2.10 Mains electricity and plugs
9 exam questions worth 21 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What are the potential difference and frequency of the UK mains supply?
Choose one answer.
Correct Not quite: the answer is 230 V, 50 Hz
Mark scheme
A: 230 V, 50 Hz
Model answer
230 V, 50 Hz (A).
Name the three wires in a mains cable and state the colour of the insulation on each.
Mark scheme
live - brown (1); neutral - blue (1); earth - green and yellow stripes (1)
Model answer
Live (brown), neutral (blue) and earth (green and yellow stripes).
State the difference between a direct current and an alternating current.
Mark scheme
direct current flows in one direction only (1); alternating current repeatedly changes direction (1)
Model answer
A direct current always flows in the same direction. An alternating current repeatedly changes direction.
Explain why the live wire is dangerous, but the neutral wire is normally not.
Mark scheme
the live wire is at a high potential difference (about 230 V) compared with the earth / a person (1); touching it lets a current flow through the body to earth, giving an electric shock (1); neutral is at about 0 V
Model answer
The live wire is at about 230 V compared with the earth, so touching it makes a current flow through your body to earth and gives you an electric shock. The neutral wire is at about 0 V.
The live wire inside a kettle comes loose and touches the metal case. Explain how the earth wire and the fuse protect the user.
Mark scheme
a large current flows through the earth wire (1); this makes the fuse melt (1); which disconnects the live wire so the case is no longer live / dangerous (1)
Model answer
A large current flows from the live wire through the case and the earth wire. This melts the fuse, which breaks the circuit and disconnects the live supply, so the case is no longer live.
Give two ways in which a three-pin plug is designed to be safe to use.
Mark scheme
plug case is plastic / an electrical insulator, so the user does not get a shock (1); fuse in the live wire / earth wire connected to the case / cable grip so wires cannot pull out (1)
Model answer
The case is made of plastic, which is an insulator, so the user is protected from a shock. It also contains a fuse in the live wire, which breaks the circuit if the current is too large.
An iron is rated at 1.2 kW and used on the 230 V mains. Fuses of 3 A, 5 A and 13 A are available. Calculate the current and state which fuse should be fitted, giving a reason.
Mark scheme
I = P ÷ V = 1200 ÷ 230 (1); 5.2 A (1); 13 A, because the 5 A fuse is lower than the current and would blow (1)
Model answer
I = 1200 / 230 = 5.2 A. This is more than 5 A, so a 13 A fuse is needed; a 5 A fuse would blow in normal use.
Explain what is meant by double insulation, and why an appliance with double insulation does not need an earth wire.
Mark scheme
the live parts are separated from the user by two layers of insulation / the case is made of an insulator (1); so the case cannot become live (1); so there is no need for an earth wire to carry a fault current away (1)
Model answer
Double insulation means that the live parts are covered by two layers of insulation, with a plastic case. The case cannot become live, even if a wire comes loose, so no earth wire is needed.
A bird sits on a bare high-voltage power line and is not harmed. Explain why.
Mark scheme
both feet are at the same potential / there is no potential difference across the bird (1); so no current flows through the bird (1)
Model answer
Both of the bird's feet are on the same wire, at the same potential, so there is no potential difference across the bird and no current passes through it.
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