MIT 8.02 Physics II: Electricity and Magnetism - Spring 2002

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: 32904
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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A Faraday cage is a container made of conducting material, such as wire mesh or metal plates, that shields what it encloses from external electric fields. In our experiments, a Faraday cage can be used to prevent external electromagnetic interference (EMI, or noise) from interfering with our neural recordings. As you know, the neural signals that we are recording are very small (on the order of micro-volts), and we use our Spikerboxes to amplify these small signals to a large enough amplitude that we can hear and record them. Depending on our environment, though, there can be electromagnetic, radio, microwave, or other types of invisible emissions that can travel through the air and interact with the metal needles and wire that we use as electrodes. The metal then propagates the noise signal like an antenna into our neural recordings, interfering with or even drowning out our recordings so that all we hear, in the worst cases, is a radio station! A Faraday cage then can be used to block many of these noise sources.

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