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.