Alternating currents

Direct current is an electric current that flows in one direction only – the electrons drifting down the wire towards a definite end. If they change direction, first moving one way and then the other we have what is called an alternating current.

We define an alternating current or voltage as one which varies with time about a mean value, usually zero.

Some examples of this variation are shown in Figure 1 which illustrates variations that are:
(a) sinusoidal, (b) square, (c) saw tooth and (d) irregular.

There are three reasons why we shall restrict ourselves mainly to considering sinusoidal variations:
(i) they may be produced by a rotating coil
(ii) the mains supply in the United Kingdom varies in this way;
(iii) all other variations can be considered as combinations of sine waves of different wavelengths and phases.