Current

Alternating and direct current

An electric current flows either as a direct current or as an alternating current.

Direct current

A direct current flows in only one direction.

On a voltage-time graph this would appear as a straight horizontal line at a constant voltage.

Car batteries, dry cells and solar cells all provide a direct current (dc) that only flows in one direction.

Alternating current

An alternating current regularly changes direction.

On a voltage-time graph, this would appear as a curve alternating between positive and negative voltages. The positive and negative values indicate the direction of current flow.

Power stations sometimes produce electricity using magnets. This provides an alternating current (ac). In the UK, the mains electrical supply is generated at a frequency of 50 Hertz (Hz) and is delivered to houses at 230 Volts (V).

Household electricity

Plugs

A plug connects a device to the mains electricity supply. The cable between the device and the three-pin plug contains three copper wires that are coated with plastic.

• copper wires are good conductors
• plastic is a good insulator

Each part of the plug has a function.

Features of a plugFunction Outer insulation All three wires in the cable are bundled together and there is extra plastic insulation wrapped round them all for safety Cable grip This holds the cable tightly in place so that wires do not become loose Live wire Copper wire coated with brown plastic along which the current enters the device Fuse A glass or ceramic canister containing a thin wire that melts if the current gets too high Neutral wire Copper wire coated with blue plastic that also connects to the cable in the wall and completes the circuit Earth wire Copper wire coated in striped plastic that provides a path for current to flow from the case of the device to the ground if there is a fault

Earthing

Without the earth wire, if a fault occurs and the live wire becomes loose, there is a danger that it will touch the case. The next person who uses the appliance could get electrocuted.

The earth wire is therefore connected to the case and is attached to a metal plate or water pipe underground. As the wire is made of copper, the earth wire provides a low resistance path to the ground. In the event of a fault, the live current passing through the case will follow this path to the ground instead of passing through a person.

Fuses

A fuse provides a built-in fail-safe to the electrical circuit for a…