In the direct current circuit, when you have a switch and you turn it on, you get the coil to generate a current to the opposite direction but the magnitude isn't big enough to offset the main one. But in the case of alternating currents, the coil seems to generate enough current up to the magnitude of the main one (because when you calculate the overall emf, its zero). Why is it different? Perhaps because there is always a change in the alternating current? But how do you know that the overall emf is zero? Also, what happens when you increase the magnitude of the voltage so that the total emf goes over zero? And what happens when you lower it?