3.1 Alternating Voltages and Currents

The waveforms shown in Figure 3.1 may represent alternating currents or voltages.

Figure 3.1: Examples of alternating voltages and currents

Thus an alternating current (AC ) is defined as a periodic current whose average value over a period is zero. Stated differently, an alternating current alternates between positive and negative values at regularly recurring intervals of time. Also, the average of the positive and negative values over a period is zero.

As shown in Figure 3.1, the period T of an alternating current or voltage is the smallest value of time which separates recurring values of the alternating waveform.

Unless otherwise stated, our subsequent discussion will be restricted to sine or cosine waveforms and these are referred to as sinusoids. Two main reasons for studying sinusoids are: (1) many physical phenomena such as electric machinery produce (nearly) sinusoidal voltages and currents and (2) by Fourier analysis, any periodic waveform which is not a sinusoid, such as the square and saw- tooth waveforms on the previous page, can be represented by a sum of sinusoids.

3.2 Characteristics of Sinusoids

Consider the sine waveform shown in Figure 3.2, where