GCSE Standard Questions: Electricity in the Home

Q11. A washing machine is connected to the mains electricity supply using a cable and three-pin plug.

(a) Here is a three-pin plug.

(i) Name the material used for the outer casing of the plug - giving the reason why that material is used.

The outer case is made of plastic (or rubber) because it is a good electrical insulator.

Be very careful when talking of insulator/conductor that you distinguish between heat conduction and electrical conduction... any mention of heat heat will lose you the mark!

[1 mark]

(ii) Name the material used for the pins of the plug - giving the reason why that material is used.

The pin is made from brass because it is hard and a good electrical conductor.

The examiner accepted copper for brass, but if you just said 'metal' it was insufficient to get you a mark. If you talked about 'heat conducting' you got no mark!

[1 mark]

 

(b) The three-pin plug contains a fuse.

The fuse is connected to one of the wires inside the cable.

(i) Which one of the wires inside the cable is the fuse connected to?

live

[1 mark]

(ii) The fuse is a thin wire inside a closed glass tube. The wire acts as a resistor. What effect does a current through a wire have on the wire?

A current flowing through it makes it hot/warm

If you said 'melt' you didn't get the mark.

[1 mark]

(iii) The power of the washing machine varies between 0.7 kW and 2 kW depending on which part of the wash cycle is operating.

Calculate the maximum current drawn from the mains electricity supply by the washing machine. The mains electricity supply is at a potential difference of 230 V.

Power = current x potential difference

P = IV

2000 = I x 230

I = 2000/230

I = 8.7 A

[4 marks]

(c) The diagram below shows how the mains electricity cable is connected to the washing machine. The earth wire is connected to the metal case of the washing machine.

If a fault makes the metal case live, the earth wire and fuse inside the plug prevent the mains cable from overheating and causing a fire. Explain how.

The earth wire is at 0V. The live wire (and therefore the metal cas) is varying from a maximum of 330V to -330V fifty times a second. As soon as the earthed case becomes live, a large current flows through the live wire to earth which causes the fuse to overheat and melt - breaking the circuit and making the case safe.

The examiners did not accept 'electricity' for current - you needed to show you know it is the flow of current that causes the fuse to 'blow' - not the size of the p.d.

The examiners accepted 'blow' for melt but 'break' was insufficient to get you a mark - as was 'snap'. Do not say 'blow up' - the fuse 'blows' it does not 'blow up' or explode!

[2 marks]

(d) New research has shown that many people underestimate the hazards of using mains electricity. It is important that people do understand the hazards of using mains electricity. Suggest why.

To reduce the chance of people getting an electric shock /electrocuted/being killed

OR to reduce the risk of an accident/ (electrical) fire.

[1 mark]

(Total 11 marks)