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Electric current What is direct current? Direct current (d.c.) is an electric current that always flows in one direction. Direct current is produced by cells and batteries. Electronic circuits such as those in computers and stereos need direct current electricity in order to work. Direct current cannot be transferred efficiently over large distances. What is alternating current? Alternating current (a.c.) is an electric current that is constantly changing direction. Alternating current is produced by most generators and is used in mains electricity. Motors often work using alternating current. The voltage of alternating current is easily changed with a transformer. Alternating current can be transferred efficiently over large distances. What is the difference between a.c. and d.c.? d.c. time voltage voltage The difference between alternating current (a.c.) and direct current (d.c.) can be seen using an oscilloscope. For each current, the oscilloscope trace is a graph showing how the voltage of an electricity supply varies with time. peak forward voltage a.c. time peak reverse voltage The voltage of a d.c. supply is steady and always in the same direction. The voltage of an a.c. supply follows a repeated pattern: it rises to a peak, returns to zero, changes direction and so on. Frequency of alternating current voltage The frequency of a.c. electricity is the number of complete cycles per second which is measured in hertz (Hz). 1 complete cycle peak forward voltage a.c. time peak reverse voltage The a.c. frequency can be determined from an oscilloscope by counting the number of complete waves per unit time. If the frequency is increased, the number of complete waves shown on the screen increases. For example, if the frequency is doubled, the number of waves doubles. Mains electricity in the UK In the UK, the frequency of mains electricity is 50 hertz: this alternating current flows backwards and forwards 50 times per second. This frequency is the same at any point in the electricity supply system but the voltage varies in different parts of the national grid. The voltage of mains electricity supplied to UK homes is 230 V. This is an effective voltage which is equal to the voltage of a d.c. supply that would produce the same heating effect. The peak (maximum) voltage is higher than this. Comparing a.c and d.c Which type of current – a.c. or d.c.?