The sine wave

At times, alternating current has a sinusoidal nature. This means that the direction of current alters at regular intervals, and current vs time curve is shaped like the trigonometric sine function. The waveform in figure is a sine wave.

Any alternating current wave which consists of a single frequency will have a perfect sine wave shape. And perfect sine wave current has 1 component frequency. Practically a wave might be so close to sine wave that it looks like exactly the sine function on an oscilloscope; in reality there are traces of other frequencies present. Imperfections are too small to be seen, but pure, single-frequency alternating current not only looks perfect, but is a perfect replication of trigonometric sine function.

The current at wall outlets in your house has an almost perfect sine waveshape, with the frequency of 60 Hz.

The square wave

It was said that there can be an alternating current the magnitude of it never changes. You might at 1st think this is impossible. How can polarity reverse without change in level? The square wave is the main example of this.

On the oscilloscope, a perfect square wave looks like a pair of parallel, dotted lines, one with the positive polarity and other with negative polarity. The oscilloscope shows graph of voltage on vertical scale, versus time on horizontal scale. The transitions maong negative and positive for the true square wave do not show up on the oscilloscope, because they are instantaneous. But perfection is quite rare. Usually, transitions can be seen as the vertical lines.

A square wave might have equal positive and negative peaks. Then the absolute magnitude of wave is constant, at the certain voltage, current, or power level. Half of the time it is +x, and the other half it is -x volts, amperes, or watts. Some square waves are lopsided, having positive and negative magnitudes differing.