## Lecture 7: Capacitance, Field Energy

author: Walter H. G. Lewin, Center for Future Civic Media, Massachusetts Institute of Technology, MIT
recorded by: Massachusetts Institute of Technology, MIT
published: Oct. 10, 2008,   recorded: February 2002,   views: 3332
released under terms of: Creative Commons Attribution Non-Commercial Share Alike (CC-BY-NC-SA)
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# Description

"...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...."

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1 fatimabpla, April 29, 2012 at 12:26 p.m.:

thank you very match Mr Walter.

2 HAYMERO WALLE, May 11, 2012 at 11:12 a.m.:

It is good, but you are prepared a question based on the topic and solve it.

3 newtspeare, September 21, 2013 at 2:40 a.m.:

This is an excellent series, and I have watched all the lectures several times. But at the start of this lecture he comes up with a very dodgy explanation as to why ‘the work done is equal to HALF the electric field …..’ If you can accept that he is talking nonsense from 2.20 to 3.40, then it should make it easier to follow the rest of the lecture. To decide on the correct explanation see my website http://squishtheory.wordpress.com/mit...