Mains electricity

Electricity can flow either as direct current (d.c.), or alternating current, (a.c.), and is used in homes to power electrical appliances.

Direct current (d.c.)

Direct current only flows in one direction.

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

An oscilloscope screen displaying the signal from a direct current (DC) supply. It is a horizontal straight line at 1.5V.

A voltage-time graph for direct current

Car batteries, simple cells, mobile phone batteries and solar cells all provide a direct current.

A simple cell or battery is d.c.

A simple cell or “battery” is d.c

Alternating current (a.c.)

The electricity supply to your home is alternating current.

The National Grid distributes electricity throughout the country and this is referred to as “mains” electricity.

The voltage and current change size and direction in a regular way which repeats as time goes on.

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.

An oscilloscope screen displaying the signal from an alternating current (AC) supply. It is a 50Hz sine wave that peaks at 230V.

A voltage-time graph for alternating current

In the UK, the mains electrical supply is generated at a frequency of 50 Hz and is delivered to houses at an average of 230 V.