Introduction to Electric Circuits

4 Single-phase a.c. circuits

4.1 ALTERNATING QUANTITIES

A quantity which is continually changing its sign from positive to negative and

back again is called an alternating quantity, usually referred to simply as an a.c.

quantity. Examples of alternating quantities are shown in Fig. 4.1(a) and (b).

The quantities shown in Fig. 4.1(c) and (d) are not alternating but are varying

direct quantities.

A graph of the quantity to a base of time is called the waveform of the

quantity and when the waveform has completed one complete series of changes

and is about to begin to repeat itself it is said to have completed one cycle. The

time for one complete cycle is termed the periodic time (T) or simply the

period. The number of complete cycles completed in one second is called the

frequency (f) and is measured in cycles per second which is called the hertz

(Hz) in honour of Heinrich Hertz (1857-94), a German Scientist.

t^0

CL

r L

L.F"

(a) (b)

Figure 4.1

V

r

t

(c)

V

t

(d)