If I take a coil, and let's just do one loop for simplicity, here's my coil and I run some current through it, I. I know that I generate a B field. Okay, but the coil itself says 'wait a minute, there's a B field being generated that is changing the flux that I see and I don't like that.' Lenz's law says I should not like a change in flux. And so there is an EMF that is developed to oppose that. Okay, so the changing B, the changing B field, through itself, through the same coil, produces an EMF. And this is in fact what we call the back EMF, because it's trying to oppose that. What is that quantity? Well, the EMF is negative L delta I over delta t, Where L is something called self inductance. Okay, and so here's the catch: any coil is an inductor. Inductors are something that you've probably played around with already in the lab. Any coil loop is an inductor. Meaning if you try to run current through it, it's going to generate a B field. The loop itself doesn't like that changing B field, and so it's going to fight back against you with this back EMF, and this is the strength of that back EMF. So we don't have to do it with just one loop, we can do it with a bunch of loops.