"...assemble charges, I have to do work, we discussed that earlier. And we call that electrostatic potential energy. Today, I will look at this energy concept in a different way, and I will evaluate the energy in terms of the electric field. Suppose I have two parallel plates, and I charge this one with positive charge, which is the surface charge density times the area of the plate, and this one, negative charge, which is the surface charge density negative times the area of the plate. And let's assume that the separation between these two is H, and so we have an electric field, which is approximately constant, and the electric field here is sigma divided by epsilon zero.

And now, I'm going to take the upper plate, and I'm going to move it up. And so as I do that, I have to apply a force, because these two plates attract each other, so I have to do work. And as I move this up, and I will move it up over distance X, I am creating here, electric field that wasn't there before...."