Question

Both alternating current and direct current are measured in amperes. But how is the ampere defined for an alternating current?

Answer 1

For a Direct Current (DC), 1 ampere = 1 coulomb/sec

Direction of AC changes with the frequency of source with the source frequency and the attractive force would average to zero. Thus, the AC ampere must be defined in terms of some property that is independent of the direction of current. Joule’s heating effect is such property and hence it is used to define rms value of AC.

So, r.m.s. value of AC is equal to that value of DC, which when passed through resistance for a given time will produce the same amount of heat as produced by the alternating current when passed through the same resistance for same time.