Alternating current and direct current

Alternating current (a.c.)

If the current constantly changes direction it is called alternating current, or a.c. Mains electricity is an a.c. supply. The UK mains supply is about 230 V. It has a frequency of 50 Hz, which means that it changes direction and back again 50 times a second. The diagram shows an oscilloscope screen displaying the signal from an a.c. supply.

An oscilloscope trace of alternating current. It shows wave shapes above and below a line labelled 0 volts.Oscilloscope trace illustrating alternating current

Direct current electricity (d.c.)

If the current flows in only one direction it is called direct current, or d.c. Batteries and solar cells supply d.c. electricity. A typical cell or battery may supply 1.5 V. The diagram shows an oscilloscope screen displaying the signal from a d.c. supply.

Diagram showing oscilloscope trace of direct current. It is a horizontal white line on a green background.Oscilloscope trace illustrating direct current