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The reason why an electromechanical alternator produces output sine- wave AC is due to the physics of its operation. The voltage produced by the stationary coils by the motion of the rotating magnet is proportional to the rate at which the magnetic flux is changing perpendicular to the coils (Faraday’s Law of Electromagntic Induction.
 

That rate is greatest when the magnet poles are closest to the coils, and least when the magnet poles are furthest from the coils. Mathematically, the rate of magnetic flux change due to a rotating magnet follows that of a sine function, so the voltage produced by the coils follows that same function. If we are o follow the changing voltage produced by a coil in arn alternator from any point on the sine wave graph to that point when the wave shape begins to repeat tsel, we would have marked exactly one cycle of that wave. This is most easily shown by spanni the distance between identical peaks, but may bc mcasured between any corresponding points on the graph.

The degree marks on the horizontal axis of the graph represent the domain of the trigonometric sine function and also the angular position of our simple two- pole alternator shaft as it rotates.

MEASUREMENTS OF AC MAGNITUDE
 

So far we know that AC voltage alternates in polarity and AC current alternates in direction. We also know that AC can alternate in a variety of different ways, and by tracing the alternation over time we can plot it as a “waveform”. We can measure the rate of alternation by measuring the time takes for a wave to evolve before it repeats itself (the “period”), and express this as cycles per unit time, of “frequency”. In music, frequency is the same as pitch, which is the essential property distinguishing one note from another.

However, we encounter a measurement problem if we try to express how large or slan AC quantity is, With DC, where quantities of voltage and current are generally stable, we have ttlerouble expressing how much voltage or current we have in any part of a t. B ho do you grant a single measurement of magnitude to something that is constantly changing? One way to express the intensity, or magnitude (also called the amplitude) of an AC quantity is to measure its peak height on a waveform graph.