## Lecture 5: E = -grad V, More on Equipotential Surfaces, Conductors, Electrostatic Shielding (Faraday Cage)

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: 32936
released under terms of: Creative Commons Attribution Non-Commercial Share Alike (CC-BY-NC-SA)
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# Description

"So today no new concepts, no new ideas, you can release a little bit and I want to discuss with you the connection between electric potential and electric fields.

Imagine you have an electric field here in space and that I take a charge Q in my pocket, I start at position A and I walk around and I return at that point A.

Since these forces are conservative forces, if the electric field is a static electric field, there are no moving charges, but that becomes more difficult, then the forces are conservative forces and so the work that I do when I march around and coming back at point A must be zero. It's clear when you uh look at the equation number three that the potential difference between point A and point A is obviously zero. I g- start at point A and I end at point A and that is the integral in going from A back to point A of E dot DL and that then has to be zero.

And we normally indicate such an integral with a circle which means you end up where you started. This is a line now this is not a closed surface as we had in equation one..."

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