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Spec links: 4.2.1.3 Current, resistance and potential difference; 4.2.3.1 Direct and alternating potential difference; 4.2.3.2 Mains electricity · Reading time: about 4 minutes

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A puzzle for visitors

Visitors to Britain are often confused. They want to dry their hair after a shower, but there is nowhere to plug in the hairdryer. In many other countries, bathrooms have ordinary plug sockets. In UK homes, the rules mean you will not find one near the bath or shower. You might see a small shaver socket, which is specially designed to be safer, but that is all. The reason is simple. Water and mains electricity are a deadly mix.

Why wet skin is dangerous

The current through a component depends on the potential difference across it and its resistance. The equation is:

V = I R, where V is the potential difference in volts (V), I is the current in amperes (A) and R is the resistance in ohms (Ω).

Your body has resistance, and dry skin gives quite a lot of it. Wet skin has a much lower resistance. Suppose that, with wet skin, the resistance of a path through your body is 1000 Ω. If you touch something at the mains potential difference of 230 V, the current through you is:

I = V ÷ R = 230 V ÷ 1000 Ω = 0.23 A.

That might sound small, but a current of less than a tenth of an amp through the chest can stop the heart. Lower resistance means a bigger current, and a bigger current means more danger. That is why bathrooms have extra rules.

Inside the cable

Mains electricity in the UK is an alternating current supply with a frequency of 50 Hz and a potential difference of about 230 V. Most appliances connect to it with a three-core cable. Each wire has an inner copper core and a coloured plastic coat:

• The live wire is brown. It carries the alternating potential difference from the supply, about 230 V.
• The neutral wire is blue. It completes the circuit and is at, or close to, 0 V.
• The earth wire has green and yellow stripes. It is at 0 V and only carries a current if there is a fault.

The live wire is the dangerous one. Even when a switch is open, touching the live wire can be deadly. Your body is at 0 V, so there is a large potential difference between you and the wire. A current flows through you to the earth.

Safety features

Imagine a loose live wire touches the metal case of a toaster. Without protection, anyone touching the case would get a shock. The earth wire is connected to the metal case. If the live wire touches the case, a large current flows through the earth wire to the ground instead.

That large current also passes through the fuse in the plug. A fuse is a thin piece of wire connected in the live wire. If the current gets too big, the fuse wire heats up and melts. This breaks the circuit and cuts off the live supply. Plugs usually contain 3 A or 13 A fuses, chosen to suit the appliance.

Rules that save lives

Bathroom lights are usually worked by a pull-cord, so you never touch a switch with wet hands. Heated towel rails and showers are wired in permanently. These rules can seem fussy. But each one is based on a simple bit of physics: keep resistance high and keep people away from the live wire.

Key words

• Potential difference: the energy transferred per unit charge passed between two points; measured in volts (V).
• Resistance: a measure of how much a component opposes the current; measured in ohms (Ω).
• Live wire: the brown wire, which carries the alternating potential difference from the supply.
• Neutral wire: the blue wire, which completes the circuit and is at or close to earth potential (0 V).
• Earth wire: the green and yellow striped wire, a safety wire at 0 V that only carries a current if there is a fault.
• Fuse: a thin wire in the live wire that melts and breaks the circuit if the current is too large.

Questions

1. What is the potential difference of the UK mains supply? [1 mark]
2. Give the colour of the live wire and the colour of the neutral wire. [2 marks]
3. How are bathroom lights usually switched on and off? [1 mark]
4. Explain why touching the live wire can be dangerous, even when the appliance is switched off. [3 marks]
5. Explain why getting an electric shock is more dangerous when your skin is wet. [2 marks]
6. A heater element has a resistance of 46 Ω and is connected to the 230 V mains. Calculate the current in the element. Give your answer in amperes. [3 marks]
7. A current of 0.5 A passes through a lamp. The potential difference across the lamp is 12 V. Calculate the resistance of the lamp. Give the unit. [3 marks]
8. A kettle has a metal case. Describe how the earth wire and fuse work together to protect a person if the live wire becomes loose and touches the case. [6 marks]

Show answers

1. About 230 V (1)
2. Live: brown (1); neutral: blue (1)
3. With a pull-cord (1)
4. The live wire is still at (about) 230 V when the switch is open (1); the body is at 0 V / earth potential, so there is a large potential difference between the body and the wire (1); so a current flows through the body (to earth), causing a shock (1)
5. Wet skin has a lower resistance (1); so for the same potential difference a larger current flows through the body (1)
6. I = V ÷ R (1); I = 230 ÷ 46 (1); I = 5 A (1)
7. R = V ÷ I = 12 ÷ 0.5 (1); R = 24 (1); Ω / ohms (1)
8. Indicative content:
   – the earth wire is connected to the metal case
   – the earth wire is at 0 V
   – if the live wire touches the case, the case would be at 230 V without the earth wire
   – the earth wire provides a low-resistance path to the ground
   – so a large current flows from the live wire through the earth wire
   – this large current also flows through the fuse
   – the fuse is in the live wire
   – the fuse wire heats up and melts
   – this breaks the circuit / disconnects the live wire
   – so the case is no longer live and a person touching it does not get a shock
   Level 3 (5–6 marks): a clear, logical description linking the earth wire, the large current, the fuse melting and the circuit breaking, explaining why the person is protected; Level 2 (3–4 marks): a description of the role of the earth wire and the fuse, with some linking; Level 1 (1–2 marks): simple statements about the earth wire or fuse with little explanation

More reading: See all 30 GCSE Physics comprehension articles