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Verbatim question for GR8677 #52
Electromagnetism}Potential

By Gauss Law, if there are no charges inside the cube, then the electric field inside would be 0. The potential \phi is related to the field \vec{E} by \vec{E} = \nabla \phi, and thus since \vec{E}=0, one infers that \phi is constant. Since the potential function has to remain continuous its value everywhere inside of the cube is the same as that at the surface of the cube, which is given as V.

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Alternate Solutions
DaveyClaus
2006-11-17 05:10:14
One cannot use Gauss's Law to determine E=0 within. Only spherical and cylindrical Gaussian surfaces (and "pillboxes") allow one to take E out of the integral; we cannot automatically assume that E=0 because \rho=0 in this situation.

\hspace{15pt} Instead, we simply recall that Laplace's equation, \nabla^{2}V=0, with boundary conditions specified allows no maxima or minima within the boundary. So if the potential is V on the cube surface, potential cannot be greater than or less than V within.

\hspace{15pt} Now we are in a position to state that E=0 within...
Alternate Solution - Unverified
Comments
DaveyClaus
2006-11-17 05:10:14
One cannot use Gauss's Law to determine E=0 within. Only spherical and cylindrical Gaussian surfaces (and "pillboxes") allow one to take E out of the integral; we cannot automatically assume that E=0 because \rho=0 in this situation.

\hspace{15pt} Instead, we simply recall that Laplace's equation, \nabla^{2}V=0, with boundary conditions specified allows no maxima or minima within the boundary. So if the potential is V on the cube surface, potential cannot be greater than or less than V within.

\hspace{15pt} Now we are in a position to state that E=0 within...
Alternate Solution - Unverified

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