of static electricity depends on the charge of the relevant bodies.

ADVANCE PREPARATION Material List

1 2 3

4 5 6

Materials Straws Beads Nylon cloth/Cotton cloth Eraser Pins Ball point pen

Required quantity 2 per group 2 per group 1 per group

1 per group 2 per group 1 per group

Things to do Not Applicable Safety Precautions Not Applicable

SESSION Link to known information/previous activity In the previous activity, we learned about some properties of static electricity, and we learned how it works in certain situations. Here we will learn how it works in a few more situations. We will see that it can even sometimes be a repulsive force!

Procedure Divide students into groups with 5 to 6 learners in each group. Distribute the materials to each group before starting the activity.

Make a hole in the centre of a straw. Connect the straw and the beads to the eraser with the pin, as the diagram below shows. The straw should turn freely in the horizontal plane. Rub the straw with a nylon cloth.

Rub a second straw with the nylon cloth, and bring it close to the straw fixed to the eraser.

Observe what happens.

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Rub a plastic pen with the nylon cloth and bring it close to the straw fixed to the eraser. Observe what happens.

Tip for the Instructor It is easy for static electricity to be lost through the human body, because the human body is connected to the ground. The rubbed parts of the straws and pen should not be touched during the experiment because the charge might leak away to the ground through the body of the person touching them.

Electric charge is not generated easily when the cotton cloth is damp. A dry cotton cloth should be used.

UNDERSTANDING THE ACTIVITY Leading questions 1. What happens when both the straws are rubbed with the nylon cloth and brought together?

2. What happens when the pen rubbed with the nylon cloth and straw rubbed with the nylon cloth are brought close to each other?

3. Can you give any reason for this?

Discussion and Explanation  The accumulation of electric charge on the surface of a body like a straw pipe or a piece of nylon is called static electricity. These electric charges are either positive or negative.  A positive charge repels another positive charge (like charge).  Similarly, a negative charge repels a like negative charge.  A positive charge and a negative charge attract each other (unlike charges).

   

Fig 3: Behaviour of electric charges When it is rubbed with the cotton cloth, the plastic straw gains some electrons, which were very loosely bound to the surface of the nylon/cotton cloth. Therefore the straw’s surface becomes more negatively charged.

The two straws repel each other, because both straws are negatively charged.

When the pen is rubbed with the nylon cloth, it becomes positively charged. Because the pen is made of a phenolic material, a different material compared to the straw, it loses negatively charged electrons to the rubbing cloth and therefore becomes positive.

The straw and the pen attract each other, because the straw is negatively charged and the pen is positively charged.

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KEY MESSAGES   

Electric charges are either positive or negative.

Like (similar) electric charges repel each other.

Unlike (opposite) electric charges attract.

LEARNING CHECK In each situation, determine whether the force applied is a pushing or a pulling force, and check the appropriate box.

Situation Attractive force Repulsive force Two electrons An electron and a proton Two balloons rubbed with a nylon cloth A rubbed straw and the wall A rubbed straw and a rubbed pen (Answers: Attractive, Repulsive, Repulsive, Attractive, Attractive)

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21 Time: 15 min

ABL 2.3

LEARNING OBJECTIVE – Understanding lightning.

The biggest and most dramatic example of static electricity seen in our lives Note to Instructor – This discussion will explain the physics behind lightning.

ADVANCE PREPARATION Material List

1

Materials Flip chart

Required quantity 1 per class, or enough to pass out to all students

Things to do Not Applicable Safety Precautions Not Applicable

SESSION Link to known information/previous activity In the previous activities, we learned about the mechanics of static electricity. Now we will apply that knowledge to investigate the physics of lightning strikes!

Procedure Show the students the flipcharts. Explain the interesting facts below.

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22

Chart 3: Lightning and Thunder

LIGHTNING AND THUNDER What is lightning and how do thunderstorms occur?

Clouds contain millions and millions of water droplets and ice particles suspended in the air, because of the continuous process of the evaporation and condensation of water from oceans and land. These droplets collide with each other, causing positive and negative charges to separate to the top and bottom of the cloud, respectively.

The continual buildup of charges at the top and bottom of the cloud increases the electric field or the force exerted on the air molecules in the cloud. This causes a breaking up of the gas molecules and a big discharge of static electricity to occur between two parts of the cloud or between the cloud and the earth. This also produces an intensely bright light flash in the surrounding air, which is known as lightning.

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23 The combination of the rapid heating of the air by the lightning and the subsequent rapid cooling creates sound waves. These sound waves are what we call thunder.

When are thunderstorms most likely to occur?

Thunderstorms can occur year-round and at all hours, but they are most likely to happen in the spring and summer months during the afternoon and evening hours.

How many thunderstorms might be happening every day?

It is estimated that there are around 1800 thunderstorms that occur across our planet every day.

Are thunderstorms dangerous?

Yes. Thunderstorms might become dangerous to people on ground and airplanes flying in the air, because of the effect of the lightning.

How do you know if lightning is nearby?

If you see dark clouds, then there may be lightning. You can also listen for thunder. If you hear thunder then you need to go indoors. Don’t be outside where lightning could occur. Take precautions.

How can we protect buildings against damage due to lightning?

A direct lightning strike can destroy or seriously damage a building due to the heavy flow of electrical charges through it in a very short time. In tall buildings and towers, a lightning arrester is provided at the highest point. This captures and conducts most of the charge from a nearby lightning strike safely to the ground, without damaging the building.

Do lightning and thunder always happen together?

The sound of thunder is always generated by a lightning flash. However, sometimes we may not hear the thunder at our location on the ground if it is caused by a lightning flash very far away from us, although we might be able to see the lightning. The sound of thunder always reaches us several seconds after the flash of lightning is seen. This is because sound travels in air at a much lower speed than light.

UNDERSTANDING THE ACTIVITY Leading Questions Not Applicable Discussion and Explanation Not Applicable

KEY MESSAGES 

Lightning is a giant discharge of static electric charge after it has built up in moisturebearing clouds.

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24

LEARNING CHECK What should you do if you are outside when a thunderstorm begins?

Why do we sometimes see lightning without hearing thunder?

TRY IT YOURSELF Below are three ways to make static electricity:2 Carpet Method Note that this method works best in the colder and drier seasons such as winter and the fall.

Find a carpet. Rub your feet against it while wearing wool socks. Continue rubbing for 30 seconds to 2 minutes.

Reach out and touch your victim. The closer your victim is to you, the better. You'll lose the charge if you have to move around too much. You should also avoid touching any metal, or you'll end up shocking yourself instead of the victim!

For added effect, jump into the air and touch your victim just before you land. This creates what is known as 'capacitive voltage multiplication' meaning a stronger shock!

Comb/Balloon Method Get a hard rubber or plastic comb. Alternatively, you can use a balloon. Also find a 12 inch piece of thread and one or two small pieces of cereal. Tie a piece of cereal to the end of the thread.

Attach the other end to the side of a table or somewhere so that the cereal is not near any other object when it is hanging.

Wash the comb or balloon. This removes any oils that may have accumulated on it. Run the comb through long, dry hair several times. This will add a charge to it. Alternatively, you can rub the comb or balloon on a wool sweater or blanket for 30 sec to 1 minute.

Slowly bring the comb or balloon near the piece of cereal. The cereal should swing to touch the comb/balloon. Keep it steady until the cereal jumps away. Try to touch the cereal again. The cereal should move away from the comb!

Wash the comb and run it through your hair again (or rub the balloon on a sweater or blanket again). This time, turn on the faucet and move the charged balloon or comb near the stream of water. Watch the stream bend away from the comb!

Slide Method

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25 Go to a park that has a slide. Go down the slide. If you feel your arm hairs sticking up that’s a good sign, and if you hear popping sounds that’s also a good sign. Touch someone. You may also shock yourself so watch out!

INTERESTING INFORMATION Static charges and the Seasons “We usually only notice static electricity in the winter when the air is very dry. During the summer, the air is more humid. The water in the air helps electrons move off you more quickly, so you cannot build up as big a static charge.”3 Triboelectric Series “When we rub two different materials together, which becomes positively charged and which becomes negative? Scientists have ranked materials in order of their ability to hold or give up electrons. This ranking is called the triboelectric series. A list of some common materials is shown here. Under ideal conditions, if two materials are rubbed together, the one higher on the list should give up electrons and become positively charged.

TRIBOELECTRIC SERIES Your hand glass your hair nylon wool fur silk paper cotton hard rubber polyester polyvinylchloride plastic”4 Ben Franklin and Lightning

“In 1751, Benjamin Franklin experimented with electricity in a thunderstorm, using a kite, a key and a Leyden jar (two conductors separated by an insulator; it is a device for storing static electricity). The thundercloud leaked electrons (negatively-charged particles) down through the kite's silk sting to the key and into the Leyden jar (on the ground). Franklin himself was insulated from the electricity; he was holding the portion of the string attached to the string but not directly to the Leyden jar. When Franklin touched the key, he got a static shock. DO NOT TRY THIS - many people have died trying it. In 1752, Franklin developed the lightning rod.”5

WEB RESOURCES http://www.physicsclassroom.com/class/estatics/ (a huge source of information about all topics relating to static electricity; contains 18 separate WebPages) Agastya International Foundation. For Internal Circulation only.

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26 http://www.electricityforum.com/static-electricity.html (an overview of static electricity with a more scientific tone; references the Bohr Model of the atom) http://www.sciencemadesimple.com/eliminatestaticelectricity.html (Static elimination and control)

VOCABULARY 1) Electrostatic force – The attractive or repulsive, non-contact force exerted by electric charges that lie on the surface of a body on other sources in their neighborhood; also called static electricity 2) Electric charge – A particle (electron or proton) that is positive or negatively charged, and exerts a force on other electric charges 3) Lightning – A giant discharge of static electricity after it has built up in moisture-bearing clouds

2http://www.wikihow.com/Make-Static-Electricity 3http://www.sciencemadesimple.com/static.html 4

Ibid.

5http://www.enchantedlearning.com/physics/Staticelectricity.shtml

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27

ABL 3 Current electricity Activity

Learning objective

3.1

1. What is current electricity 2. What is a simple electric circuit?

3. What is the conventional direction of electric current in a circuit

3.2

What is a cell? What are the types of cells?

What are the parts of a dry cell?

3.3

What is an electric bulb?

Key messages 

An electric current is the drift of electrons in a direction in a medium.  A simple electric circuit is an arrangement consisting of a source of electricity (like a battery), one or more devices that consume electricity (like bulbs, fans, and radios), the conductors that connect these devices, and sometimes a switch.  The direction of electric current in a circuit is such that the current leaves the battery at the positive terminal.

Electrons flow in the opposite direction from the current.  A dry cell is a source of electric current. It consists of a carbon cathode, a zinc anode, and dry ammonium chloride paste as an electrolyte inside.  The carbon electrode is the positive terminal for the current flowing in the circuit and the zinc electrode is the negative terminal.  A dry cell is a primary cell, which cannot be recharged after it has supplied all the energy it contained.  The filament-type electric bulb is a device that converts electric energy to useful light energy by passing a current through it.  New types of electric bulbs are currently available based on other principles (CFL – Compact Fluorescent Lamp; LED - Light Emitting Diode), which do not waste energy through heat. Therefore they are more energy-efficient.

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Time (min) 15

15

15

28

ABL 3.1

Time: 15 min

LEARNING OBJECTIVE – 1. What is current electricity 2. What is a simple electric circuit?

3. What is the conventional direction of electric current in a circuit Note to Instructor – Students will learn how to assemble a simple electric circuit.

ADVANCE PREPARATION Material List Material 1 2 3 4 5

Cells or battery boxes 1.5V bulbs Connecting wires (conductors) Switches Torch with 3 cells

Required Quantity 5 per group 5 per group 5 per group 5 per group 1 per class

Things to do Not Applicable Safety Precautions Students should not connect the circuit, or turn the switch to ON or OFF, until the instructor gives the appropriate instruction.

NEVER connect circuit parts to the plug points present in the classroom.

SESSION Link to known information/previous activity In the previous ABL, we investigated static electricity, but you probably associate electricity with wires and circuits. Now we will begin our investigation of current electricity by discussing electric circuits.

Procedure Divide students into groups and facilitate the group activity, then complete the instructor demo.

3.1a Group activity

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29 Connect the battery, bulb and switch in series, as shown the circuit diagram below (draw the circuit diagram on the board).

Fig 4: Simple Circuit Flip the switch to on and observe the bulb.

Fig 5: Direction of current in a circuit 3.1b Instructor demonstration Show how the torch is a simple electric circuit.

UNDERSTANDING THE ACTIVITY Leading questions 1. What is this arrangement called and what are the parts of the simple circuit?

2. What happened to the bulb when you turned on the switch after connecting the bulb, battery, and conductor?

3. What is the direction of current?

Discussion and Explanation Discussion Item 1 – Parts of a circuit  A simple circuit is an arrangement consisting of o A source of electricity o A device that uses electricity (called a load) o A device that controls the flow of current (called a switch or a fuse) o The conducting copper wires that connect these devices In the circuit we assembled, the load is a bulb, which consumes electricity to produce light.

Electrical load is different from mechanical load, which is used to lift objects.

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30 A cell/battery is the source of electricity in the circuit we created. A battery usually contains several cells. Other sources are generators, dynamos, windmills, and solar energy sources.

 





Chart 4: Battery The bulb glows when the switch is turned on Current in a circuit is the passage of electrons from negative to positive terminal of battery through different parts of circuit. If the current carriers were positive particles instead of electrons they would move from positive to negative terminal in the external circuit. This by convention is considered as the direction of current.

The symbols of several common electrical devices are included in the diagram below.

Chart 5: Symbols to denote Electrical Devices Direction of current – The current leaves the positive terminal, flows round the circuit and enters the negative terminal

KEY MESSAGES  



An electric current is the drift of electrons in a direction in a medium.

A simple electric circuit is an arrangement consisting of a source of electricity (like a battery), one or more devices that consume electricity (like bulbs, fans, and radios), the conductors that connect these devices, and sometimes a switch.

The direction of electric current in a circuit is such that the current leaves the battery at the positive terminal. Electrons flow in the opposite direction from the current.

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LEARNING CHECK 1. Name the parts of a simple electric circuit.

2. What is a common source of electricity in a circuit? What are three common electric loads?

3. Utilize your understanding of the requirements of an electric circuit to state whether charge would flow through the following arrangements of cells, bulbs, wires and switches.

If there is no charge flow, then explain why not.6 a.

b.

c.

d.

(Answer: a. No charge flow. The switch is in the open position, so there is no closed conducting loop. b. No charge flow. There is no closed conducting loop from the + to the - terminal. c. Yes, there is charge flow. In this case, there is a closed conducting loop stretching from the + to the - terminal. Thus, there is a charge flow. However, the flow does not pass through the light bulb and so the bulb will not light. d. Maybe. If two cells are pumping charge in opposite directions as indicated by the fact that there + terminals are connected to each other. If they have different voltage ratings (e.g., 1.5 V and 9 V), then there will be a charge flow. However, if their voltage ratings are identical, then there will be no charge flow.)

4. The diagram at the right shows a light bulb connected to a 12-V car battery. The + and terminals are shown. a. As a + charge moves through the battery from D to A, it ________ (gains, loses) potential energy and ________ (gains, loses) electric potential. The point of highest energy within a battery is the ______ (+, -) terminal. b. As a + charge moves through the external circuit from A to D, it ________ (gains, loses) potential energy and ________ (gains, loses) electric potential. The point of highest energy within the external circuit is closest to the ______ (+, -) terminal.

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32 c. Use >, <, and = signs to compare the electric potential (V) at the four points of the circuit.

VA VB VC VD (Answers: a. as a positive charge moves through the battery from D to A, it gains potential energy and gains electric potential. The point of highest energy within a battery is the positive terminal. b. As a positive charge moves through the external circuit from A to D, it loses potential energy and loses electric potential. The point of highest energy within the external circuit is closest to the positive terminal. c.

)

5. Which of the following is true about the electrical circuit in your flashlight?7 a. Charge moves around the circuit very fast - nearly as fast as the speed of light. b. The battery supplies the charge (electrons) that moves through the wires. c. The battery supplies the charge (protons) that moves through the wires. d. The charge becomes used up as it passes through the light bulb. e. The battery supplies energy that raises charge from low to high voltage. f. ... Nonsense! None of these are true.

(Answer: E. as emphasized on this page, the battery supplies the energy to move the charge through the battery, thus establishing and maintaining an electric potential difference. The battery does not supply electrons or protons to the circuit; those are already present in the atoms of the conducting material. In fact, there would be no need to even supply charge at all since charge does not get used up in an electric circuit; only energy is used up in an electric circuit.)

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ABL 3.2

Time: 15 min

LEARNING OBJECTIVE â&#x20AC;&#x201C; What is a cell? What are the types of cells? What are the parts of the dry cell?

Note to Instructor â&#x20AC;&#x201C; These demonstrations and diagrams will introduce students to dry cells, voltaic cells, and secondary cells, and will give them a more detailed understanding of the functioning of a dry cell.

ADVANCE PREPARATION Material List

1 2 3 4 5

Material Used dry cell Cutter Gloves Charts of various types of cells Torch with at least 3 cells

Required quantity 1 per class 1 per class 1 set per class 1 per class 1 per class

Things to do Print observation sheets and diagrams for your students.

Safety Precautions Not Applicable

SESSION Link to known information/previous activity One of the parts of a simple electric circuit is a source of electricity. Here we will investigate the most common source of electricity, a cell, to determine how it produces electricity.

Procedure Complete 3.2a, an instructor demo, then distribute the observation sheet and diagram to your students and have them each complete 3.3b, an individual activity.

3.2a Instructor demo Show the symbols chart (C5) from ABL 3.1 to show the symbol of a cell and explain the basic functions. Highlight the positive and negative terminals and the direction of current in the circuit.

Device name

Symbol

Description

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34 1

Cell

The longer vertical line denotes the positive terminal, and the shorter line denotes the negative terminal. Current always leaves the battery at the positive terminal.

Show the pictures of the three main types of cells: the LeclerchĂŠ cell (dry cell), the voltaic cell, the secondary cell, and the solar cell.

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8

9

Chart 6: Different types of cells Show students the dry cell. Point out the positive and negative terminals, and show how the can is sealed to prevent any leakage of substances from inside and to prevent moisture from outside getting inside.

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36 Explain that we can connect 2, 3, or 4 dry cells to form a battery, as we see in the torchlight. (A detailed explanation of this will be given in ABL 5.) Show how the cells, bulb, and switch in the torch form a simple electric circuit.

Finally, cut open the cell and display the parts.

Tip for the instructor Please observe that in the picture of the dry cell above, carbon is marked as the cathode and zinc is marked as the anode. This is for the following reason. Current is generated in the outside circuit by excess electrons reaching the zinc electrode during the process of electrolysis inside the cell.

For this electrolytic process, the carbon acts as the source of the electrons (the cathode) and the zinc as the recipient of the electrons (the anode). Thus, electrons flow from carbon to zinc inside the cell and from the negative terminal of the cell to the outside circuit returning to the cell through the positive (carbon) terminal. The direction of electron flow is opposite to the direction of the electric current in the circuit.

3.2b Student activity Read the observation sheet and observe the displayed parts of the cell.

1 2 3

Parts Positive terminal Negative terminal Paste electrolyte

Chart 7: Parts of a cell Made up of Carbon rod and carbon powder Zinc Ammonium chloride

UNDERSTANDING THE ACTIVITY Leading questions 1. What are the parts of the dry cell?

2. What is the positive terminal of the cell?

3. What is the negative terminal of the cell?

4. What is the electrolyte used in the cell?

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37 5. How many types are cells are there?

Discussion and Explanation Discussion Item 1 – Dry cells  A dry cell is derived from a cell based on an ammonium chloride electrolyte originally invented by Leclanché in 1866 (more than 150 years ago, in your grandfather’s grandfather’s time).  A dry cell is a source of electric current to a circuit.  It contains two terminals, a positive terminal and a negative terminal.  The positive terminal is a carbon rod in the top centre of the cell and is surrounded by carbon powder.  The negative terminal is the zinc plate at the bottom of the cell.  A dry paste of ammonium chloride forms an electrolyte medium between the 2 electrodes.  A dry cell described above has a specified capacity to generate current. The dry cell becomes “dead” after it has supplied that amount of electrical energy. It cannot be recharged to produce more current. Such cells are known as primary cells.  A dry cell supplies electrical energy to an external load like a bulb or motor, by the electrolytic process that takes place inside it.  In order to drive electric current through the external circuit, the cell generates a driving force across its terminals, called the Electromotive Force (EMF). This is measured in volts.

Discussion Item 2 – Voltaic cells  A voltaic cell (also known as a galvanic cell) is an electrochemical cell that uses spontaneous redox reactions to generate electricity. A redox reaction is a reaction involving the transfer of electrons between two substances.  It consists of two separate half-cells.  A half-cell is composed of an electrode (a strip of metal – which is good conductor) within a solution (electrolyte).  The two half-cells are linked together by a wire running from one electrode to the other.

Discussion Item 3 – Secondary cells  There are also many types of cells that can be reactivated or recharged after they have supplied all of their stored energy, by passing a controlled electric current through them.

These are secondary cells, which can be used again and again by recharging.  The lead acid battery commonly used in cars and motorcycles, the nickel cadmium batteries used in calculators and cameras, and the lithium ion batteries in mobile phones are examples of secondary (rechargeable) cells.

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10

Chart 8: Lithium Ion Battery Discussion Item 4 – Solar cells  There is another source of electricity, called a solar cell, which generates an electric current when kept in sunshine.  It draws light energy from sunshine and converts it to electricity.  As there is plenty of sunshine everywhere in a country like India, electricity can be generated easily even in remote areas like hills and forests where there is no electric supply.

KEY MESSAGES   

A dry cell is a source of electric current. It consists of a carbon cathode, a zinc anode, and dry ammonium chloride paste as an electrolyte inside.

The carbon electrode is the positive terminal for the current flowing in the circuit and the zinc electrode is the negative terminal.

A dry cell is a primary cell, which cannot be recharged after it has supplied all the energy it contained.

LEARNING CHECK What are the parts a dry cell? A voltaic cell?

What is the difference between a primary cell and a secondary cell?

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39

ABL 3.3

Time: 15 min

LEARNING OBJECTIVE â&#x20AC;&#x201C; What is an electric bulb?

Note to Instructor â&#x20AC;&#x201C; In this activity, students will learn to distinguish between LED bulbs and standard bulbs.

ADVANCE PREPARATION Material List

1 2 3 4 5

Material Screw-type or bayonettype bulbs LED bulb (same wattage) Cell Connecting wires Switch

Required Quantity 1 per group 1 per group 1 per group 2 per group 1 per group

Things to do Check that all of the bulbs are working.

Safety Precautions Students should not connect the circuit, or turn the switch to ON or OFF, until the instructor gives the appropriate instruction.

NEVER connect circuit parts to the plug points present in the classroom.

SESSION Link to known information/previous activity Simple circuits contain devices that are sources of electricity and devices that consume electricity.

In the previous activity we investigated electricity sources; now we will investigate electricity consumers.

Procedure Divide students into groups for the activity, and distribute the materials to each group. Facilitate the following group activity.

Take the two bulbs and touch them to determine the difference between them.

Connect the cell, bulb, and the switch using connecting wires. Flip on the switch, so that bulb glows.

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Replace the bayonet bulb by LED, connect the P and N terminals properly. (P to the positive and N to the negative). Switch on foe few minutes, touch the bulb and switch off.

UNDERSTANDING THE ACTIVITY Leading questions 1. Have you seen or touched a light bulb before?

2. What does a bulb do?

3. Why does it get hot?

4. Which bulb is hotter?

5. Why is LED bulb is not hot?

6. What is the symbol for a bulb?

Discussion and Explanation

1

Device name Cell

2

Bulb

       

Symbol

Description The longer vertical line denotes the positive terminal, and the shorter line denotes the negative terminal. Current always leaves the battery at the positive terminal.

The glowing of a bulb indicates the flow of current. An electric bulb gives off light and heat. The bulb has two leads, filaments that emit light and heat energy.

A bulb is an electrical device that converts electrical energy into light and heat energy.

It has two terminals. In a circuit, these two terminals are connected to the positive and negative terminal of a cell.

A filament-type bulb has a thin, coiled wire of a special metal, called a filament. The inside of the bulb is filled with an inert (non-reactive) gas without any oxygen in it, to increase the life of the filament.

When a current is passed through the filament, it gets hot. As more and more current is passed, the filament gets hotter and starts glowing, first in red colour and finally in white colour. This white-hot filament emits both light and heat energy.

In practice, we use only the light energy output, which is a small portion of the total energy produced by the bulb. Heat produced by the filament makes the bulb hot and the surrounding air warm.

LED does not generate much of the thermal radiation. It does not become hot.

The bulbs are designed as screw-type or bayonet-type.

Screw-type bulbs can be fitted into the holder by directly screwing them in.

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41    

A bayonet-type bulb has two pins, which have to be properly aligned inside the holder and fixed by giving the bulb a push and small turn.

Each type of bulb can be fitted only into the holder meant for it.

Currently, new types of electric bulbs are in the market (CFL – Compact fluorescent Lamp; LED- Light Emitting Diode) that are based on other principles and do not waste energy by heat.

The symbol for a bulb in an electric circuit is represented as given in Chart 5, ABL 3.1

Note to Instructor In the experiment shown to children, a small bulb is used, which is designed to produce a small amount of light using a small amount of electric power (1 W or less). These bulbs can work with a single cell as a source, providing a small voltage (1.5 V). Circuits using such low voltage sources do not give electric shocks.

However, bulbs normally used at home are bigger and they provide much more light and consume large amounts of electrical power (60 W). They are designed to work at higher voltages (230 V). We have to be very careful to avoid getting dangerous electric shocks when working with bulbs and other devices at 230 volts.

KEY MESSAGES  

The filament-type electric bulb is a device that converts electric energy to useful light energy by passing a current through it.

New types of electric bulbs are currently available based on other principles (CFL – Compact Fluorescent Lamp; LED - Light Emitting Diode), which do not waste energy through heat. Therefore they are more energy-efficient.

LEARNING CHECK When a light bulb is turned on, it produces two kinds of electricity. What are those types? Which is unwanted?

TRY IT YOURSELF Do you have any LED light bulbs at home? Try to find non-standard lights in stores you shop in.

How many different kinds of light bulbs can you find?

INTERESTING INFORMATION Testing a Switch “To determine if an on/off switch is operating properly, it can be removed from a circuit and tested with an ohmmeter. … When the switch is open, the ohmmeter reads infinite resistance.

When the switch is closed, the ohmmeter reads zero ohms.

To test an on/off switch without removing it from the circuit, a voltmeter can be used. [Attach the voltmeter across the switch.] When the switch is closed, 0 volts is read by the voltmeter. Many Agastya International Foundation. For Internal Circulation only.

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42 beginning students assume that it should measure the supply voltage instead. However, a closed switch is similar to a straight wire which has no voltage drop. When the switch is open, the supply potential [for example, 3 volts] is read on the voltmeter. An open switch is no different than a break in a series circuit in which the supply voltage is dropped across.” 11

WEB RESOURCES http://www.dummies.com/how-to/content/what-is-an-electronic-circuit.html (another introduction to circuits) http://people.sinclair.edu/nickreeder/eet114/mod04.htm (everything you would ever want to know about circuits, condensed onto one page and presented clearly http://www.dummies.com/how-to/content/find-thevenins-and-nortons-equivalents-forcomplex.html (an introduction to circuit transformations, an aspect of a theoretical way to analyze circuits) http://electronics.howstuffworks.com/everyday-tech/lithium-ion-battery.htm (an in-depth look at lithium-ion batteries, one of the more useful but expensive battery types)

VOCABULARY 1) Simple electric circuit – An arrangement consisting of a source of electricity, one or more devices that consume electricity, the conductors that connect these devices, and sometimes a switch 2) Source of electricity – A device such as a battery or generator that generates electricity 3) Load – A device such as a light bulb or fan that consumes electricity 4) Conductor – A material, usually a metal, that allows electricity to pass through it 5) Switch – A spring-loaded device used to manually make or break the supply of power to the connected load so that it can be activated or deactivated at will 6) Direction of electric current – Current in a circuit leaves the battery at the positive terminal and travels around the circuit to the battery’s negative terminal; electrons flow in the opposite direction 7) Battery – A collection of dry cells 8) Negative terminal – The part of a dry cell that receives the excess electrons from electrolysis in the cell and acts as the source of electrons into the circuit 9) Positive terminal – The part of a dry cell that releases electrons during electrolysis and receives electrons returning to the cell from the circuit 10) Dry cell – A source of electricity consisting of a carbon cathode, a zinc anode, and dry ammonium chloride paste as an electrolyte inside 11) Voltaic cell – A source of electricity based on spontaneous redox reactions, consisting of two half-cells that each contain an electrode within an electrolyte solution and that are linked by a wire connecting the two electrodes 12) Redox reaction – A reaction involving the transfer of electrons between to substances, so that one is reduced and one is oxidized 13) Secondary cell – A source of electricity that is rechargeable Agastya International Foundation. For Internal Circulation only.

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43 14) Solar cell â&#x20AC;&#x201C; A source of electricity that converts light energy from sunshine into electric energy 6http://www.physicsclassroom.com/Class/circuits/u9l2b.cfm 7http://www.physicsclassroom.com/Class/circuits/u9l1c.cfm 8http://spectrum.ieee.org/images/sep07/images/lithf2.gif

9http://suryaurza.com/wp-content/uploads/2013/02/solar_cell.png

10http://www.snupeel.com/wp/wp-content/uploads/2009/01/03_secondary.jpg 11http://www.wisc-online.com/objects/ViewObject.aspx?ID=DCE1102

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ABL 4 Measuring current and voltage Activity

Learning objective

4.1

What are the devices used to measure current and voltage across a DC circuit?

Key messages 





4.2

What are conductors and insulators?







Time (min) 40

Current and voltage are measured by an ammeter and a voltmeter, respectively.

The current flowing in a circuit and the voltage appearing across the bulb can be varied with a rheostat, which is a variable resistance.

A dry cell produces Direct Current (DC), which flows in such a direction that the current always leaves the cell at its positive terminal.

Materials that allow electric 15 current to pass through them are called conductors.

Materials that do not allow electric current to pass through them are called insulators.

Safety precautions must be taken when working with electrical voltages of 110 volts or higher.

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ABL 4.1

Time: 40 min

LEARNING OBJECTIVE – What are the devices used to measure current and voltage in a DC circuit?

Note to Instructor – This activity will show students how to use a rheostat, an ammeter, and a voltmeter. It will also help them understand the direction of current in a circuit.

ADVANCE PREPARATION Material List Materials 1 2 3 4.

5 6 7

Voltmeter (0 - 3 V) Cell Connecting wires Rheostat Bulb Ammeter (0 – 300 mA) Switch

Required quantity 1 per group 1 per group 3 per group 1 per group 1 per group 1 per group 1 per group

Things to do Not Applicable Safety Precautions Students should not connect the circuit, or turn the switch to ON or OFF, until the instructor gives the appropriate instruction.

NEVER connect circuit parts to the plug points present in the classroom.

SESSION Link to known information/previous activity In the previous activities, we learned about batteries and bulbs. Now we will learn how to measure the current and voltage across them.

Procedure Divide students into groups of 4-6 students. Distribute the materials to each group, and facilitate the following group activity.

Take the cell, the bulb, and the switch; connect them together with the connecting wires. Complete the circuit with an ammeter and a rheostat, as shown in the circuit diagrams in fig/ Chart 9. Keep the voltmeter free so that its probes can be made to touch the ends of any one of the components in the circuit, as required. The positive terminal of the voltmeter should be connected to the Agastya International Foundation. For Internal Circulation only.

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46 positive side of the potential difference in the component being measured, and the negative terminal of the voltmeter should be connected to the other side of the component.

Connect the voltmeter across the bulb as shown in the figure.

Move the rheostat key to different positions as shown in figures/ Chart 9 (one end, middle and other end) and observe the voltmeter and ammeter readings.

Notice that the current direction indicated by the ammeter remains the same all the time, irrespective of the position of the rheostat.

The readings of both ammeter and voltmeter change.

Set the rheostat position so that the ammeter reads about Âź of the full-scale value. Note how the bulb is glowing. Now interchange the connections to the positive and negative terminals of the cell.

Note the behavior of the ammeter. The meter indicates zero and is kicking towards less. The bulb is still glowing. That means that current continues to flow â&#x20AC;&#x201C; but the ammeter is not able to read it.

This is because you have reversed the direction of the current in the circuit by changing the terminals of the cell. Now the ammeter will only indicate the current properly if you interchange the connections to the ammeter also. Do that and check.

Tabulate the readings.

Fig 6: Ammeter, Voltmeter, Rheostat

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Chart 9: Circuit Diagrams for ABL 4.1 Experiment Position of Rheostat No. 1 2 3

Voltmeter reading across bulb (V2 volts)

Ammeter reading (I amps)

End 1 (Fig 2) Middle position (Fig 3) End 2 (Fig 4)

Note to Instructor: The placement of voltmeters and ammeters is important. Students often connect an ammeter as if it were a voltmeter (see the diagram on the left below), or they connect a voltmeter as if it were an ammeter (diagram on the right). Connecting a meter in these ways will certainly give an incorrect reading and may also damage the meter.12

Fig 7: Examples of incorrect circuits

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UNDERSTANDING THE ACTIVITY Leading questions 1. What is a voltmeter?

2. How do you identify the positive and negative terminals of the voltmeter, the ammeter, and the cell?

3. How do you make connections to the ammeter and the voltmeter?

4. In which direction is the current flowing in the circuit?

Discussion and Explanation Discussion Item 1 – Voltmeter  A voltmeter is a device used to measure the potential difference (voltage) between any two points. When a voltmeter is connected to a cell in an open circuit, it shows the emf of the cell.  The emf of the cell used is 1.5 volts.  The positive and negative terminals of the voltmeter are clearly marked with (+) and (-) signs.  A voltmeter is always connected in parallel, to any element.  The voltage across the bulb is called potential differences (PD). It will vary depending on the position of the rheostat, which determines the amount of current flowing in the circuit.  The voltage across any component in the circuit is represented by the letter V. The unit of voltage is the volt.

Discussion Item 2 – Ammeter  An ammeter is a device used to measure the current flowing through any component in a circuit.  The positive and negative terminals are marked (+) and (-), respectively.  An ammeter should always be connected in series with the source and the component, to measure the current through the component. The current indicated by the ammeter is the same amount of current flowing through each component in the series circuit.  The letter used to represent current is I.  Current is the amount of charge flowing per second. It is given by I = q/t, where q is the charge in coulombs and t is the time in seconds.  The unit of current is the ampere.  If 1 coulomb of charge flows through a wire in 1 second, then the current is said to be 1 ampere.

Discussion Item 3 – Rheostat  A rheostat is an adjustable or variable resistor. It is used to control the electrical resistance of a circuit without interrupting the flow of current.  A rheostat has 3 terminals and usually consists of a resistive wire wrapped to form a torrid coil, with a wiper that slides along the surface of the coil. It is most often designed with a ceramic core.

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49 

Rheostats are used in applications that require high voltage and high current.

Discussion Item 4 – Direction of current  The current produced by a dry cell is Direct Current (DC). It is the current which flows in one direction at all times in a circuit.  The direction of the current in an external circuit is always such that it leaves the positive terminal of the battery and returns to the battery at its negative terminal, after passing through the other circuit components, which may include bulb, a rheostat, or an ammeter.  That is why, when the connections to the terminals of the battery are interchanged, the current begins flowing in the opposite direction through the other circuit components.  A bulb and a rheostat will continue to work properly even when the direction of the current flowing in the circuit is reversed, but the ammeter is designed to work with current flowing through it in one direction only, so it works properly only when its connections are also interchanged.

KEY MESSAGES   

Current and voltage are measured by an ammeter and a voltmeter, respectively.

The current flowing in a circuit and the voltage appearing across the bulb can be varied with a rheostat, which is a variable resistance.

A dry cell produces Direct Current (DC), which flows in such a direction that the current always leaves the cell at its positive terminal (as opposed to Alternating Current, which will be discussed in a later ABL).

LEARNING CHECK13 1. A current is said to exist whenever _____. a. a wire is charged b. a battery is present c. electric charges are unbalanced d. electric charges move in a loop 2. Current has a direction. By convention, current is in the direction that ___. a. + charges move b. - electrons move c. + electrons move 3. A current of one ampere is a flow of charge at the rate of _______ coulomb per second.

4. When a charge of 8 C flows past any point along a circuit in 2 seconds, the current is ________ A. 5. If 5 C of charge flows past a point in a circuit in 10 seconds, then the current is _________ A. (Answers: d; a; 1; 4; 0.5)

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50

ABL 4.2

Time: 15 min

LEARNING OBJECTIVE â&#x20AC;&#x201C; What are conductors and insulators?

Note to Instructor â&#x20AC;&#x201C; This activity will teach children to distinguish the characteristics of conductors and insulators.

ADVANCE PREPARATION Material List

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Material

Required Quantity

Bulb Bulb holder Connecting wire Cell Hair pin Safety pin Pencil lead Eraser Plastic scale Match stick Iron nail Copper rod Paper clip Acrylic Connecting stand Voltage Tester

1 per group 1 per group 1 per group 1 per group 1 per group 1 per group 1 per group 1 per group 1 per group 1 per group 1 per group 1 per group 1 per group 1 per group 1 per group 1 per class

Things to do Not Applicable Safety Precautions This activity should only be done with a low voltage source such a single cell (1.5 V). It should never be attempted at regular supply voltage (230 V) as this could lead to dangerous electric shock.

SESSION Link to known information/previous activity In the previous ABL, we learned how to assemble a simple electric circuit. This requires wires to connect the circuit elements; in this activity, we will discover why wires are necessary. We will also learn why a loop of rope cannot be used as an electric circuit.

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Procedure Divide the students into groups and distribute the materials. Facilitate the following group activity.

Connect the bulb and battery to the connecting stand with connecting wires. This arrangement forms an open circuit.

Fig 8: Open Circuit Insert the remaining materials one by one into the connecting stand to form closed circuits.

Observe the bulb. For which materials does the bulb glow, and for which materials does the bulb not glow? Fill the observation sheet with your findings.

For those materials that make the bulb glow, write the name of the material under the conductor column in the observation sheet. For those materials that do not make the bulb glow, write the name of the material under the insulator column in the observation sheet.

Object 1 2 3 4 5 6 7 8 9 10

Name of the material

Does the bulb glow? (Y/N)

Conductors

Insulators

Hair pin Safety pin Pencil lead Eraser Plastic scale Match stick Iron nail Copper rod Paper clip Acrylic

UNDERSTANDING THE ACTIVITY Leading questions 1. What happens to the bulb when the materials are placed in the connecting stand?

2. Based on this activity, materials can be grouped into how many types?

3. What are the materials that make the bulb glow called?

4. What are conductors and insulators?

5. What is the use of an insulator in an electrical device?

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52 6. Where do we use conductors and insulators together?

7. What is a voltage tester (also called a line tester)?

Discussion and Explanation Discussion Item 1 – Conductors and insulators  When materials like copper and iron are inserted in the stand, the bulb glows, because they allow current to pass through them. They are called conductors.  When materials like plastic and acrylic are kept in the stand, then the bulb does not glow.

These materials do not allow current to pass through them, and are called insulators.  Sometimes, insulators which have become wet may also conduct electricity. For example, tree branches may short-circuit the electric power line by brushing against live wires during a storm.  We use conductors and insulators together, in the cables in our houses. The outer layer of the cable is an insulator (like plastic or rubber), which surrounds the inner wire, which is a conductor made of copper or aluminum. The insulation ensures that the current-carrying inner conductor does not come in contact with other similar conductors in the circuit or with external objects or persons. Thus it prevents short circuits and shocks. (Show the wire and inner leads to the students.)

Discussion Item 2 – Voltage testers  A tester is used to verify whether electricity is present in a circuit. It contains a neon bulb connected with a high resistance in series.  When we put the tester in its socket (in the circuit with current flowing), a very small amount of current flows through our body and completes the circuit. This makes the bulb glow, indicating that current is present.  The outer portion of a tester is made with insulating material, like glass or plastic.  A detailed description of a voltage tester is given in the next ABL. Discussion Item 3 – Dangers of electricity and precautions to be taken  Generally, electricity flows to the earth through anything that will conduct electrical current.  Wood and glass do not conduct electricity; there are other materials that do not conduct electricity.  Electricity passes through the human body. Sometimes electric shock can have serious effects on people; it can even cause death. The severity of the damage caused to the human body by electric shock depends upon several factors, which include the amount of current flowing through the body (in amperes), the specific path of the current in the body, and the amount of time the current takes in passing through the body.  Precautions to avoid electric shock: o Whenever working or conducting experiments with electric voltages greater than what is provided by individual cells and small batteries, it is important to ensure that you do not touch a live wire or part, directly or through any conducting objects like nails or rods. o All tools used in electrical work, like screwdrivers and pliers, should have a proper insulated handle made of wood or plastic. o Be careful not to work with electrical gadgets on tables or floors that are wet.

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

o Become familiar with first aid procedures in case an electric shock occurs by accident.

Only trained electricians can work with live electric wiring. Even trained electricians must wear safety equipment, like rubber shoes and rubber gloves, when they have to climb up the electric poles to do repair work.

KEY MESSAGES   

Materials that allow electric current to pass through them are called conductors.

Materials that do not allow electric current to pass through them are called insulators.

Safety precautions must be taken when working with electrical voltages of 110 volts or higher.

LEARNING CHECK Name two common materials that are conductors, and two materials that are insulators, that you did not analyze in this activity. (Possible Answers: gold, aluminum, (impure) water; glass, air, porcelain)

TRY IT YOURSELF If you see an electric wire with the outer coat of plastic worn off, tell an adult about it and you could save someone from a dangerous electric shock!

The next time you plug an electric cord into the wall, try to name each part as a conductor or insulator. Are there very many conductors that you can see? Why or why not?

INTERESTING INFORMATION Resistivity Insulators tend to have high resistance to the flow of current (resistivity) and conductors tend to have low resistivity. The resistivity of some common materials is as follows.14 Resistivity (ohm m) Glass 1012 Mica 9 x 1013 Quartz (fused) 5 x 1016 Copper 1.7 x 10-8 Conductivity “[The] relative mobility of electrons within a material is known as electric conductivity. Conductivity is determined by the types of atoms in a material (the number of protons in each atom's nucleus, determining its chemical identity) and how the atoms are linked together with one another.

Materials with high electron mobility (many free electrons) are called conductors, while materials with low electron mobility (few or no free electrons) are called insulators. …

It must be understood that not all conductive materials have the same level of conductivity, and not all insulators are equally resistant to electron motion. Electrical conductivity is analogous to the Agastya International Foundation. For Internal Circulation only.

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54 transparency of certain materials to light: materials that easily "conduct" light are called "transparent," while those that don't are called "opaque." However, not all transparent materials are equally conductive to light. Window glass is better than most plastics, and certainly better than "clear" fiberglass. So it is with electrical conductors, some being better than others. …

It should also be understood that some materials experience changes in their electrical properties under different conditions. Glass, for instance, is a very good insulator at room temperature, but becomes a conductor when heated to a very high temperature. Gases such as air, normally insulating materials, also become conductive if heated to very high temperatures. Most metals become poorer conductors when heated, and better conductors when cooled. Many conductive materials become perfectly conductive (this is called superconductivity) at extremely low temperatures.”15 Insulators, semiconductors, and conductors “Solid-state materials can be classified into three groups: insulators, semiconductors and conductors. Insulators are materials having an electrical conductivity diamond: 10-14 S/cm); semiconductors have a conductivity silicon it can range from 10-5S/cm to 103S/cm); [and] conductors are materials with high conductivities:

(like (for

(like silver: 106S/cm.)

WEB RESOURCES http://electronicsclub.info/voltage.htm (an unusual but very clear and helpful explanation of the meaning of voltage and current) http://www.s-cool.co.uk/a-level/physics/current-charge-and-voltage/revise-it/current-electricityand-conventional-current (a review of the principles of electric circuits, with a fabulous interactive diagram for the direction of current flow) http://www.engineersgarage.com/tutorials/dc-voltage-current-testing(an introduction to multimeters with lots of pictures) http://www.ndt-ed.org/EducationResources/HighSchool/Electricity/conductorsinsulators.htm (a review of conductors and insulators) http://www.allaboutcircuits.com/vol_1/chpt_1/2.html (further details about conductors and insulators, especially regarding relative conductivity, conductivity at varying temperatures, and the speed of electron flow)

VOCABULARY 1) Current – A measure of the speed of electric charges moving through a circuit, measured in amps (A) 2) Voltage – A measure of the potential difference across a device or a section of a circuit; it describes the strength of the inducement for electric charges to travel across that circuit section and is measured in volts (V) 3) Voltmeter – A device used to measure the voltage across a part of a circuit Agastya International Foundation. For Internal Circulation only.

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55 4) Ammeter – A device used to measure the current through a circuit 5) Rheostat – A resistor with variable resistance that is used to determine the amount of current flowing in a circuit 6) Direct Current – Current that flows in such a direction that the current always leaves the cell at its positive terminal 7) Conductor – A material that allows electric current to pass through it 8) Insulator – A material that does not allow electric current to pass through it 9) Voltage tester – A device used to verify whether electricity is present in a circuit 10) Electric shock – A condition in which a large amount of electric current passes through the human body, potentially causing serious harm

12http://people.sinclair.edu/nickreeder/eet114/mod04.htm

13http://www.physicsclassroom.com/class/circuits/u9l2c.cfm

14http://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.html 15http://www.allaboutcircuits.com/vol_1/chpt_1/2.html

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56

ABL 5 Ohm’s law Activity

Learning objective

5.1

What is Ohm’s law?

5.2

What is the relationship between the resistance and the length of a conductor?

What is the relationship between the resistance and the cross sectional area of a conductor?

5.3

Key messages 

Ohm’s law - The electric current in the conductor is directly proportional to the potential difference across the conductor when the temperature remains a constant.  The resistance of a conductor is a constant at any given temperature.  The resistance of a conductor increases as the length increases.

It can be shown that R L 

The resistance of a conductor decreases as thickness (or area of cross section) increases. It can be shown that Resistance is directly proportional to 1/cross sectional area.

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Time (min) 30

15

15

57

ABL 5.1

Time: 30 min

LEARNING OBJECTIVE – What is Ohm’s law?

Note to Instructor – Students will construct a series of circuits and for each one record rheostat, ammeter, and voltmeter readings. They will then plot graphs of voltage vs. current.

ADVANCE PREPARATION Material List

1

2 3 4 5 6

Material Agastya Electricity Board No. 4 or No. 5, with accessories Battery or eliminator Bulb Rheostat Ammeter (0 to 500 mA) Voltmeter (0 to 3 V)

Required Quantity 1 per group

1 per group 1 per group 1 per group 1 per group 1 per group

Things to do Try the activity before class to ensure that everything is working.

Note to Instructor: If the board odel is not available, connect individual devices as explained in the procedure.

Fig 9: Agastya Electricity Board no.4 model Safety Precautions Students should not connect the circuit, or turn the switch to ON or OFF, until the instructor gives the appropriate instruction.

Never connect circuit parts to the plug points present in the classroom.

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SESSION Link to known information/previous activity Our previous investigation of circuits has been entirely qualitative. In this activity we will learn the first of the quantitative relationships that govern electric circuits.

Procedure Divide students into groups of 5-6 students. Distribute the materials to each group. Facilitate the following group activity.

Connect the battery, switch, rheostat, bulb, and ammeter in series, as show in Figure 1. Connect the voltmeter parallel to the bulb as circuit diagram. Put on the switch. When current flows in the circuit the bulb glows, bring the rheostat key to the middle position (see Figure 1).

Tabulate the voltmeter and ammeter readings.

Change the position of the rheostat, take the voltmeter and ammeter readings and tabulate.

Confirm that you have tabulated readings for 4 to 5 positions of rheostat.

Fig 10: Circuit Diagram for ABL 5.1 Plot a graph by taking voltage on the x-axis and current on the y-axis. Calculate the reciprocal of the slope of the graph. Also calculate the ratio of V and I in each case.

Trail No.

Voltmeter reading across the bulb/unknown resistance (V volts)

Ammeter reading(I amps)

Resistance of bulb (R2 = V/I)

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59 4 5 6 Worksheet 1 Observations for Ohm’s Law

UNDERSTANDING THE ACTIVITY Leading questions 1. What happens to the brightness of the bulb when the voltage is increased or decreased by adjusting the rheostat?

2. What happens to the current when the voltage is increased or decreased using the rheostat?

3. What can you conclude from this experiment?

Discussion and Explanation Discussion Item 1 – Ohm’s law  When the voltage across a circuit is increased, the brightness of the bulb increases. Thus we can say that the current in the circuit increases as the voltage increases. Current through any element in the circuit is directly proportional to the voltage across that element, assuming that its resistance is constant.  Ohm’s law - The electric current in the conductor is directly proportional to the potential difference across the conductor when the temperature remains a constant.  Voltage (V) α Current (I), i.e. V α I and V/I = R, a constant  The resulting graph will appear as follows:

Fig 11: Ideal graph for Ohm’s Law Note that the slope of the line (measured as the change in the value of current divided by the change in the value of voltage, I/V) is constant at all points on the graph. The reciprocal of this slope (V/I) is the value of resistance in ohms.

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KEY MESSAGES  

Ohm’s law - The electric current in the conductor is directly proportional to the potential difference across the conductor when the temperature remains a constant.

The resistance of a conductor is a constant at any given temperature.

LEARNING CHECK16 1. Which of the following will cause the current through an electrical circuit to decrease?

Choose all that apply. a. decrease the voltage b. decrease the resistance c. increase the voltage d. increase the resistance (Answers: A and D. The current in a circuit is directly proportional to the electric potential difference impressed across the circuit and inversely proportional to the resistance of the circuit. Reducing the current can be done by reducing the voltage (A) or by increasing the resistance (D).)

2. A certain electrical circuit contains a battery with three cells, wires and a light bulb. Which of the following would cause the bulb to shine less brightly? Choose all that apply. a. increase the voltage of the battery (add another cell) b. decrease the voltage of the battery (remove a cell) c. decrease the resistance of the circuit d. increase the resistance of the circuit (Answers: B and D. The bulb will shine less brightly if the current in it is reduced.

Reducing the current can be done by reducing the electric potential difference impressed across the bulb (B) or by increasing the resistance of the bulb (D).)

3. Use the Ohm's law equation to provide numerical answers to the following questions: a. An electrical device with a resistance of 3.0 Ω will allow a current of 4.0 amps to flow through it if a voltage drop of ________ volts is impressed across the device. b. When a voltage of 120 V is impressed across an electric heater, a current of 10.0 amps will flow through the heater if the resistance is ________ Ω. c. A flashlight that is powered by 3 Volts and uses a bulb with a resistance of 60 Ω will have a current of ________ amps.

(Answers: a. 12; b. 12; c. 0.05) 4. The sticker on a compact disc player says that it draws 288 mA of current when powered by a 9-volt battery. What is the power (in watts) of the CD player?

(Answer: P = 2.59 W) 5. Calculate the resistance and the current of a 1500-Watt electric hair dryer plugged into a 120 V outlet.

(Answer: I = 12.5 Amp and R = 9.6 Ω) 6. A color TV has a current of 2.0 amps when connected to a 120-volt household circuit. What are the resistance (in ohms) and the power (in watts) of the TV set?

(Answer: 60 Ω and 240 W)

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ABL 5.2

Time: 15 min

LEARNING OBJECTIVE â&#x20AC;&#x201C; What is the relationship between the resistance and the length of a conductor?

Note to Instructor â&#x20AC;&#x201C; Students will assemble circuits with two different lengths of resistors, and will compare the values of current through the two circuits.

ADVANCE PREPARATION Material List

1 2 3 4 5

Material Baluragi-sir kit with a 6V battery Bulb Ammeter Switch Nichrome resistor of a different length than the resistor in the kit

Required Quantity 1 per group 1 per group 1 per group 1 per group 1 per group

Things to do Try the activity before class to determine that everything is working properly.

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Fig 12: Electricity kit Safety Precautions Students should not connect the circuit, or turn the switch to ON or OFF, until the instructor gives the appropriate instruction.

Never connect circuit parts to the plug points present in the classroom.

SESSION Link to known information/previous activity We will use the V=IR relationship that we learned about in the last activity to explain the relationship between the resistance and the length of a resistor.

Procedure Facilitate the following group activity. Divide the class into groups and distribute the materials.

Connect the battery, the bulb, the switch, the ammeter, and the resistor of shorter length in series, as shown in the circuit diagram. Put on the switch and observe the brightness of the bulb.

Note the ammeter reading.

Now remove the wire and introduce the longer resistor into the circuit. Put on the switch and record the brightness and ammeter reading.

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Fig 13: Circuit diagrams for ABL 5.2 Wire

Brightness of the bulb (less/more)

Current in the ammeter

Resistance (less/more)

Shorter Wire Longer wire Worksheet 2: Observation sheet for relation between Resistance and Length of Conductor

UNDERSTANDING THE ACTIVITY Leading questions 1. What happens to the brightness of the bulb when the longer wire is connected in the circuit?

2. What happens to the ammeter reading when the longer wire is connected in the circuit?

3. What is the relationship between the length of the wire and the resistance of the conductor?

Discussion and Explanation  When a longer wire is connected in the circuit, the brightness of the bulb decreases, indicating that the current has decreased in the circuit.  We know from Ohm’s law that the current decreases when the circuit resistance increases, for a constant value of voltage.  When the length of a wire is increased, its resistance also increases.  If R is measured and length is known it can be shown that R/L is constant.  We use this principle in working with rheostats.

KEY MESSAGES 

The resistance of a conductor increases as the length increases. It can be shown that R L

LEARNING CHECK (See Learning Check after 6.3)

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64

ABL 5.3

Time: 15 min

LEARNING OBJECTIVE â&#x20AC;&#x201C; What is the relationship between the resistance and the cross sectional area of a conductor?

Note to Instructor â&#x20AC;&#x201C; Students will assemble two circuits, one with a thicker resistor and the other with a thinner resistor. They will discover the relationship between the amounts of current passing through each circuit.

ADVANCE PREPARATION Material List

**1 2 3 4 5 6 7

Material Bulb Connecting wires Battery Switch Ammeter Thin nichrome wire Thick nichrome wire

Required Quantity 1 per group 1 per group 1 per group 1 per group 1 per group 1 per group 1 per group

Things to do Try the activity before class and check that everything is working properly.

Safety Precautions Students should not connect the circuit, or turn the switch to ON or OFF, until the instructor gives the appropriate instruction.

Never connect circuit parts to the plug points present in the classroom.

SESSION Link to known information/previous activity In the last activity we related the resistance of a resistor to its length. Now we will relate it to its cross-sectional area. This will give us a full understanding of the resistances of differently shaped objects.

Procedure Facilitate the following group activity. Divide the students into groups and distribute the materials.

Connect the bulb, the battery, the switch, the ammeter, and a thin nichrome wire, as shown in the circuit diagram. Put on the switch and observe the brightness of the bulb. Note the current in the circuit.

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65 Now put off the switch. Remove the thin wire and replace it with the thick wire of the same material. Put on the switch and observe the brightness of the bulb. Note the ammeter reading.

Record the brightness and ammeter readings in the observation sheet.

Fig 15: Circuit diagrams for ABL 5.3 Wire

Brightness of the bulb (less/more)

Current in the ammeter

Resistance (less/more)

Thinner wire Thick wire Worksheet 3: Observation sheet for relation between Resistance and cross sectional area of a Conductor

UNDERSTANDING THE ACTIVITY Leading questions 1. What happens to the brightness of the bulb and the current when the thicker resister is connected in the circuit?

2. What happens to the resistance when thicker wire is connected in the circuit?

3. What is the relationship between the resistance and the thickness of the wire?

4. Why do we use thicker wires for high-voltage transmission (HT) lines?

Discussion and Explanation  When a thick wire is connected in a circuit, the resistance in the circuit decreases and hence the current increases, so the brightness of the bulb will be greater.  A larger cross sectional area allows more electrons to flow through the conductor.  When a thin wire is connected in the circuit, the resistance increases and hence current decreases. The brightness of the bulb will be lower.

KEY MESSAGES 

The resistance of a conductor decreases as thickness (or area of cross section) increases. It can be shown that Resistance is directly proportional to 1/cross sectional area.

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66

LEARNING CHECK17 1. Household circuits are often wired with two different widths of wires: 12-gauge and 14gauge. The 12-gauge wire has a diameter of 1/12 inch while the 14-gauge wire has a diameter of 1/14 inch. Thus, 12-gauge wire has a wider cross section than 14-gauge wire. A 20-Amp circuit used for wall receptacles should be wired using 12-gauge wire and a 15Amp circuit used for lighting and fan circuits should be wired using 14-gauge wire. Explain the physics behind such an electrical code.

(Answer: A 12-gauge wire is wider than 14-gauge wire and thus has less resistance. The lesser resistance of 12-gauge wire means that it can allow charge to flow through it at a greater rate - that is, allow a larger current. Thus, 12-gauge wire is used in circuits which are protected by 20-Amp fuses and circuit breakers. On the other hand, the thinner 14gauge wire can support less current owing to its larger resistance; it is used in circuits which are protected by 15-Amp fuses and circuit breakers.)

2. You have a 12-guage wire and a 14-guage wire that have the same total resistance. Which one must be longer?

3. Two wires - A and B - with circular cross-sections have identical lengths and are made of the same material. Yet, wire A has four times the resistance of wire B. How many times greater is the diameter of wire B than wire A? (Answer: DB = 2 * DA. If wire A has four times the resistance, then it must have the smaller cross-sectional area since resistance and cross-sectional area are inversely proportional. In fact, A must have one-fourth the cross-sectional area of B. Since the cross-sectional area of a circular cross-section is given by the expression PI•R 2, wire A must have one-half the radius of wire B and therefore one-half the diameter. Put another way, the diameter of wire B is two times greater than the diameter of wire A.)

TRY IT YOURSELF Ohm’s law describes a directly proportional relationship and an inversely proportional relationship. Proportionality is a concept that you use all the time without even realizing it – distance is directly proportional to time at a constant speed, the circumference of a circle is directly proportional to its diameter, on a map drawn to scale the distance between two points is directly proportional to the distance between the two locations those points represent. Try to identify other proportional relationships that you use in real life. (Some things to investigate are weight scales, the price of petrol, and the salary of someone who works on commission.)

If you want to be a smarty-pants, the next time you are in science class and the teacher introduces a new equation, try to identify it as a directly proportional relationship, an inversely proportional relationship, and inverse square law, or none of those!

INTERESTING INFORMATION The resistance of a conductor is inversely proportional to its cross-sectional area: R α 1/A. Thus there is a constant ρ such that R = ρ/A. ρ is the constant called resistivity. Its unit is the ohmmeter. Every material has a unique resistivity constant. Here are the resistivity values for some common materials at 20 degrees Celsius: Agastya International Foundation. For Internal Circulation only.

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Material Silver Copper Gold Aluminum Tungsten Iron Platinum Lead Nichrome Carbon Polystyrene Polyethylene Glass Hard rubber

Resistivity (ohm-meters) 1.59 x 10-8 1.7 x 10-8 2.4 x 10-8 2.8 x 10-8 5.6 x 10-8 10 x 10-8 11 x 10-8 22 x 10-8 150 x 10-8 3.5 x 10-5 107 – 1011 108 – 109 1010 – 1014 1013

WEB RESOURCES http://electronicsclub.info/ohmslaw.htm (how to perform calculations with Ohm’s law) http://www.ndt-ed.org/EducationResources/HighSchool/Electricity/ohmslaw.htm (an explanation of what Ohm’s law tells us) http://www.hamuniverse.com/ohmslaw.html (Ohm’s law in terms of water flow) http://www.physics.uoguelph.ca/phyjlh/Fendt/phe/ohmslaw.htm (an applet demonstrating Ohm’s law) http://www.physicsclassroom.com/class/circuits/u9l3b.cfm (a review of the relationship between resistance and length/area of a wire)

VOCABULARY 1) Ohm’s law – The electric current in the conductor is directly proportional to the potential difference across the conductor when the temperature remains a constant.

2) Resistance – A measure of how well a material obstructs the movement of charges through it, measured in ohms (Ω) 3) Directly proportional – A relationship between two parameters such that a graph plotting their mutual variation is a straight line; also called a linear relationship 4) Inversely proportional – A relationship between two parameters such that one varies linearly with the reciprocal of the other 5) Inverse square law – A relationship between two parameters such that one is inversely proportional to the square of the other 6) Resistor – A circuit component that has a fixed resist 16http://www.physicsclassroom.com/Class/circuits/u9l3c.cfm

17http://www.physicsclassroom.com/Class/circuits/u9l3b.cfm

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68

ABL 6 Series and parallel connections Activity

Learning objective

6.1

What is a battery? How are sources like cells connected in series or parallel arrangements to form batteries?

Key messages 





6.2

An example of a battery using both series and parallel arrangements of cells.



6.3

How are loads like bulbs connected in series or in parallel? How does the current flow in these circuits?



Batteries are formed by connecting two or more cells together.

Two or more cells are said to be connected in series if the negative terminal of the first cell is connected to the positive terminal of the second cell, the negative terminal of the second is connected to the positive terminal of the third, and so on. Series connections are used when the current is to be delivered to the load at a high voltage.

In a parallel arrangement of cells, all of the positive terminals are connected to one point, which becomes the positive terminal of the combination of cells (called the battery). Similarly, all the negative terminals are brought together to form a common negative terminal of the battery.

Parallel connections of cells will deliver currents at low voltage for a long time.

A solar cell array is usually a series–parallel combination of solar cells designed to meet the requirements of the voltage and current to be supplied.

In series connections of bulbs, the first end of the first bulb is connected to the positive terminal of the cell, the second end of the first bulb is connected to the first end of the second bulb, and the second end of the second bulb is

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Time (min) 25

10

25

69

ď&#x201A;§

connected to the negative terminal of the cell.

In parallel connections of bulbs, all of the first terminals of the bulbs are connected to one wire and all of the second terminals of bulbs are connected to another wire.

These two wires are connected to the two ends of the cell.

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70

ABL 6.1

Time: 25 min

LEARNING OBJECTIVE â&#x20AC;&#x201C; What is a battery? How are sources like cells connected in series or parallel arrangements to form batteries?

Note to Instructor â&#x20AC;&#x201C; Students will learn that batteries are made of multiple cells, and they will discover how series and parallel arrangements of cells affect the intensity of a light bulb connected in the circuit.

ADVANCE PREPARATION Material List

1 2 3 4 5 6

Material

Required quantity

Bulbs Bulb holders Connecting wires Switch Cells Cycle rubber bands

1 per group 1 per group 1 per group 1 per group 2 per group 2 per group

Things to do Check that the bulbs and other equipment are all working.

Safety Precautions Students should not connect the circuit, or turn the switch to ON or OFF, until the instructor gives the appropriate instruction.

Never connect circuit parts to the plug points present in the classroom.

SESSION Link to known information/previous activity In previous ABLs, we have described batteries as comprised of electric cells. Here we will further investigate how best to arrange the cells in a battery.

Procedure Divide students into groups of 4-6 students. Distribute the materials to each group. Facilitate the following activity.

Connect the bulb, the cell, and the switch as shown in Figure 1. Put on the switch and observe the brightness of the bulb.

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71 Now put off the switch and add one more cell in the circuit as shown in Figure 2. (The negative terminal of the first cell should be connected to the positive terminal of the second cell.) Put on the switch and observe the brightness of the bulb.

Put off the switch. Remove one of the cells and connect them in parallel as shown in Figure 3.

(Note: The positive terminals of the two cells should be connected to one common end point and the negative terminals of the cells should be connected to another common end point.)Put on the switch and observe the brightness of the bulb.

Fig 16: Circuit diagrams for ABL 6.1

UNDERSTANDING THE ACTIVITY Leading questions 1. When is the intensity of the bulb greater–when there is a single cell or when there are two cells connected in series in the circuit?

2. What is this type of connection called?

3. What is the reason for the change in brightness when two cells are connected in the circuit?

4. When is the intensity of the bulb greater – when the cells are connected in series or in parallel?

Discussion and Explanation Discussion Item 1 – What is a battery?  A battery is a combination of electric cells created to obtain more current and voltage from the source. For example, torchlight usually has a combination of 2, 3, or 4 dry cells, giving a Agastya International Foundation. For Internal Circulation only.

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72



total voltage of 3, 4.5, or 6 volts. The battery in a motorcycle used to start the engine and to operate the lights and the horn is also a combination of cells.

When a single cell is connected in the circuit, the current flowing through the bulb is determined by the voltage of that single cell.

Discussion Item 2 – Cells connected in series  Two cells are said to be connected in series if the negative terminal of the first cell is connected to the positive terminal of the second cell. The positive terminal of the first cell and negative terminal of the second cell are connected to the bulb and the switch. In series circuits, the voltage generated by each cell will be added to get the total voltage applied to the load. The same amount of current flows through every component of the circuit –cell 1, cell 2, the bulb, and the switch.  When two cells are connected in series, the current through the bulb becomes double compared to a circuit with a single cell. (In a series connection, current flows along only one path in the circuit.)  With two cells are in series, the brightness of the bulb becomes much greater.  With two cells connected in series, their voltages add up to twice the voltage of a single cell.

The total voltage appearing across both the cells is the sum of the EMFs of individual cells.

Discussion Item 3 – Cells connected in parallel  In a parallel connection of the cells, all the positive terminals are connected to one point and all the negative terminals are connected to another point. These two points are separately connected to the two terminals of the bulb.  When a load is connected across the parallel combination of cells, the load current is the sum of the currents supplied by each cell. However, the current supplied by each cell to the load may be different.  In parallel connections, the same voltage appears across each cell and across the load, and the net voltage appearing across the load will be the voltage of the single cell.  In this activity, we have taken cells of equal voltage, so the intensity of the bulb remains the same with a single cell as with two cells in parallel.

Discussion Item 4 – The difference between series and parallel connections of cells  When cells are connected in series, the bulb draws higher current and glows brighter. The bulb will glow as long as the charge in the cells lasts.  When cells are connected in parallel, the bulb draws less current and the brightness is less than with a series connection, but the bulb will glow for a longer time because the charge in the battery will last longer.

KEY MESSAGES  

Batteries are formed by connecting two or more cells together.

Two or more cells are said to be connected in series if the negative terminal of the first cell is connected to the positive terminal of the second cell, the negative terminal of the second is connected to the positive terminal of the third, and so on. Series connections are used when the current is to be delivered to the load at a high voltage.

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73 

In a parallel arrangement of cells, all of the positive terminals are connected to one point, which becomes the positive terminal of the combination of cells (called the battery).

Similarly, all the negative terminals are brought together to form a common negative terminal of the battery. Parallel connections of cells will deliver currents at low voltage for a long time.

LEARNING CHECK Observe the electrical wiring below. Indicate whether the connections are series or parallel connections. Explain each choice.18

(Answers: A. parallel; B. series) For each of the following questions, answer “series” or “parallel.”

Which type of electric cell arrangement will give greater light intensity?

Which type of electric cell arrangement will yield a longer-lasting charge?

(Answers: series; parallel) If you have an appliance that asks for a 24-volt battery but you have only two 12-volt batteries, how should you connect them to power the appliance?

(Answer: series)

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74

ABL 6.2

Time: 10 min

LEARNING OBJECTIVE – An example of a battery using both series and parallel arrangements of cells.

Note to Instructor – In this demonstration, students will observe the functioning of a solar panel.

ADVANCE PREPARATION Material List

1

Material

Required quantity

Solar energy kit model

1 per class

Things to do Check that every part of the model, including the fan, speaker, bulb, and voltmeter, is working.

Safety Precautions Circuits using low voltage sources (such as 1.5 V) do not give electric shocks. However, we have to be very careful to avoid getting dangerous electric shocks when working with devices at higher voltages (such as 230 volts).

SESSION Link to known information/previous activity In the previous activity, we saw that batteries can series or parallel arrangements of cells. Here we will see a battery – in this case, a solar battery – that contains cells arranged both in series and in parallel.

Procedure Take the students outside and ask them to sit in a semicircle. Call on one of the students and ask him/her to hold the solar panel and expose it to sunlight. Ask another student to connect the solar panel pin to the fan, the bulb, the voltmeter, and finally the speaker, one by one. The other students should carefully observe what goes on.

Now ask the student holding the solar panel to cover the panel with a hand. Ask the second student to connect the pin to the fan, the bulb, the voltmeter, and the speaker in succession. Ask the students what they observe.

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Fig 17: Solar energy kit

UNDERSTANDING THE ACTIVITY Leading questions 1. What is the source of energy here for the devices like the fan, bulb, and speaker?

2. Why do the devices not work when the solar panel is blocked?

3. How many cells are there in the panel, and how they are connected?

Discussion and Explanation Discussion Item 1 – A series-parallel connection of cells  In order to construct a battery which can deliver adequate current at adequate voltage, it is necessary to make an array of cells which contains several strings connected in parallel, each string consisting of several cells connected in series.  A solar cell array or panel is a good example of such an arrangement of cells.

Discussion Item 2 – How solar panels work  A typical silicon PV cell is composed of a thin wafer consisting of an ultra-thin layer of phosphorus-doped (called N-type because phosphorous tends to gain extra electrons and become negatively charged) silicon on top of a thicker layer of boron-doped (called P-type because boron tends to get rid of electrons and become positively charged) silicon.  An electrical field is created near the top surface of the cell where these two materials are in contact, called the P-N junction. When sunlight strikes the surface of a PV cell, this electrical field provides momentum and direction to light-stimulated electrons, resulting in a flow of current when the solar cell is connected to an electrical load.

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76

Chart 10: Diagram of a photovoltaic cell.   

Regardless of size, a typical silicon PV cell produces about 0.5 volts of DC under open-circuit, no-load conditions.

The current (and power) output of a PV cell depends on its efficiency and size (surface area), and is proportional to the intensity of sunlight striking the surface of the cell.

For example, under peak sunlight conditions, a typical commercial PV cell with a surface area of 100 cm2will produce about 1 to 1.5 watts of electrical power.19

Discussion Item 3 – How the cells inside a solar panel are connected  Each cell of a solar panel is called a Photovoltaic (PV) cell. These are connected electrically in series and/or parallel circuits to produce higher voltages, currents, and/or power levels.

Photovoltaic modules consist of PV cell circuits sealed in an environmentally protective laminate, and are the fundamental building blocks of PV systems.  Photovoltaic panels include one or more PV modules assembled as a pre-wired, fieldinstallable unit.  A photovoltaic array is the complete power-generating unit, consisting of any number of PV modules and panels.

Chart 11: Photovoltaic cells, modules, panels and arrays.

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 Today’s photovoltaic modules are extremely safe and reliable products, with minimal failure rates and projected service lifetimes of 20 to 30 years. Most major manufacturers offer warranties of 20 or more years for the maintenance of a high percentage of the initial rated power output.20

KEY MESSAGES 

A solar cell array is usually a series–parallel combination of solar cells designed to meet the requirements of the voltage and current to be supplied.

LEARNING CHECK Ask learners to list the key things they have learnt. Guide them to the key messages listed and then put up the chart of key messages. If you have time during the class, make up a small game, quiz or match the following as a learning check. This may have to be done as part of advance preparation.

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78

ABL 6.3

Time: 25 min

LEARNING OBJECTIVE â&#x20AC;&#x201C; How are loads like bulbs connected in series or in parallel?

How does the current flow in these circuits?

Note to Instructor â&#x20AC;&#x201C; In this activity, students will create electric circuits with two light bulbs in series and in parallel. They will discover how the arrangement of bulbs is related to the intensity of the light.

ADVANCE PREPARATION Material List

1 2 3 4 5 6

Material

Required quantity

Bulbs Bulb holders Connecting wires Switch Cells Cycle rubber bands

2 per group 2 per group 4 per group 1 per group 2 per group 2 per group

Things to do Check that the bulbs are working.

Safety Precautions Students should not connect the circuit, or turn the switch to ON or OFF, until the instructor gives the appropriate instruction.

Never connect circuit parts to the plug points present in the classroom.

SESSION Link to known information/previous activity We have seen cells connected in series and parallel. What happens if we connect other circuit component in series and parallel arrangements?

Procedure Divide the class into groups of 4-6 students and distribute the materials to each group. Facilitate the following group activity.

Take two bulbs. Connect one terminal of one bulb to the positive terminal of the cell. Connect the second terminal of the bulb to the first terminal of the other bulb. Connect the second terminal of the second bulb to the negative terminal of the cell. This configuration is shown in Figure 1.

Connect the switch between a bulb and the cell. Consider this to be a Case 1 connection.

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79 Put on the switch and observe the brightness of the bulbs.

Remove the first bulb and observe whether the other bulb glows or not. Reinsert the bulb and observe what happens to the second bulb.

Put off the switch and remove the two bulbs.

Connect both of the first terminals of the bulbs to the positive terminal of the cell. Connect both of the second terminals of the bulbs to the negative terminal of the cell. This configuration is shown in Figure 2. Connect the switch between the cell and bulbs. Consider this to be a Case 2 connection.

Now put on the switch and observe the brightness of the bulbs.

Remove the first bulb and observe whether the other bulb glows or not. Reinsert the bulb and observe what happens to the other bulb.

Fig 18: Bulbs in series and parallel

UNDERSTANDING THE ACTIVITY Discussion Item 1 â&#x20AC;&#x201C;Series connection of bulbs Leading questions 1. When you remove one of the bulbs from the Case 1 connection, what happens to the other bulb?

2. When you insert the bulb again, what happens to the other bulb?

3. What is this connection called?

4. In how many paths does current flow in a series circuit?

5. What is a series connection of the bulbs?

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Discussion and Explanation  In a series connection, when one of the bulbs is removed the other bulb also goes out.

When you insert the bulb again then both bulbs glow. Current flows in only one path because there is no other path.

Discussion Item 2 –Parallel connection of bulbs Leading questions 6. When you remove one of the bulbs from the Case 2 connection, what happens to the other bulb?

7. What is a Case 2 connection called?

8. In how many paths does current flow in a parallel circuit?

9. What is a parallel connection of the bulbs?

Discussion and Explanation  In a parallel connection, you can remove one bulb while the other bulb still glows, because current flows through more than one path in the circuit – current flows separately through the two bulbs.  In our houses we use parallel connections so that we can switch off and on whichever circuits are required.

Discussion Item 3 –The brightness of the bulbs in parallel and series connections Leading questions 10. Compare the brightness of the bulbs connected in series and connected in parallel.

Discussion and Explanation  The brightness of the bulb depends upon on the current flowing in the circuit. And the current flowing in the circuit depends on the resistance in the circuit. If the resistance is greater, the current will be less, and vice versa. Here, bulbs can be considered as resistors.  When the bulbs are connected in series, the total resistance in the circuit will be sum of the resistances of the individual bulbs. The resistance will be greater than with a single bulb, so the current will be less, resulting in less brightness.  When we connect bulbs in parallel, then the reciprocal of the effective resistance is equal to the sum of the reciprocals of the individual resistors. This results in a decrease of resistance compared to a circuit with a single bulb. More current will pass through the main circuit but branch currents are same as in single bulb. The bulbs glow with the same intensity.

Discussion Item 4 –Decorative lights (serial sets) Leading questions 11. Give an everyday life application of series and parallel connections.

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81 Discussion and Explanation  For decorative purposes, we use several sections of bulbs, each section containing a large number of bulbs connected in series.



Chart 12: Serial sets Serial sets contain two to five sections of bulbs connected in parallel, where each section contains at least 50 bulbs connected in series.

Chart 13: Bulbs in Series and parallel

KEY MESSAGES 

In series connections of bulbs, the first end of the first bulb is connected to the positive terminal of the cell, the second end of the first bulb is connected to the first end of the second bulb, and the second end of the second bulb is connected to the negative terminal of the cell.

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82 ď&#x201A;§

In parallel connections of bulbs, all of the first terminals of the bulbs are connected to one wire and all of the second terminals of bulbs are connected to another wire. These two wires are connected to the two ends of the cell.

LEARNING CHECK 1. If you put more light bulbs into a series circuit without changing the battery, what will happen? (Answer: The bulbs will be dimmer.)

2. If a light bulb breaks in a series circuit, what will happen? (Answer: The circuit is broken; all other bulbs will stop working.)

3. If you put more light bulbs into a parallel circuit without changing the battery, what will happen? (Answer: The bulbs will stay bright.)

4. If a light bulb breaks in a parallel circuit, what will happen? (Answer: The other bulbs keep working.)

5. Are the appliances in your kitchen connected in series or in parallel? Why? (Answer: Parallel. Otherwise the dishwasher, toaster, garbage disposal, and overhead light would all have to be turned on for the refrigerator to function.)

TRY IT YOURSELF We have learned that serial sets use several parallel connections of strings of light bulbs in series.

Try to determine another series or parallel connection of batteries or loads that you use regularly.

In a torch light with two or more batteries, are the batteries connected in series or parallel? In a wall clock with multiple batteries, how are they connected? A table fan?

INTERESTING INFORMATION Indicator bulbs â&#x20AC;&#x153;A study of the overall current for parallel connections requires the addition of an indicator bulb.

The indicator bulb is placed outside of the branches and allows one to observe the affect of additional resistors upon the overall current. The bulbs that are placed in the parallel branches only provide an indicator of the current through that particular branch. So if investigating the affect of the number of resistors upon the overall current and resistance, one must make careful observations of the indicator bulb, not the bulbs that are placed in the branches. The diagram below depicts the typical observations.

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Fig 19: Parallel connection of light bulbs It is clear from observing the indicator bulbs in the above diagrams that the addition of more resistors causes the indicator bulb to get brighter. For parallel circuits, as the number of resistors increases, the overall current also increases. This increase in current is consistent with a decrease in overall resistance. Adding more resistors in a separate branch has the unexpected result of decreasing the overall resistance!”21 Why do batteries seem to go dead and then come back to life if you let them rest? 22 “The "self-recharging" features of batteries is most noticeable in a car battery. In some cases you can crank the engine until the battery seems totally dead, then come back an hour later and crank it again. The higher the drain on the battery (a car's starter motor is an incredibly high-drain device!), the greater the effect.

To understand why this happens, it is helpful to understand what's going on inside the battery.

Let's take the simplest zinc/carbon battery as an example. If you take a zinc rod and a carbon rod, connect them together with a wire, and then immerse the two rods in liquid sulfuric acid, you create a battery. Electrons will flow through the wire from the zinc rod to the carbon rod.

Hydrogen gas builds up on the carbon rod, and over a fairly short period of time coats the majority of the carbon rod's surface. The layer of hydrogen gas coating the road blocks the reaction occurring in the cell and the battery begins to look "dead". If you let the battery rest for awhile, the hydrogen gas dissipates and the battery "comes back to life".

In any battery, be it an alkaline battery found in a flashlight or a lead acid battery in a car, the same sort of thing can happen. Reaction products build up around the two poles of the battery and slow down the reaction. By letting the battery rest, you give the reaction products a chance to dissipate. The higher the drain on the battery, the faster the products build up, so batteries under high drain appear to recover more.”

WEB RESOURCES http://www.ee.buffalo.edu/faculty/paololiu/edtech/roaldi/References/sp.htm (a reference on what it means to be connected in series or in parallel) http://www.facstaff.bucknell.edu/mastascu/elessonshtml/Resist/Resist2.html (a reference, written as a lesson plan, with lots of information about series and parallel connections with questions to check your understanding mixed in) Agastya International Foundation. For Internal Circulation only.

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84 http://www.zbattery.com/Connecting-Batteries-in-Series-or-Parallel (Connecting batteries in series or parallel)

VOCABULARY 1) Battery – A source of electricity formed by connecting two or more cells together 2) Solar cell array – A series–parallel combination of solar cells designed to meet the requirements of the voltage and current to be supplied 3) Solar cell – An electric cell comprised of a layer of phosphorous-doped silicon on top of a layer of boron-doped silicon, with an electric field where these two materials are in contact, that takes in sunlight to stimulate electrons, resulting in a flow of current when the solar cell is connected to an electrical load 4) Serial set – An arrangement of light bulbs used for decorative purposes that consists of two to five sections of bulbs connected in parallel with each section containing at least 50 bulbs connected in serie 18http://www.physicsclassroom.com/class/circuits/U9L4b.cfm 19http://www.fsec.ucf.edu/en/consumer/solar_electricity/basics/how_pv_cells_work.htm 20http://www.fsec.ucf.edu/en/consumer/solar_electricity/basics/cells_modules_arrays.htm 21http://www.physicsclassroom.com/class/circuits/u9l4b.cfm 22http://electronics.howstuffworks.com/everyday-tech/question390.htm

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85

ABL 7 Effective resistance in series and parallel connections

Activity

Learning objective

7.1

What is the effective resistance when two resistors are connected in series?

What is the effective resistance when resistors are connected in parallel?



What is power, and how can we calculate the power of electrical devices?



7.2

7.3

Key messages





7.4

What is the electricity tariff? How is an electricity bill compiled?

 

When resistances are connected in series, the effective resistance in the circuit is equal to the sum of the individual resistances.

When resistors are connected in parallel, the reciprocal of the effective resistance is equal to the sum of the reciprocals of the individual resistances.

Power is defined as the rate at which electrical energy is dissipated or consumed in an electric circuit.

The power consumed by a load is the product of the voltage applied to the load by the current flowing though the load.

Power is measured in watt.

An electricity bill consists of a fixed charge related to the connected load capacity (in kW) and an energy consumption charge related to the actual amount of energy used (in kWh).

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Time (min) 15

15

15

20

86

ABL 7.1

Time: 15 min

LEARNING OBJECTIVE – What is the effective resistance when two resistors are connected in series?

Note to Instructor – In this activity students will assemble circuits with two resistors in series, measure the voltage and current using a voltmeter and an ammeter, and then use Ohm’s law to calculate the total resistance in the circuit.

ADVANCE PREPARATION Material List

1 2 3 4 5 6 7

Material Battery Bulb Switch Two resistors of 5 ohms, or one resistor of 5 ohms and one of 2 ohms Voltmeter (0–3 V) Milli-ammeter (0–300 mA) Rheostat

Required Quantity 1 per group 1 per group 1 per group 2 resistors per group 1 per group 1 per group 1 per group

Things to do Try the activity before class to ensure that it is working.

Safety Precautions Students should not connect the circuit, or turn the switch to ON or OFF, until the instructor gives the appropriate instruction.

Never connect circuit parts to the plug points present in the classroom.

SESSION Link to known information/previous activity In ABL 5, we learned that circuit components can be connected in series and in parallel. When cells are connected in series, the voltage in the circuit doubles and the current doubles, so a light bulb in the circuit becomes brighter. When bulbs are connected in series, the current is decreased, so each of them becomes less bright. In this activity, we will connect resistors in series and determine the total resistance in the circuit.

Procedure Facilitate the following group activity with 5 to 6 learners in each group.

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87 Connect the battery, rheostat, bulb, switch, and known resistors R 1 & R2, as shown in the figure.

Connect the voltmeter parallel to the resisters. Connect the ammeter in series with the resisters.

Put on the switch and note down the ammeter and voltmeter readings by adjusting the rheostat.

Record the readings in the observation sheet.

Calculate the value of the effective resistance, R=V/I. Find the sum of the values R 1 and R2.Compare R with R1+R2.

Fig 20: Circuit diagram for ABL 7.1 Voltmeter reading in volts

Ammeter reading in amps

Effective resistance in ohms (R=V/I)

Worksheet 4: Observation sheet for effective resistance- Resistors in parallel

UNDERSTANDING THE ACTIVITY Leading questions  Compare R with R1 + R2.

Discussion and Explanation  R = R1 + R 2

KEY MESSAGES 

When resistances are connected in series, the effective resistance in the circuit is equal to the sum of the individual resistances.

LEARNING CHECK (See Learning Check after 7.2.)

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88

ABL 7.2

Time: 15 min

LEARNING OBJECTIVE â&#x20AC;&#x201C; What is the effective resistance when resistors are connected in parallel?

Note to Instructor â&#x20AC;&#x201C; In this activity, students will measure the effective resistance across two resistors connected in parallel in a circuit, and compare that to the value based on the formula.

ADVANCE PREPARATION Material List

1 2 3 4 5 6 7

Material Bulb Battery Rheostat Switch Ammeter Voltmeter Resistor

Required Quantity 1 per group 1 per group 1 per group 1 per group 1 per group 1 per group 2 per group

Things to do Try the activity before class to ensure that everything is working properly.

Safety Precautions Students should not connect the circuit, or turn the switch to ON or OFF, until the instructor gives the appropriate instruction.

Never connect circuit parts to the plug points present in the classroom.

SESSION Link to known information/previous activity In the previous activity, we connected resistors in series and determined the total resistance. Now we will do the same thing with resistors connected in parallel. Remember from ABL 5 that when two cells are connected in parallel, the current through a bulb doubles so the bulb glows brighter, but the voltage across the bulb does not change. When two bulbs are connected in parallel, the current through the battery increases and the bulbs glow brighter.

Procedure Facilitate the following group activity. Divide the class into groups and distribute the materials.

Connect the bulb, battery, rheostat, switch, and ammeter in series, as shown in the figure. .

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23

Fig 21: Circuit diagram for ABL 7.2 Connect resistors R1and R2 parallel to each other, and connect the voltmeter across both resistors.

Put on the switch and observe the voltmeter and ammeter readings.

Record the values in the table and find the value of the effective resistance.

Verify the value of the effective resistance using the formulađ?&#x2018;&#x2026;đ?&#x2018;&#x2019;đ?&#x2018;&#x201C;đ?&#x2018;&#x201C; (đ?&#x2018;?) = Voltmeter reading in volts

Ammeter reading in amps

Effective resistance in ohms (R=V/I)

đ?&#x2018;&#x2026;1 Ă&#x2014; đ?&#x2018;&#x2026;2 đ?&#x2018;&#x2026;1 +đ?&#x2018;&#x2026;2

.

Effective resistance from theoryđ?&#x2018;&#x2026;đ?&#x2018;&#x2019;đ?&#x2018;&#x201C;đ?&#x2018;&#x201C; (đ?&#x2018;?) = đ?&#x2018;&#x2026;1 Ă&#x2014; đ?&#x2018;&#x2026;2 đ?&#x2018;&#x2026;1 +đ?&#x2018;&#x2026;2

.

Worksheet 5: Observation sheet for effective resistance- Resistors in parallel

UNDERSTANDING THE ACTIVITY Leading questions đ?&#x2018;&#x2026; Ă&#x2014;đ?&#x2018;&#x2026; ď&#x192;&#x2DC; Compare R with đ?&#x2018;&#x2026;1 +đ?&#x2018;&#x2026; 2.

1

2

Discussion and Explanation đ?&#x2018;&#x2026; Ă&#x2014;đ?&#x2018;&#x2026; ď&#x201A;ˇ đ?&#x2018;&#x2026; = đ?&#x2018;&#x2026;1 +đ?&#x2018;&#x2026; 2 1

2

KEY MESSAGES ď&#x201A;§

When resistors are connected in parallel, the reciprocal of the effective resistance is equal to the sum of the reciprocals of the individual resistances.

LEARNING CHECK 1. Three 3- Ί resistors in series would provide a resistance equivalent to one resistor of what resistance?

(Answer: 9 Ί) 2. Three resistors with values of 5 Ί, 6 Ί, and 7 Ί are placed in series. What is the effective resistance?

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90 3. Compare circuits X and Y below. Each is powered by a 12-volt battery. The voltage drop across the 12-ohm resistor in circuit Y is ____ the voltage drop across the single resistor in X. a. smaller than b. larger than c. the same as (Answer: a.)

4. Given three resistors, R1 = 20 Ω, R2 = 30 Ω, and R3 = 30 Ω connected in parallel, what is the effective resistance?

(Answer: Req = 1/{(1/20)+(1/30)+(1/30)} = 1/{(3/60)+(2/60)+(2/60)} = 1/(7/60)=60/7 Ω = approximately 8.57 Ω.)

5. Given three resistors connected in parallel with values R 1 = 8 Ω, R2 = 8 Ω, and R3 = 4 Ω, what is the effective resistance?

(Answer: 1/Req = 1/8 + 1/8 + 1/4 Ω = 1/2 Ω. Thus Req = 2 Ω.)

6. Use the concept of equivalent resistance to determine the unknown resistance of the identified resistor that would make the circuits equivalent.

a.

b.

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c.

(Answers: a. 3 Ω, 11 Ω; b. 4 Ω, 18 Ω; c. 4 Ω, 18 Ω)

7.

24

(Answer: Req = 100 + 300 + 1/ (1/20 + 1/20) + 1/ (1/40 + 1/10) + 10 Ω = 400 + 1/ (1/10) + 1/ (1/8) + 10 Ω = 400 + 10 + 8 + 10 Ω = 428 Ω.)

8. A 12-V battery and a 12-ohm resistor are connected as shown in circuit. A 6-ohm resistor is added to the 12-ohm resistor to create circuit Y as shown. The voltage drop across the 6-ohm resistor in circuit Y is ____ that across the resistor in X. a. larger than b. smaller than c. the same as (Answer: c.)

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ABL 7.3

Time: 15 min

LEARNING OBJECTIVE â&#x20AC;&#x201C; What is power, and how can we calculate the power of electrical devices?

Note to Instructor â&#x20AC;&#x201C; Students will assemble a simple circuit with one battery cell and one bulb, and calculate its power. They will do the same thing for a circuit with two bulbs in parallel and a circuit with two cells in series.

ADVANCE PREPARATION Material List

1 2 3 4 5 6 7

Material Battery (1.5 V) Bulb Conducting wire Switch Ammeter Voltmeter Rheostat

Required Quantity 2 cells per group 2 per group 1 set per group 1 per group 1 per group 1 per group 1 per group

Things to do Not Applicable Safety Precautions Students should not connect the circuit, or turn the switch to ON or OFF, until the instructor gives the appropriate instruction.

Never connect circuit parts to the plug points present in the classroom.

SESSION Link to known information/previous activity In our prior investigation of electricity, we have studied voltage, current, and resistance. Another word that is sometimes used to describe electric output is power; what is power and how do we calculate it?

Procedure Facilitate the following group activity. Divide the students into groups of 4-5 and distribute the materials to each group.

Connect the battery, bulb, switch, ammeter, and rheostat as shown in the figure. Connect the voltmeter in parallel with the bulb. Close the switch.

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Fig 22: Circuit diagram for ABL 7.3 Note down the values of voltage (voltmeter reading) and current (ammeter reading) in the first row of the observation sheet. Calculate the power of the bulb.

Open the switch and connect two bulbs in parallel with one cell. Close the switch and repeat the above experiment, recording the readings and calculating the power.

Open the switch and connect two cells in series with one bulb. Close the switch and repeat the experiment.

Experiment Number of cells No. and bulbs connected 1 2 3

Voltmeter reading (V) in volts

Ammeter reading (I) in amperes

Power consumed by bulbs (P = V Ă&#x2014; I) in watts

1 cell and 1 bulb in series 1 cell and 2 bulbs in parallel 2 cells and 1 bulb in series Worksheet 6: Observation sheet for calculation of power

UNDERSTANDING THE ACTIVITY Leading questions 1. What is the voltmeter reading in volts for each experiment?

2. What is the ammeter reading in amps for each experiment?

3. What is the power used (or consumed) by the bulb?

4. Define power.

5. What are the units of power?

Discussion and Explanation ď&#x201A;ˇ Power is the amount of energy generated by a source or used by a load every second. Thus, electric power (P) is defined as the rate at which electrical energy is dissipated or consumed in an electric circuit.

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94     

Power is represented by P = V×I, where V is the voltage in volts, I is the current in amps, and P is the power in watts. Substituting this into V = IR, we get P = I 2R or P = V2/R. As power measures the rate at which energy is produced or consumed, we can also represent power by P = E/t, where E is the electrical energy and t is time during which energy is generated or spent.

Electrical energy is measured in joules (J). If 1 ampere of current is drawn while a potential difference of 1 volt is applied across a circuit, then 1 watt of power is consumed. If this current flows for 1 second, then the circuit has consumed 1 joule of electrical energy.

1 watt (W) = 1 volt-ampere (VA), and 1 watt = 1 joule/second (J/s). Alternatively, 1 joule (J) = 1 watt-sec (Ws).

One watt of power is very small; therefore, in actual practice we use a much larger unit called a kilowatt (kW). It is equal to 1000 watts. Similarly, the amount of energy consumed in one second is very small. In practice we use the amount of energy consumed in one hour, the watt-hour (Wh). Thus, the commercial unit of electrical energy is the kilowatt-hour (kWh). It is commonly known as the “electrical unit” or “unit”. 1 kWh = 1000 watt × 3600 second = 3600000 J.

KEY MESSAGES   

Power is defined as the rate at which electrical energy is dissipated or consumed in an electric circuit.

The power consumed by a load is the product of the voltage applied to the load by the current flowing though the load.

Power is measured in Watt.

LEARNING CHECK25, 26 1. A lamp with a resistance of 3 Ω is connected in a circuit with a battery of 18 V. What is the power in this circuit?

2. What would happen to the power in the last question if we doubled the battery voltage while keeping the resistance constant?

3. A tired squirrel (mass of approximately 1 kg) does push-ups by applying a force to elevate its center-of-mass by 5 cm in order to do a mere 0.50 Joule of work. If the tired squirrel does all this work in 2 seconds, then determine its power.

4. Your household's monthly electric bill is often expressed in kilowatt-hours. One kilowatthour is the amount of energy delivered by the flow of l kilowatt of electricity for one hour.

Use conversion factors to show how many joules of energy you get when you buy 1 kilowatt-hour of electricity.

(Answers: (1) I = 18/3 = 6 A; power = 6*18 = 108 W; (2) power would quadruple; (3) 0.5 J/2 s = 0.25 W; (4) 1 kWh = 1000 W x 3600 s = 3.6 x 106 J)

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ABL 7.4

Time: 20 min

LEARNING OBJECTIVE â&#x20AC;&#x201C; What is the electricity tariff? How is an electricity bill compiled?

Note to Instructor â&#x20AC;&#x201C; Students will read an electricity bill and relate the amount of energy consumed to its cost.

ADVANCE PREPARATION Material List

1

Material Electricity bill copy

Required Quantity 1 per student

Things to do Print copies of the electricity bill to give to students.

Safety Precautions Not Applicable

SESSION Link to known information/previous activity In the last activity we investigated electric power. Power is a concept that is used for commercial electricity applications. In this activity we will determine how power and electrical energy come into play in an electricity bill.

Procedure Divide the students into groups. Distribute one copy of the bill to every student and facilitate the following individual activity.

Go through the bill thoroughly. Look at every component of the bill.

Find the cost for one unit. Find the total number of units consumed as per the bill. Find the total cost of the consumed units.

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Worksheet 7: Electricity Bill

UNDERSTANDING THE ACTIVITY Discussion and Explanation  When a new building is constructed, an application is made to the local electricity office requesting them to provide an electric connection from the nearest street pole. The connection is provided by the electricity authority for an approved value of the peak power (peak load) to be drawn. Usually it is between 1 to 10 kW, depending on how big the building is and how many points are provided to connect electrical appliances. This is called the connected load, in kW.  The electricity bill has a fixed charge per kW for this connected load. This amount has to be paid, whether we switch on the loads and use the electricity or not.  In addition, the electricity bill also mentions the actual amount of electrical energy used last month, as the difference in meter reading between the last month and the current month. This is energy in Units or kWh.  Electrical energy used is charged at Rs. 2.30 per unit for the first 30 units every month and at Rs. 3.50 per unit for the next 70 units per month. Higher rates are charged in higher usage slabs. These rates may be changed from time to time.  The total electricity bill to be paid every month is the sum of the fixed charges for the power connection and the actual energy usage, plus some taxes.

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98 Note to the instructor: I (KGN) have noticed a mistake in BESCOM bills (English) in Bangalore. In the column on Energy Usage charges, it is printed as kW instead of kWh. What the energy meter reads is kilowatt hours (Units) and not kilowatts (kW). Please examine the bills and point out this mistake to learners, if necessary.

KEY MESSAGES 

An electricity bill consists of a fixed charge related to the connected load capacity (in kW) and an energy consumption charge related to the actual amount of energy used (in kWh).

LEARNING CHECK Calculate the electrical energy consumed by the various electrical appliances given in the observation sheet and calculate the charges to be paid.

Wattage 60 watt bulb 200 watt fan 2000 watt refrigerator

1500 watt grinder 1000 watt water heater 150 watt television 5 watt computer

Hours used daily 10

Number of days 30

8 2 (It switches on and off; the total ON time is 2 hours.)

0.5

30 25

1.0

25

3

30

10

30

Consumed electrical energy units (kWh) 60×10×30 = 18 kWh

Charge, at 2 rupees per unit 18×2 =36 Rs.

20

TRY IT YOURSELF If your parents wouldn’t mind, ask them if you can see the electricity bill for your house next month. Find the cost of one unit at your house, and see how many units your family used in the month. Look at the breakdown of which appliances used the most energy and try to find a way to reduce your household’s energy consumption!

If your parents are not comfortable showing you the electricity bill, ask a teacher if you can see the breakdown of electricity use for your school.

INTERESTING INFORMATION

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99 â&#x20AC;&#x153;Adding more resistors in series means that there is more overall resistance; yet adding more resistors in parallel means that there is less overall resistance. The fact that one can add more resistors in parallel and produce less resistance is quite bothersome to many. An analogy may help to clarify the reason behind this initially bothersome truth.

The flow of charge through the wires of a circuit can be compared to the flow of cars along a toll way system in a very crowded metropolitan area. The main sources of resistance on a toll way system are the tollbooths. Stopping cars and forcing them to pay a toll at a tollbooth not only slows the cars down, but in a highly trafficked area, will also cause a bottleneck with a backup for miles. The rate at which cars flow past a point on that toll way system is reduced significantly by the presence of a tollbooth. Clearly, tollbooths are the main resistor to car flow.

Now suppose that in an effort to increase the flow rate the Toll way Authority decides to add two more tollbooths at a particular toll station where the bottleneck is troublesome to travelers. They consider two possible means of connecting their tollbooths - in series versus in parallel. If adding the tollbooths (i.e., resistors) in series, they would add them in a manner that every car flowing along the highway would have to stop at each tollbooth in consecutive fashion. With only one pathway through the tollbooths, each car would have to stop and pay a toll at each booth. Instead of paying 60 cents one time at one booth, they would now have to pay 20 cents three times at each of the three tollbooths. Quite obviously, adding tollbooths in series would have the overall effect of increasing the total amount of resistance and decreasing the overall car flow rate (i.e., current).

The other means of adding the two additional tollbooths at this particular toll station would be to add the tollbooths in parallel fashion. Each tollbooth could be placed in a separate branch. Cars flowing along the toll way would stop at only one of the three booths. There would be three possible pathways for cars to flow through the toll station and each car would choose only one of the pathways. Quite obviously, adding tollbooths in parallel would have the overall affect of decreasing the total amount of resistance and increasing the overall car flow rate (i.e., current) along the toll way. Just as is the case for adding more electrical resistors in parallel, adding more tollbooths in parallel branches creates less overall resistance. By allowing for more pathways (i.e., branches) by which charge and cars can flow through the bottleneck areas, the flow rate can be increased.â&#x20AC;?27

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28

WEB RESOURCES http://physics.bu.edu/py106/notes/Circuits.html (resistors in series and parallel) http://www.physicsclassroom.com/class/energy/u5l1e.cfm (more information about how to calculate power) http://science.howstuffworks.com/electricity7.htm (practical information about electricity usage)

VOCABULARY 1) Effective resistance â&#x20AC;&#x201C; The resistance across an entire circuit, measured from one terminal of the battery to the other 2) Power â&#x20AC;&#x201C; The rate at which electrical energy is dissipated or consumed in an electric circ 23http://www.uq.edu.au/_School_Science_Lessons/32.2.54.GIF

24http://www.wikihow.com/Image:Calculate-Series-and-Parallel-Resistance-Step-3.jpg 25http://www.allaboutcircuits.com/vol_1/chpt_2/4.html

26http://www.physicsclassroom.com/class/energy/u5l1e.cfm

27http://www.physicsclassroom.com/class/circuits/u9l4b.cfm 28

Ibid.

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