# Alternating quantities

An alternating quantity is one which reverses in direction repeatedly.

A cycle is a complete period of events, which then repeats itself. The time for one cycle is the periodic time or period, T. The number of complete cycles in one second is the frequency, f. The unit of frequency is the Hertz (Hz). Thus,

For a sinusoidal waveform (sine wave) r.m.s. (root mean square) value of current is that current which will have the same heating effect as a direct current of the same value. is denoted by I, and unless stated the r.m.s. value is always

assumed. , where , is the peak or maximum value of current. The peak-to-peak value will be twice the peak value, i.e. .

A waveform can be said to lag, lead, or be in phase with another waveform. Unless otherwise stated, an AC waveform is assumed to be sinusoidal.

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