The sine wave
Sometimes, alternating current has a sine-wave, or sinusoidal, nature. This means
that the direction of the current reverses at regular intervals, and that the
current-versus time curve is shaped like the trigonometric sine function. The waveform
in Fig. 9-1 is a sine wave.
Any ac wave that consists of a single frequency will have a perfect sine waveshape.
And any perfect sine-wave current contains only one component frequency. In practice,
a wave might be so close to a sine wave that it looks exactly like the sine function on an
oscilloscope, when in reality there are traces of other frequencies present. Imperfections
are often too small to see. But pure, single-frequency ac not only looks perfect, but
actually is a perfect replication of the trigonometric sine function.
The current at the wall outlets in your house has an almost perfect sine waveshape,
with a frequency of 60 Hz.
The square wave
Earlier in this chapter, it was said that there can be an alternating current whose magnitude
never changes. You might at first think this is impossible. How can polarity reverse
without some change in the level? The square wave is an example of this.
On an oscilloscope, a perfect square wave looks like a pair of parallel, dotted lines,
one with positive polarity and the other with negative polarity (Fig. 9-2A). The oscilloscope
shows a graph of voltage on the vertical scale, versus time on the horizontal scale.
The transitions between negative and positive for a true square wave don’t show up on
the oscilloscope, because they’re instantaneous. But perfection is rare. Usually, the
transitions can be seen as vertical lines (Fig. 9-2B).
A square wave might have equal negative and positive peaks. Then the absolute
magnitude of the wave is constant, at a certain voltage, current, or power level. Half of
the time it’s +x, and the other half it’s -x volts, amperes, or watts. Some square waves
are lopsided, with the positive and negative magnitudes differing.
