AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Students should be able to explain: that a live wire may be dangerous even…
Full specification wording
Mains electricity: Students should be able to explain: that a live wire may be dangerous even when a switch in the mains circuit is open the dangers of providing any connection between the live wire and earth.
Opening a switch only breaks the loop; it does not disconnect the live core from the supply.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 8 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Opening a switch only breaks the loop; it does not disconnect the live core from the supply. The length of live wire between the consumer unit and the switch is still at about 230 V relative to earth, so touching it is just as dangerous with the appliance off as with it on. The second danger is any accidental connection between live and earth. You are at earth potential whenever you stand on the ground, touch a metal pipe or lean on a damp wall, so completing that path puts the full 230 V across your body and charge passes through you. Water makes it worse, because damp skin has a much lower resistance and the same 230 V then drives a larger current. Isolate at the consumer unit before touching any mains wiring.
What earns the marks
- The live wire remaining connected to the supply when a switch is open.
- The live wire still being at about 230 V relative to earth.
- A person at 0 V completing a path from live to earth.
- The resulting current through the body, increased by the lower resistance of damp skin.
Common mistakes and how to put them right
Saying an open switch makes the whole circuit safe because there is no current anywhere.
Arguing that the danger comes from current already in the wire rather than from the potential difference created.
Treating the neutral wire as equally dangerous to touch.
Saying water conducts so the electricity goes into the water, without mentioning lower resistance and larger current.
Exam tips
- Explain the open switch case by saying which side of the switch is still connected to the supply.
- For any shock question, state both potentials before you mention current.
- Mention resistance explicitly when water appears in the question, because that is where the extra mark usually sits.
Try the lesson
The start of the Mains electricity: Students should be able to explain: that a live wire may be dangerous even when a switch in the mains circuit is open the dangers of providing any connection between the live wire and earth. lesson, made from checked teaching notes for this specification point.
Have a go first
A mains circuit has a switch that is currently open, so no current is flowing. Why can it still be dangerous to touch the live wire before the switch?
Listen
Opening a switch only breaks the loop; it does not disconnect the live core from the supply. The length of live wire between the consumer unit and the switch is still at about 230 V relative to earth, so touching it is just as dangerous with the appliance off as with it on. The second danger is any accidental connection between live and earth. You are at earth potential whenever you stand on the ground, touch a metal pipe or lean on a damp wall, so completing that path puts the full 230 V across your body and charge passes through you. Water makes it worse, because damp skin has a much lower resistance and the same 230 V then drives a larger current. Isolate at the consumer unit before touching any mains wiring. A strong answer earns marks for these points. the live wire remaining connected to the supply when a switch is open. the live wire still being at about 230 V relative to earth. a person at 0 V completing a path from live to earth. the resulting current through the body, increased by the lower resistance of damp skin. Exam tip: Explain the open switch case by saying which side of the switch is still connected to the supply.
Key words
Say what each word means before you turn the card.
open switch still live
Try answering aloud or in your head before turning the card.
live to earth connection
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body at 0 V
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resistance of damp skin
Try answering aloud or in your head before turning the card.
See every definition
- open switch still live
- Opening a switch only breaks the loop, so the length of live wire between the consumer unit and the switch is still at about 230 V relative to earth.
- live to earth connection
- Any accidental connection between live and earth completes a path that puts the full 230 V across your body and charge passes through you.
- body at 0 V
- You are at earth potential whenever you stand on the ground, touch a metal pipe or lean on a damp wall.
- resistance of damp skin
- Damp skin has a much lower resistance, so the same 230 V drives a larger current.
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