Electric Generators
DV = D[ (N B A cos(qBA) ] / Dt .
When we change the angle, qBA, with respect to time (qBA=wt) , the calculus gives us the following relation: DV = N B A w sin(qBA) , or VAC = Vo sin(wt) where Vo=NBAw, and where w = DqBA/Dt = 2pf.
This kind of voltage is an alternating voltage (AC voltage) since the sine function alternates between positive and negative.