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Solving Laplace's Eqn: Setting Zero of Potential 
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#1
Nov2010, 01:56 PM

P: 374

1. The problem statement, all variables and given/known data
I have two quick related questions that I think will help clear something up for me. (1) An uncharged metal sphere of radius R is placed in an otherwise uniform electric field E = E0 zhat. The field induces charge. Find the potential in the region outside the sphere. (2) Find the potential outside an infitely long metal pipe of radius R placed at right angle to an otherwise uniform electric field E0. 2. Relevant equations 3. The attempt at a solution Okay, so I am mostly comfortable with the solutions to these problems, with the exception of one key concept. Problem 1: If E=E0 zhat (for r>>R, then V for r>>R is E0z + C) The one hangup I have is that for problem (1), you set V=0 on the xyplane "by symmetry." Thus we can say C=0. I'm not sure I understand the reasoning behind setting V=0 for all z=0. Similarly, from problem (2), if we suppose that the electric field is pointing along z direction, so again, the form of the potential is E0z + C. Again, one should set V=0 on the xyplane, so C=0. I don't have a clear picture in my head of why you set V=0 on the xyplane. Any explanations of this concept would be greatly appreciated 


#2
Nov2010, 02:15 PM

Sci Advisor
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P: 2,603

Since the electric field is the gradient of the potential, [tex]\vec{E} =  \nabla V[/tex] (for electrostatics), a constant shift in the potential [tex]V' = V + c[/tex] doesn't change the electric field. Therefore we are usually free to define the location of [tex]V=0[/tex] by adding a constant term to the potential. In your examples, choosing [tex]V(z=0) = 0[/tex] is just the most convenient choice, but it was not the only one.



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