Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Module 3.1.2

Sensing 3.1.2.A Fundamental concepts; 3.1.2.B Ohmâ&#x20AC;&#x2122;s Law & I-V Characteristics

Topic Notes Name:__________ -1-

Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Important resources for this module: All prezi presentations, booklets, homeworks and practical sheets are all available on the departmental website: http://www.aquinasphysics.com/312sensing1.html

https://www.alevelphysicsonline.com/electricity Excellent video tutorials made by an A level physics teacher for A level physics students. If you need to go over any concepts again, this is the first place that you should look. Free access to the course textbook (via the departmental website). Follow the instructions on the website for how to log in.

http://www.aquinasphysics.com/kerboodle.html Challenging questions from GCSE level to Undergraduate physics problems. If you are hoping for a B, A or A* you must be visiting this site and regularly practicing the problems. They also run excellent workshops. Look out for these!!

https://isaacphysics.org/

Multiple-choice practice revision questions on your phone. Revise on the bus on the way in to college!!

http://www.gojimo.com/ -2-

Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Key terms, definitions and equations ................................................................................................................... - 5 Charge,Q ................................................................................................................................................................... - 6 Current, I ................................................................................................................................................................... - 7 Measuring current ................................................................................................................................................ - 8 Conventional current vs. electron flow................................................................................................................. - 8 Stretch & challenge: electron number density, n and drift velocity v .................................................................. - 9 Potential difference, V ............................................................................................................................................ - 10 Measuring potential difference .......................................................................................................................... - 11 Resistance, R & Conductance, G ............................................................................................................................. - 12 Power in electrical circuits ...................................................................................................................................... - 14 Power equations ................................................................................................................................................. - 14 Power of everyday electrical appliances............................................................................................................. - 16 Electrical equation wheel .................................................................................................................................... - 16 Space for your own notes ................................................................................................................................... - 17 -

2.

Circuit symbols & circuit diagrams...................................................................................................................... - 18 -

3.

Ohmâ&#x20AC;&#x2122;s law & I-V characteristics .......................................................................................................................... - 19 Finding I-V characteristics experimentally .............................................................................................................. - 20 Fixed resistors & Ohmâ&#x20AC;&#x2122;s Law ................................................................................................................................... - 20 Filaments ................................................................................................................................................................. - 22 Diodes ..................................................................................................................................................................... - 24 Space for your own notes ................................................................................................................................... - 25 -

4.

A (tough) exam question..................................................................................................................................... - 26 -

5.

Isaac Physics Mastery Questions ........................................................................................................................ - 28 Questions on Charge Carriers (I) ............................................................................................................................. - 28 -

Space for your own notes ........................................................................................................................................... - 30 -

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Learning Objectives 3.1.2 (a) Describe and explain (i) current as the flow of charged particles (ii) potential difference as the energy per unit charge (iii) resistance & conductance (including series and parallel combinations) (v) dissipation of power in electrical circuits (vi) the relationship between potential difference and current in ohmic resistors (Ohmâ&#x20AC;&#x2122;s Law) (b) Make appropriate use of: (i) the terms: potential difference, current, charge, resistance, conductance. (ii) recognise standard circuit symbols by sketching, plotting from data and interpreting: (iii)

graphs of current against potential difference and graphs of resistance or conductance against temperature for ohmic and non-ohmic devices or components.

(c) Make calculations and estimates involving: 2 (i) đ?&#x2018;&#x2026; = đ?&#x2018;&#x2030;â &#x201E;đ??ź ; đ??ş = đ??źâ &#x201E;đ?&#x2018;&#x2030; ; đ?&#x2018;&#x2030; = đ?&#x2018;&#x160;â &#x201E;đ?&#x2018;&#x201E; = đ?&#x2018;&#x192;â &#x201E;đ??ź ; đ?&#x2018;&#x192; = đ??źđ?&#x2018;&#x2030; = đ??ź 2 đ?&#x2018;&#x2026; = đ?&#x2018;&#x2030; â &#x201E;đ?&#x2018;&#x2026; ; đ?&#x2018;&#x160; = đ??ź đ?&#x2018;&#x2030; đ?&#x2018;Ą

(ii) đ??ź = â&#x2C6;&#x2020;đ?&#x2018;&#x201E;â &#x201E;â&#x2C6;&#x2020;đ?&#x2018;Ą (d) Demonstrate and apply knowledge and understanding of the following practical activities (i)

investigating electrical characteristics for a range of ohmic and non-ohmic devices using voltmeters and ammeters.

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

3.1.2 Electricity – Basic terms & ideas, component properties These notes coordinate with the Prezi 3.1.2.A & 3.1.2.B on the departmental website & pages 42-47(ish) in the course textbook. Prezi 3.2.1.A Introduction to electricity

Prezi 3.2.1.B Ohm’s Law & I-V characteristics

https://goo.gl/4jVVbm

https://goo.gl/4Vjsgj

Electricity is traditionally one of the toughest topics for students to fully understand when studying for their A levels. Perhaps this is because it is very difficult to visualise what is going on inside a circuit when you press a switch and something magical happens – a heater turns on, the lights turn on and off, or your mobile phone sends a message. In these booklets we will aim to build a model to help you understand how circuits work, and in doing so help you become more able to solve questions on electricity that commonly come up in exams.

1. Key terms, definitions and equations As with any topic, but perhaps even more so, a good grasp of the fundamental terms and ideas behind the concepts being taught is vital to ensure a thorough understanding. In this first section, we will explore a number of ideas relating to electrical circuits. You may have covered some before at school, but you now must aim to get a more thorough understanding and holistic overview of how all these concepts map together. The first two really important concepts to get a grasp on are as follows: (1) electricity is all about transfers of energy  Energy is transferred into the circuit in components such as cells, batteries, photovoltaic cells (solar panels), dynamos etc.  Energy is transferred out of the circuit at components such as resistors, bulbs, motors, LEDs etc. (2) charge carriers – electrons and ions – are the vehicles by which this energy is transferred from one part of the circuit to another.

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Charge,Q A good explanation of the concept of charge from www.alevelphysicsonline.com can be found at the link below (and QR code left) https://goo.gl/LxiMDf

Charge in circuits is for the vast majority of the time carried by electrons, although it may also be carried by charged ions in cases where circuits are not confined to wires (we will look at some examples of current flow through fluids next year). For a simple circuit, answer the questions below: (1) what is the unit for charge, Q?

(2) what is the charge on a single electron, in terms of the units you have defined above?

(3) How many electrons make up one unitâ&#x20AC;&#x2122;s worth of charge?

(4) How much charge is carried by 2.5Ă&#x2014;1019 electrons?

A really important equation (which you are not given in the formula booklet) that links the total charge present Q to the number of electrons n and charge on one electron q is (complete out the box to the right).

Q=

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Current, I A good explanation of the concept of electric current from www.alevelphysicsonline.com can be found at the link below (and QR code left)

https://goo.gl/eHVr3r

Current, I, is another fundamental concept in electricity. In the space below, give a definition of the term current, including an equation and the units.

Current is defined as…

I= It has units of…

In terms of the charge Q passing a point in a time t

To practice your basic understanding of current, answer the questions below. Show all working throughout. (1) Complete the following sentence to define the Coulomb: “ One Coulomb is the total charge passing a point when a current of …………. flows for a time of ………….” (2) How much current flows if 28 C of charge is delivered in 7 minutes?

I = ……………… (3) Calculate the charge passed through a torch bulb in 5 minutes when the torch bulb caries a steady current of 0.3 A.

Q = ……………. (4) How much current flows if 8.5×1017 electrons passes a point in 6 seconds?

I = …………….. (5) Calculate the number of electrons hitting the screen of an old TV tube each second (flat screens never used to exist, believe it or not!!) when the beam current is 1.0 mA.

n = ……………….

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Measuring current We measure the current flowing through a circuit using a component known as an ammeter. Answer the following questions on ammeters. (1) Do ammeters need to be put in a circuit in series or in parallel with the component that it is being used to measure the current through? Explain why. In series

In parallel

(delete as appropriate)

Explanation:

(2) What is the ideal resistance of an ammeter? Explain why this is ideal.

Conventional current vs. electron flow One of the trickier issues that you may have to deal with is an acceptance of the fact that conventional current â&#x20AC;&#x201C; the idea that electrical current flows from positive to negative â&#x20AC;&#x201C; is opposite to the direction of electron flow around a circuit (as negatively charged particles, electrons are repelled from the negative terminal of a power supply and attracted to the positive. You do no really need to worry about this until we look at electric fields in the second year. You will, however, be expected to describe the path taken by an electron within a wire. Illustrate a possible path taken by annotating the diagram below, showing a wire and some of the metal ions that make it up.

electron

metal ions

wire

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Stretch & challenge: electron number density, n and drift velocity v Some materials have more conduction electrons â&#x20AC;&#x201C; that is electrons that are not tied to a particular atom but which are free to move throughout the structure of the material â&#x20AC;&#x201C; than others. Metals, for example, have more free conduction electrons than conductors such as polymers, and hence are better conductors of electricity. The property of a material that describes the number and availability of electrons to carry a current is the electron number density n.

An excellent video on electron number density n, mean drift velocity v and current I from www.alevelphysicsonline.com is available at https://goo.gl/XLH7dz

In the space below, watch the video linked to above and take notes (this is an important concept to appreciate, but you will not be asked questions on it in an exam.

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Potential difference, V Two short videos giving excellent explanation of the concept of potential difference and its unit from www.alevelphysicsonline.com can be found at the links below. https://goo.gl/RbKNSU

and

https://goo.gl/3KYFxS

The potential difference V is also a fundamental concept in understanding any electric circuit. This is possibly the most forgotten definition in this unit, but also probably the most useful – MAKE SURE YOU REMEMBER IT!!

Potential difference V is defined as…

V=

In terms of the energy transferred W and the charge passing a point Q

It has units of…

To practice your basic understanding of potential difference (and electrical current), answer the questions below. Show all working throughout. (1) From the definition for potential difference given above, suggest an alternative unit to the volt. 1 Volt is equivalent to … (2) What is the potential difference when… (a) 120 J of energy is transferred when 15 C of charge passes through a component?

V= ……………… (b) 60 J of energy is transferred when a charge of 5 C passes through a component?

V= ……………… (3) How much energy is transferred when… (a) A current of 5 A flows for 20 mins through a 120 V heater?

W= ……………… (b) A current of 3 A flows for 3 hours across a component with a p.d. of 4.0 V?

W= ……………… - 10 -

Aquinas College Physics

Module 3.1.2: Sensing (electricity)

(c) A current of 10 mA flows for 20 seconds across a component with a p.d. of 6 mV?

W= ……………… (4) A 12 V car battery supplies a current of 5 A for 10 mins to a headlamp. How much energy is transferred in total?

W = ……………… (5) What is the p.d. across a 60 W bulb, when a current of 5 A passes though it?

V = ………………

Measuring potential difference We measure the p.d. across a component or section of an electric circuit using a component known as a voltmeter. Answer the following questions on voltmeters. (1) Do voltmeters need to be put in a circuit in series or in parallel with the component that we are measuring the p.d. across? Explain why. In series

In parallel

(delete as appropriate)

Explanation:

(2) What is the ideal resistance of a voltmeter? Explain why this is ideal.

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Resistance, R & Conductance, G An explanation of the concept of resistance (including a debunking of a very common misconception from GCSE) & conductance from www.alevelphysicsonline.com can be found at the links below (and QR codes) https://goo.gl/FttAv6 and https://goo.gl/APHKcw

The resistance R of a component or part of a circuit is again another key idea to understand. Complete the section below.

Resistance R is defined as…

R= In terms of the current I through a component and p.d. V across it.

It has units of…

As well as considering a component’s resistance, we could also think about how easily it allows charge carriers to pass through it. Rather than considering the resistance, we may choose to consider the component’s conductance G. This concept become particularly useful when we consider circuit laws – specifically parallel circuits (more on this later). Conductance G is defined as…

G=

=

In terms of (1) the resistance R, and (2) the current I through a component and p.d. V across it.

It has units of…

To practice your basic understanding of resistance & conductance, answer the questions below. Show all working throughout.

(1) From the definition for resistance given above, suggest two possible alternative units to the ohm (). 1 Ohm is equivalent to … 1 Ohm is also equivalent to … (2) From the definition for conductance, suggest an alternative unit to the Siemen (S). 1 Siemen is equivalent to … 1 Siemen is also equivalent to …

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

(3) The current through a lamp is 2.0 A and the p.d. across it is 6 V. Calculate both the resistance and conductance.

R= ……………… G= ……………… (4) A 12 V battery passes a current of 0.3 A through a component. What is the resistance and conductance?

R= ……………… G= ……………… (5) What is the potential difference when a current of 500 mA passes through a component of resistance 24 ?

V= ……………… (6) How much current passes through a component of 15 k resistance with a potential difference of 240 V across it?

V= ………………

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Power in electrical circuits More details on this can be found on the video at https://goo.gl/ic7Kai (see QR code right) In many questions you will be asked to calculate the power P dissipated at an electrical component. Define what is meant when we talk about the power dissipated at a resistor or electrical component: Power P dissipated isâ&#x20AC;Ś

What are the units for electrical power?

Power equations There are three key equations that allow us to work out the power dissipated across an electrical component. In the box right, state an equation giving the power dissipated in terms of the current I passing through the resistor and the p.d. V across the resistor.

P= In terms of the current I through a component and p.d. V across it.

Answer the question below. If the potential difference V across a fixed resistor is doubled: (1) What happens to the current I flowing through it?

(2) What happens to the power P dissipated in the resistor?

(3) Explain how the equation above is derived from previous equations relating to the definition of potential difference V and current I (see notes earlier in the booklet and the video linked to at the top of the page.

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Because of your answer to question (2) above, it is often useful not to think of the power in terms of two co-varying quantities, but instead to consider how the power of a component varies in terms of the componentâ&#x20AC;&#x2122;s resistance R and either the current I flowing through it or the p.d. V across it. State these equations in the box below.

P=

P=

In terms of the resistance R and current I

In terms of the resistance R and p.d. V

Answer the questions below on electrical power. (1) A charge of 10 C passes through a conductor in 2 s. The potential difference across the two ends of the conductor is 12 V. Calculate the power dissipated in the conductor.

(2) A resistor is labelled 10 kď &#x2014; 0.60 W. The power is the maximum safe power output. What is the maximum current it can take without overheating?

(3) 3 V; 0.5 A is written on the packet of torch bulbs. The lifetime of a torch bulb is about 10 hours. Calculate how much energy a torch bulb will have dissipated before it becomes unusable.

In question (3) above, a single equation can be used to find the energy W transferred by a component runs with a p.d. V and current I, for a time t. State the equation for energy transferred by the component in the box left.

W= In terms of the current I, p.d. V , and time t

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Power of everyday electrical appliances A useful piece of general knowledge that may come up in exam questions is an appreciation of the approximate power of a number of different common electrical items. Do a bit of internet research completing the table below. Appliance

Typical Power

Typical Voltage

Typical Current

Fuse you should use (circle appropriate option))

Kettle

230 V

3A

5A

13A

Microwave Oven

230 V

3A

5A

13A

Electric Grill

230 V

3A

5A

13A

Hoover

230 V

3A

5A

13A

Laptop computer

230 V

3A

5A

13A

Hand-held flashlight

3V

------------------------

Electrical equation wheel Over the last few pages we have derived a number of different equations relating, amongst other quantities, current I, potential difference V, power P and resistance R. Practise your recall of these equations, and your skills at algebraic manipulation, by completing the wheel below. Here, the idea is to state each of these quantities in terms of just two of the other three mentioned above. Two examples have been given for you.

In terms of V & P In terms of V & R

In terms of I&R

In terms of P & R

In terms of V&R In terms of I & R In terms of R & P In terms of P & I

In terms of V & P

In terms of I & P

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Space for your own notes …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

2. Circuit symbols & circuit diagrams You will expected to draw circuits using the correct circuit symbols. You need to memorise each of the circuit symbols below. State which components each circuit symbol represents in the box.

A video discussing the most important circuit symbols can be found at www.alevelphysicsonline.com and is available at https://goo.gl/wM5FUv

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

3. Ohm’s law & I-V characteristics Explanation of Ohm’s Law and I-V characteristics from www.alevelphysicsonline.com can be found at the links below (and QR codes) https://goo.gl/6KSdc1

and

https://goo.gl/8huduY

One useful method of considering how different electrical components behave is by investigating how the current I passing through them varies as we change the potential difference V across the component. These graphs show something known as the component’s I-V characteristics. There are three main components that you will be expected to recall and understand the I-V characteristics of:   

Fixed (or Ohmic) Resistors Filaments (such as those used in old-fashioned light bulbs) Diodes (one example being LEDs)

Sketch the I-V characteristics of each of these components on the axes below.

We will now look at how we would find the I-V characteristics for each different components experimentally. We will then consider these three main components one at a time and consider what information we can infer from the graphs about the component’s behaviour.

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Finding I-V characteristics experimentally In the space below, give an explanation of the experiment you would conduct in order to experimentally determine the I-V characteristics of different components. Include a clear circuit diagram and a detailed description of the data you would collect, including the instruments you would use to take the measurements. Explain how you would minimise and account for any uncertainties when taking your readings.

Fixed resistors & Ohmâ&#x20AC;&#x2122;s Law The first component we will consider are fixed resistors. These are also known as Ohmic resistors, as they follow Ohmâ&#x20AC;&#x2122;s Law. For the graph of the fixed resistor that you have sketched on the previous page, describe the relationship between the current I and potential difference V:

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

In the space below, give a definition of Ohm’s Law. In an exam, this would be a two-mark question, so what are the two marks for? Ohm’s law states that…

State Ohm’s Law as an equation in this box.

1st mark is for…

2nd mark is for…

The graph shows the I-V characteristics of an ohmic resistor. Answer the questions below:

(1) Calculate the resistance of the ohmic resistor.

Resistance = ……………  (2) On the graph sketch the I-V characteristics of a component with twice this resistance.

(3) Plotted below are two other sets of axes. Complete these plots to show how: (a) The resistance R of an ohmic component varies with the current I flowing through it (b) The conductance G of an ohmic resistor varies with the potential difference V across it.

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Filaments Filaments lamps (obviously) increase in temperature as you pass a current through them. An excellent explanation of why that is can be found on the video at https://goo.gl/LUBSJG from www.alevelphysicsonline.com. See the QR code right. Answer the questions below on the effect this has on the resistance of a conductor. Use bullet points for structure.

(1) Explain why conductors (such as filaments) increase in temperature as the current through them increases.

(2) Explain what happens to the resistance R of a conductor as the current through it increases.

An understanding and ability to clearly explain how and why the resistance/conductance across a conductor varies with p.d./ current is vitally important. Make sure you commit the explanations you have given above to memory ready to pull out if ever asked about it in an exam.

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

It is also important to be able quantitatively explain how this concept relates to the I-V characteristics of different components. Answer the questions below. (1) Explain how the I-V graph of a filament bulb shows that this is happening to the resistance.

(2) For the filament shown right, explain how you would find the resistance of the filament at a p.d. of +3.0 V. You may want to annotate the diagram.

(3) What is the resistance of the filament lamp at: a. A p.d. of 3.0 V

R = ………………..  b. A p.d. of 6.0 V

R = ………………..  c. A p.d. of -4.0 V

R = ……………….. 

(4) On the axes below, sketch how the (a) resistance; and (b) conductance vary with temperature for a filament lamp.

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Diodes Answer the questions below on the behaviour of diodes:

(1) On the axes right, mark in the typical I-V characteristics for diodes.

(2) With reference to the graph, explain the behaviour of the diode in terms of how its resistance changes at different values of p.d.

(3) Explain what is meant by the term forward bias when discussing the behaviour of diodes

(4) Explain what is meant by the term striking voltage when discussing diodes

(5) Research and discuss a number of common uses of diodes in electrical circuits. Why are they useful?

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

Space for your own notes …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

4. A (tough) exam question Attempt the following question on many of the concepts covered in this booklet. The figure right shows two I-V graphs A and B, for a diode and a resistor of fixed resistance. (a) (i) State which of the two graphs relate to the diode. Explain your decision.

(1) (ii) Calculate the resistance R of the fixed resistor

R = ………… 

(1)

(b) (i) Suggest why the diode should not be connected directly across a 1.5 V supply in its conducting direction.

(1) (ii) The diode and resistor are connected in series across a 1.5 V supply. The diode is connected in its conducting direction. Use the graph to predict the current I drawn from the supply. Make your method clear. (Hint: in a series circuit, the components draw the same current but share the p.d.).

I = ……………….. mA (2)

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Aquinas College Physics

Module 3.1.2: Sensing (electricity)

(c) A sensitive ammeter has a full scale deflection of 100 A and a resistance of 1000 . The meter remains undamaged for voltages across it of up to 0.6 V. (i) Explain what is meant by the term full scale deflection

(1) (ii) To protect the ammeter against accidental misconnection, two diodes are connected in parallel across the meter as shown in the figure below. The diodes have the same I-V graph as the diode shown in the graph earlier.

Suggest how the diodes protect the meter from accidental current overload, and why they have little effect during normal operation. You may (will) wish to make numerical calculations with your reasoning.

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5. Isaac Physics Mastery Questions These questions have their background in the ethos that anything is hard when you don’t spend much time on it, and that things become much easier the more your practice. You will know this if you are learning to drive, or when you pick up any other new skill for the first time. I would say that playing the piano is really really hard. But I’ve probably not spent more than a few hours when I was very young trying. Someone who has spent hours practising playing the piano would probably tell you that it is easy as it comes naturally to them – neglecting to mention all the hours of practice that they have put in.

PHYSICS IS NO DIFFERENT!! THE MORE YOU PRACTICE THE BETTER YOU GET.

Have a go at the mastery questions on the following pages. You can input your answers and check they are right on the isaacphysics.org website (see separate links for each section). You should be aiming to get correct at least the number given at the top of each section before you can consider yourself to have mastered each of these core concepts.

Questions on Charge Carriers (I) *Starts easy, and gets harder – aim for 8/10 at least to gain a mastery of these concepts. These questions come from the Isaac Physics skills mastery book (buy this and the CGP revision guide through ParentPay for just £10!!). You can enter your answers and complete these questions with the IsaacPhysics board at https://isaacphysics.org/s/nJLiZl. Data you will need for these questions: Magnitude of the charge on an electron = 1.60 × 10-19 C Electron number density of copper [Cu] = 1029 m-3 Electron number density of germanium [Ge] = 1020 m-3 The current through a conductor can be found by the equation I = nAve where I = current (A), n = electron number density (m-3), A = cross-sectional area of the wire (m2), v = mean drift velocity of the electrons (m s-1) and e is electron charge (C). (1) How many electrons are needed to carry a charge of -6.00 C?

(2) How many electrons flow past a point each second in a 5.0 mA electron beam?

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Module 3.1.2: Sensing (electricity)

(3) Alpha particles have twice the charge of an electron. What is the current caused by a radioactive source which emits 3000 alpha particles per second?

(4) An electron gun emits 3.0 × 1021 electrons in two minutes. What is the beam current?

(5) Assume all wires have a circular cross section. Complete the table below filling in the gaps where the dotted lines fall. Careful with your units. Diameter / mm

Cross-sectional area / mm2

Material

Current / A

Drift velocity / ms-1

2.5

Copper

13

………………..

0.75

Copper

6.0

……………….

Copper

…………………

0.005

Copper

2.0

0.20

Germanium

2.0

0.20

1.0 ……………… …………………

(6) In an experiment, a current of 3.5 A is being passed through a copper sulphate solution in a 10 cm cubical container, with the electrical terminals being opposite faces. This contains equal numbers of Cu2+ and SO42ions which have respectively +2 and -2 electron charge units. Assuming that the two ions have equal speed in the solution, and that there are 6.0 × 1026 of each per cubic metre of solution, work out their mean speed.

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Module 3.1.2: Sensing (electricity)

Space for your own notes …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………………………….. ……………………………………………………………………………………………………………………………………………………………………………………..

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3.1.2.A&B - Basic concepts; I-V characteristics
3.1.2.A&B - Basic concepts; I-V characteristics