I encountered this text while reading about AC currents:
Both alternating current and direct current are measured in amperes. But how is the ampere defined for an alternating current? It cannot be derived from the mutual attraction of two parallel wires carrying ac currents, as the dc ampere is derived. An ac current changes direction with the source frequency and the attractive force would average to zero.
But if the ac currents in the two wires are always in phase, then they should always attract, since parallel currents attract each other; and the net force should not be 0. So why don't we use this to define an ampere for ac current?
