The live wire carries the alternating potential difference from the supply.

The idea

The live wire is the core connected to the alternating supply, so its electrical potential swings above and below earth potential 50 times a second, and the potential difference between it and earth is about 230 V. That is what makes it the dangerous core. Your body sits at roughly earth potential, so touching the live wire puts about 230 V across you, and charge then flows through you to earth as a shock. The live core stays at that potential all the way to the switch even when the switch is open, because opening a switch only breaks the loop, it does not disconnect the supply. This is why an electrician isolates a circuit at the consumer unit rather than trusting a wall switch before starting work.

What earns the marks

• The live wire being at about 230 V because it is connected to the alternating supply.
• The person being at earth potential, 0 V.
• The large potential difference between the live wire and the person.
• Charge or current passing through the body to earth.

Common mistakes and how to put them right

• Describing the live wire as positive, which ignores that an ac supply reverses.

• Saying the shock happens because the live wire has a lot of current in it, rather than because of the potential difference across the body.

• Assuming a live wire is safe whenever the appliance is switched off.

• Claiming the person is positively charged rather than at 0 V.

Exam tips

• Shock explanations need three steps: live at 230 V, body at 0 V, large potential difference driving current through the body.
• Use 'potential' for a single point and 'potential difference' for the gap between two points; examiners mark that distinction.
• Quote the 230 V figure rather than writing 'a high voltage'.