Alternating Current

In ac, the polarity reverses at regular intervals. The instantaneous amplitude (that is, the amplitude at any given instant in time) of ac usually varies because of the repeated reversal of polarity. But there are certain cases where the amplitude remains constant, even though the polarity keeps reversing. The rate of change of polarity is the variable that makes ac so much different from dc. The behavior of an ac wave depends largely on this rate: the frequency.