An alternating quantity is one which acts in alternate positive and negative directions, whose magnitude undergoes a definite series of changes in in definite intervals of time and in which the sequences is identical in negative and positive. Figure (a) shows alternating quantity which is sinusoidal in nature and is quite common with alternating current and voltages. The shape of an alternating quantity may however be any non-sinusoidal also so but the variation in positive and negative half with reference to time has to be symmetrical and it should repeat itself in the same manner.
The graph repeats itself at regular time intervals. The time during which it repeats is called ‘periodic time‘ T. one repetition is called a ‘cycle‘. The number of repetitions for cycles per second is called ‘frequency‘.
Frequency f = 1/T (cycles/sec.)
or T = 1/f (seconds)