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Science

Mains electricity

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# Direct current and alternating current

You should know the differences between direct current (d.c.) and alternating current (a.c.) electrical supplies.

## Direct current

Direct current

If the current [current: Moving electric charges, for example, electrons moving through a metal wire. ] flows in only one direction it is called direct current, or d.c. Batteries and cells supply d.c. electricity, with a typical battery supplying maybe 1.5V. The diagram shows an oscilloscope screen displaying the signal from a d.c. supply.

## Alternating current

Alternating current

If the current constantly changes direction, it is called alternating current, or a.c.. Mains electricity is an a.c. supply, with the UK mains supply being about 230V. It has a frequency [frequency: The number of repetitions per second of a wave. The unit of frequency is the hertz, 'Hz'. ] of 50Hz (50 hertz), which means it changes direction, and back again, 50 times a second. The diagram shows an oscilloscope screen displaying the signal from an a.c. supply.

## Alternating current - higher

Alternating current

The potential difference of the live terminal varies between a large positive value and a large negative value. However, the neutral terminal is at a potential difference close to earth, which is zero. The diagram shows an oscilloscope screen displaying the signals from the mains supply. The red trace is the live terminal and the blue trace the neutral terminal. Note that, although the mean voltage of the mains supply is about 230V, the peak voltage is higher.

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