Mains electricity and the National Grid · GCSE Physics

Direct and alternating current

Cells and batteries supply direct current (dc): the current flows in one direction only.
Mains electricity is an alternating current (ac) supply: the current repeatedly changes direction.
In the UK, the mains supply has a frequency of 50 Hz and a potential difference of about 230 V.

Most appliances are connected to the mains with a three-core cable, with colour-coded insulation.
Live wire (brown): carries the alternating potential difference from the supply.
Neutral wire (blue): completes the circuit and is at or close to earth potential (0 V).
Earth wire (green and yellow stripes): a safety wire that stops the appliance becoming live. It only carries a current if there is a fault.

The potential difference between the live wire and earth is about 230 V.
The live wire can be dangerous even when a switch in the mains circuit is open, because it is still at 230 V.
Touching the live wire makes a connection between it and earth through your body, so a large current flows through you: an electric shock.
A fuse or circuit breaker in the live wire cuts off the current if it gets too large.

The National Grid is a system of cables and transformers that links power stations to homes and businesses.
Step-up transformers increase the potential difference from the power station to the transmission cables.
A higher potential difference means a lower current for the same power, so less energy is lost heating the cables. This makes the grid efficient.
Step-down transformers decrease the potential difference to a much lower, safer value for use in homes.

Direct current (dc): Current that flows in one direction only.
Alternating current (ac): Current that repeatedly changes direction.
Live wire: The brown wire that carries the alternating pd from the mains supply.
Neutral wire: The blue wire that completes the circuit, at about 0 V.
Earth wire: The green and yellow safety wire that stops an appliance becoming live.
National Grid: The network of cables and transformers that distributes electricity.
Step-up transformer: A transformer that increases the potential difference.
Step-down transformer: A transformer that decreases the potential difference.