Why induced emf alternates?

Here is A Simple answer for this question

as you know that emf is induced in the conductor when there is change of flux linkage to the conductor which gives rise to the inducement of electric field in the conductor that provide drift velocity to the electrons hence emf is induced in the conductor but the last only till the change in flux in progress and becomes zero as soon as the flux linkages becomes constant hence we summaraize that for inducement of current there must be change in change of flux hence change in electric field that gives rise to allternating emf.