AQA

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PHYSICS HIGHER7 Electricity: The domestic electricity supply

Learning objectives

• To apply the equations relating energy and power to electric circuits• To recall how a domestic plug is wired • To understand the difference between ac and dc supplies• To understand the structure of the National Grid, including the use of

transformers to reduce energy loss

Starter activity

• What’s in a plug?; 5 minutes; page 54Show the student the inside of a plug and explain the purpose of each of the wires. Discuss the hazards when using electricity from a mains supply, emphasising the danger posed by the live wire, and explain how the earth wire helps to keep an electrical device safe. The idea that a potential of zero is safe is usually obvious, so you might work on that to reinforce the idea of potential differences. You could discuss why the power switch on a mains device should be in the neutral wire and not the live wire, and why the metal casing of an appliance such as a washing machine is connected to earth.

Main activities

• Electrical power; 20 minutes; page 55The concepts underlying this activity should be familiar to the student from previous lessons on energy, power and potential difference. The idea that the potential difference pushes the charge around the circuit can be linked to work done as force × distance. The movement of charge against a resistance changes the energy store. Introduce the equations to be used: E = Pt, E = QV, P = VI = I 2R.

• The National Grid; 20 minutes; page 56Show the student an image of a sinusoidal oscilloscope trace. Ensure they are confident that a direct current (dc) supply is constant, whereas an alternating current (ac) supply varies with time in the same way as the oscilloscope trace does. The UK mains supply is alternating with an average value of 230 V and a frequency of 50 Hz. It looks like a sine wave with a peak of 325 V and a trough of –325 V, and it completes 50 full up-and-down cycles every second. At this stage, the student does not have to worry about the precise operation of a transformer as this will be covered in lesson 35. It will help to treat the transformer as a box that changes the potential difference, stepping it up or down. Ensure the student realises that a transformer works only with ac supplies.

Plenary activity

• A bird on the wire; 5 minutesAsk the student to explain why a bird standing on a 25 kV cable does not get an electric shock.

Homework activity

• Electricity in use; 40 minutes; page 57Full instructions are given on the activity sheet.

Support idea

• Electrical power The student needs to remember the equations for electrical energy transfer and power and be confident using them. It might help if initially, the equations are available on paper to consult. Provide support in choosing the correct equation. If the student struggles to rearrange equations, introduce formula triangles.

Extension idea

• The National Grid Test the student on how transformers work.

Progress and observations

Specification links

• 4.2.3.1• 4.2.3.2• 4.2.4.1• 4.2.4.2• 4.2.4.3• MS 1a, 1c, 2a, 3b, 3c, 4a

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AQA

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PHYSICS HIGHER

1. Look at the plug below.

a) Colour the wires in the diagram correctly.

b) Label the three wires.

Starter activity: What’s in a plug?

Learning objectives

• To learn how a three-core cable is wired

Equipment

• plug attached to cable of appliance• screwdriver• coloured pencils

Time 5mins

2. Complete the table to describe the purpose of each of the three wires.

Wire name Purpose

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AQA

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PHYSICS HIGHERMain activity: Electrical power

Learning objectives

• To use the equations for electrical power and energy

Equipment

none

Time 20mins

For every question you answer correctly, your tutor will add 30 seconds to the time you have to solve the crossword.

1. Calculate the power used when 100 J of energy is changed in 4 s.

2. Calculate the energy transferred by a 20 W light bulb in one minute.

3. A heater is rated at 2 kW. How long will it take to deliver 4 MJ of thermal energy?

4. A device is supplied by a 12 V battery. 1 kJ of energy is transferred. How much charge flows?

5. A 170 W television is supplied from the mains at 230 V. Calculate the current in the television.

6. An electric motor uses 1500 W supplied from the 230 V mains. Calculate the current it draws and its electrical resistance.

7. Complete the crossword in the time you tutor gives you.

2

3 4 5

7

1

6

9

8Across4 The name of the blue wire in a plug (7)6 If too much current flows through an appliance, a will break the circuit. (4)7 Current that flows in one direction (6)8 The name of the green and yellow wire in a plug (5)9 The electricity network that connects electrical energy producers with consumers (8,4)

Down1 Plastic is a good electrical (9)2 Current that reverses direction (11)3 Copper is a good electrical (9)5 Device used for measuring current (7)

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AQA

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PHYSICS HIGHER

1. Sketch two graphs on these axes to show an ac and a dc supply voltage.

Main activity: The National Grid

Learning objectives

• To understand the difference between ac and dc• To describe the use of step-up and step-down transformers in

the National Grid

Equipment

none

Time 20mins

2. This diagram shows part of the National Grid network.

a) State which of the transformers, T1, T2 and/or T3 step up the potential difference, and which are step-down transformers.

T1 T2 T3

The generator produces electricity at 25 kV. Typical values for the other four potential differences are as follows:

V1: 132 kV V2: 132 kV V3: 11 kV V4: 230 V

b) Discuss with your tutor why:

• in practice, V2 will be smaller than V1

• V3 is very much larger than V4.

c) Suppose the generator supplies 1000 A at 25 kV and that the resistance of the cables is 50 Ω. Calculate the power loss in the cable if there was no step-up transformer T1.

d) T1 is a 1 : 5 step-up transformer. When the potential difference is increased, the current is decreased by the same ratio. Calculate the new current in the cable and hence, the new power loss.

e) Power loss occurs in the cables because energy is transferred to thermal energy. Use the definition of efficiency and your answers to a) and b) to explain why the National Grid is an efficient way of transferring energy.

voltage

time0

generator transformer

cablesT1

V125 kV V2 V3 V4

transformer transformer

factory

housesT2 T3

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AQA

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PHYSICS HIGHERHomework activity: Electricity in use

Learning objectives

• To practise using the equations for power, energy and charge flow

Equipment

none

Time 40mins

1. A computer uses 40 W of power. Calculate the current it draws when connected to a 230 V mains supply.

2. A battery moves 150 C of charge through a potential difference of 12 V in 1 minute. Calculate the power developed in the battery.

3. A power shower has a power rating of 7 kW.

a) Calculate the charge flowing through the unit in 1 minute.

b) The supply cables have a resistance of 0.6 Ω. Calculate the power dissipation in the cables.

4. Explain why an electrical device with a lower resistance will draw more power from the mains supply.

5. A 12 V light bulb uses 24 W of power.

a) Calculate its resistance and the current at full power.

b) Calculate the resistance of a 100 W mains light bulb and the current at full power.

c) Two bulbs are connected in series and supplied from the mains at 230 V as shown in the diagram. Calculate the current in the circuit. Assume that both bulbs operate at their working resistance.

d) Use your answer to part c) to comment on the brightness of the bulbs.

230 V

12 W 100 W

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PHYSICS HIGHER7 Answers

Starter activity: What’s in a plug?

1.

Main activity: Electrical power

1. 25 W2. 1200 J3. 2000 s or 33.33 minutes 4. 83.33 C5. 0.74 A6. I = 6.52 A, R = 35.27 Ω 7. down: 1. insulator, 2. alternating, 3. conductor, 5. ammeter; across: 4. neutral, 6. fuse, 7. direct, 8. earth,

9. National Grid

Main activity: The National Grid

1. Check the student’s graphs. Correct sketches may vary, but the ac graph should be a sine wave with peaks and troughs of the same amplitudes, and the dc graph should be a horizontal line.

2. a) T1: step-up; T2: step-down; T3: step-downb) V2 will be smaller than V1 due to energy losses in the cables. V3 needs to be high in order to maintain efficient

transfers of large amounts of energy to industrial units, whereas V4 needs to be low to improve safety in the home.c) 50 MWd) I = 200 A, so the new power loss is 2.0 MW.

e) efficiency = useful power outpower in

The National Grid includes a step-up transformer so that the supply cables are at a very high voltage. The current is reduced proportionally. This reduces the rate of energy loss as thermal energy, making the network efficient.

Homework activity: Electricity in use

1. 0.17 A2. 30 W3. a) 1830 C

b) 460 W4. P = I2R = V

R )( 2 × R = V

2

R , so for a constant voltage, the lower the resistance, the greater the power.

5. a) R = 6 Ω, I = 2 Ab) R = 529 Ω, I = 0.43 Ac) 0.047 Ad) The 12 W bulb is fairly bright, but the 100 W bulb is dim.

2. Wire name Purposelive Carries supply voltage (230 V) to the appliance

neutral Completes the circuit (is at about 0 V)

earth Safety wire (kept at 0 V), route to earth if a fault occurs

neutral wire (blue)

live wire(brown wire)

earth wire(yellow and green wire)

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