Electrical power and safety — GCSE Physics practice questions

Question 1

The cable of a food mixer is crushed under a heavy cupboard door. The live and neutral wires inside the cable touch, and the current in the cable becomes very large.

What does the fuse in the plug of the mixer do?

Accepted Answer

A: Its thin wire melts and breaks the circuit.. The very large current heats the thin wire inside the fuse until it melts. The circuit is then broken, so there is no current, and the cable cannot overheat and start a fire.

Question 2

A hand blender has a plastic case and is marked with a symbol of one square inside a larger square. Its cable has only two wires.

Which statement about the hand blender is correct?

Accepted Answer

A: It has no earth wire, but there is still a fuse in its plug to protect the cable.. The symbol means the blender is double insulated: no exposed metal, so no earth wire is needed and the cable has only live and neutral wires. A fuse is still fitted in the live side of the plug, because a fault could still make the current too large and overheat the cable.

Question 3

The battery of a camera drone has an electromotive force (e.m.f.) of 15 V.
During a flight lasting 24 minutes, the battery transfers 0.060 kW h of energy to the drone.

What is the average current in the battery during the flight?

Accepted Answer

B: 10 A. The energy is 0.060 × 3.6 × 10⁶ = 216 000 J and the time is 24 × 60 = 1440 s, so the power is 216 000 / 1440 = 150 W. The current is P / V = 150 / 15 = 10 A.