Mains electricity
Direct and alternating current
flows in one direction only. Cells and batteries supply direct potential difference.
keeps changing direction, many times a second.
In the UK, the mains electricity supply is ac, with a (it changes direction back and forth 50 times a second) and a .
Key terms
The three wires in a cable
Most appliances connect to the mains with a . Each wire has its own colour of insulation:
| Wire | Colour | Potential difference | Job |
|---|---|---|---|
| Live | Brown | About 230 V | Carries the alternating pd from the supply |
| Blue | At or close to 0 V | Completes the circuit | |
| Earth | Green and yellow stripes | 0 V | A safety wire; only carries a current if there's a fault |
Remember
Learn each wire's colour and pd together: live is brown at 230 V, neutral is blue near 0 V, earth is green and yellow at 0 V.
Key terms
Why the live wire is dangerous
Your body is at 0 V, the same as the earth. If you touch the live wire, there is a across your body, so a current flows through you to the ground. This gives you an electric shock, which can kill.
The live wire can be dangerous , because it is still connected to the supply.
Any connection between the live wire and earth is dangerous, because a large current can flow through it.
How the earth wire and fuse protect you
Appliances with a metal case have the earth wire connected to the case. The sits in the live wire, inside the plug.
If a fault makes the live wire touch the metal case:
- The earth wire gives a to the ground.
- A flows from the live wire, through the case and earth wire.
- This large current makes the fuse melt, breaking the circuit.
- The live wire is now cut off, so the case is safe to touch.
The earth wire only carries a current when there is a fault. Normally no current flows in it.