Electricity
4 marks · EasyThe lamp in a student’s desk light has a voltage of 12 V across it and a current of 0.5 A flowing through it when switched on. The lamp is connected to the mains using a plug that contains a fuse and a live, neutral and earth wire.
Calculate the power output of the lamp.
Name which wire the switch and fuse are always connected to and give a reason.
Explain how the fuse works to protect the appliance.
State one reason double insulation can make the earth wire unnecessary.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) 6 W
(b) Live wire; to ensure the appliance is disconnected from the high voltage supply if the switch/fuse operates.
(c) The fuse melts (or 'blows') if the current is too high, cutting off the circuit and protecting the appliance from damage or fire.
(d) Double insulation stops any live parts being exposed, so the earth wire is not needed.
Show mark scheme and worked solution
Marking points
M1 (a) · 0.5 marks · Uses P = I × V for power calculation.
A1 (a) · 0.5 marks · Power is calculated as 6 W. (follow-through)
B1 (b) · 1 mark · Names correct wire (live) and gives a suitable reason about disconnection from supply.
B1 (c) · 1 mark · Explains melting of fuse and breaking the circuit to prevent danger.
B1 (d) · 1 mark · States that double insulation prevents touching metal parts so earth wire is not needed.
Worked solution
a) P = I × V = 0.5 A × 12 V = 6 W
b) Live wire; so switching off or breaking the fuse disconnects the high voltage from the appliance, making it safe.
c) If the current is too high, the fuse melts, breaking the circuit so no current can flow to the appliance, preventing damage or fire.
d) With double insulation, all metal parts are covered by insulation so users cannot touch live parts, making the earth wire unnecessary.