Question

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

Answer 1

In a DC circuit, 1 Ampere is defined as 1 Coulomb per second, but for AC circuit, the direction of current keeps changing with the source frequency and the attractive force would average to zero.
Instead, Joule’s heating is used to define the rms value of AC current. RMS value of AC is equal to the value of DC required to generate the same amount of heat through a given resistor in a given time.