Explain why the live wire in a mains plug can be dangerous even when the appliance is switched off.

AQA GCSE Physics (8463) — 4.2.3 Domestic uses and safety · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student switches off a lamp at its on/off switch but leaves the plug in the socket. A friend tells her the lamp is now completely safe to carry out repairs on.

Model answer (4 marks)

The on/off switch only opens the circuit on the live side; the live wire remains connected to the mains supply.
The live conductor still carries the full 230 V alternating potential.
If a person touches the live wire (or any part connected to it) a conductive path is created to earth.
Current can then flow through the body, producing a potentially fatal shock.

Examiner tips

Use the word "live" and "230 V" to show understanding of mains voltage.
Explain that the switch does not disconnect the live wire from the supply.
Mention the risk of a current path through the body to earth.
Show the causal chain from switch position to shock.

Common mistakes

Saying the lamp is safe because the switch is off, ignoring the live wire remains energized.
Using the term "neutral" instead of "live" for the dangerous conductor.
Failing to mention the 230 V potential difference or the path to earth.

Mark scheme (4 marks)

The switch breaks the circuit on the live side only (or: switching off does not disconnect the live wire from the supply)
The live wire carries the alternating potential difference of 230 V
Touching the live wire (or any part connected to it) provides a path for current to flow through the person to earth
This causes a serious / fatal electric shock

Key terms in this question

live wire