Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Explain the difference in function between the live and the neutral mains…
Full specification wording
Topic 10 – Electricity and circuits: 10.37 Explain the difference in function between the live and the neutral mains input wires
The live wire carries the alternating potential difference from the supply to the appliance, so it is the wire that is dangerous to touch: it alternates between a positive and a negative potential…
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 35 of 39
Study notes
From the checked teaching notes for this specification point.
The idea
The live wire carries the alternating potential difference from the supply to the appliance, so it is the wire that is dangerous to touch: it alternates between a positive and a negative potential relative to earth, at about 230 V in the UK mains supply. The neutral wire completes the circuit back to the supply and is held at a potential close to earth, so it normally carries current but has little potential difference relative to earth. Both wires carry the same current in a working series circuit, so the difference is about potential, not about current. A switch or fuse belongs in the live wire so that the appliance can be isolated from the high potential.
What earns the marks
- States that the live wire carries the alternating potential difference from the supply to the appliance.
- States that the neutral wire completes the circuit and returns current to the supply.
- Explains that the live wire is at a potential that alternates relative to earth, making it dangerous to touch.
- Explains that the neutral wire is held at a potential close to earth, so it is normally safe to touch.
- Recognises that both wires carry the same current in a working circuit, so the difference concerns potential difference rather than current.
Common mistakes and how to put them right
Claims that the neutral wire carries no current.
Put it right by noting that the neutral carries the same current as the live wire in a working series circuit.
Says the live wire is positive and the neutral is negative.
Put it right by explaining that the live potential alternates between positive and negative relative to earth.
Assumes the neutral wire is always safe.
Put it right by noting that a fault can raise the neutral potential, which is why switches and fuses go in the live wire.
Exam tips
- Link each wire to its potential relative to earth, not just to the direction of current.
- Use the phrase 'alternating potential difference' for the live wire to show the a.c. nature of the supply.
- Explain why safety devices are fitted in the live wire, since this shows understanding of the difference in function.
Try the lesson
The start of the Topic 10 – Electricity and circuits: 10.37 Explain the difference in function between the live and the neutral mains input wires lesson, made from checked teaching notes for this specification point.
Have a go first
In a mains circuit, which input wire carries the alternating potential that changes between positive and negative relative to earth?
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