The Physics Question Bank

A plug connects an electrical appliance to the mains electricity supply. In the UK, mains electricity is about 230 V and uses alternating current, or AC.

A three-pin plug contains three wires:

live wire
neutral wire
earth wire

Each wire has a different job. The colours help people connect them correctly and safely.

The live wire is brown.

The live wire carries energy from the mains supply to the appliance. It is at about 230 V compared with the ground, so it is the most dangerous wire in the plug.

If someone touches the live wire, current may pass through their body to the ground. This can cause a serious electric shock.

The live wire carries current to the appliance when the appliance is switched on. It is also connected to the fuse. This is important because the fuse must be on the live side of the circuit. If the current becomes too large, the fuse melts and breaks the circuit, cutting off the supply to the appliance.

The neutral wire is blue.

The neutral wire completes the circuit. It provides a path for current to return to the mains supply after it has passed through the appliance.

In normal conditions, the neutral wire is close to 0 V compared with the ground. This means it is usually at a much lower potential difference than the live wire.

However, the neutral wire should not be called safe. It can still carry current when the appliance is working. It can also become dangerous if there is a fault, if the wiring is incorrect, or if the neutral connection is broken.

For GCSE Physics, the key point is this:

The live wire is the most dangerous because it is at a high potential difference compared with the ground. The neutral wire is usually close to 0 V, but it is still part of the mains circuit and should never be touched.

The earth wire is green and yellow.

The earth wire is a safety wire. It is connected to the metal case of an appliance. It does not normally carry current.

The earth wire is needed if a fault occurs. For example, if the live wire touches the metal case, the metal case could become live. Anyone touching the case could then receive an electric shock.

The earth wire provides a low-resistance path to the ground. This causes a large current to flow through the live wire and the fuse. The fuse then melts and breaks the circuit.

This disconnects the appliance from the mains supply and makes the appliance safer to touch.

This is why appliances with metal cases usually need an earth wire. Appliances with plastic cases often do not need an earth wire because plastic is an insulator.

The fuse is another important safety feature.

The fuse is connected to the live wire. Its job is to protect the appliance, the cable and the user if the current becomes too large.

If too much current flows, the thin wire inside the fuse heats up and melts. This breaks the circuit and stops the current. This helps prevent overheating, damage and fire.

The cable grip is also important.

The cable grip holds the outer insulation of the cable firmly in place. It stops the wires being pulled loose if someone pulls on the cable.

This matters because loose wires inside a plug can be very dangerous.

A simple way to remember the wires is:

live brings energy to the appliance
neutral completes the circuit
earth protects the user if there is a fault

The main GCSE points are:

The live wire is brown and is about 230 V.

The neutral wire is blue and is usually close to 0 V.

The earth wire is green and yellow and is a safety wire.

The fuse is connected to the live wire.

The earth wire and fuse work together to protect the user if the live wire touches the metal case.

A good GCSE exam answer would be:

The live wire carries current to the appliance and is at about 230 V, so it is the most dangerous wire. The neutral wire completes the circuit by carrying current back to the mains supply. It is usually close to 0 V, but it can still carry current and should not be touched. The earth wire is connected to the metal case and provides a low-resistance path to ground if a fault occurs. This causes a large current to flow, melting the fuse and breaking the circuit.