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E and M -Griffiths 3.20

  1. Mar 10, 2008 #1
    1. The problem statement, all variables and given/known data
    Find the potential outside a charged metal sphere (charge Q, radius R) placed in an otherwise uniform electric field E_0. Explain clearly where you are setting the zero of potential.

    2. Relevant equations

    3. The attempt at a solution
    If I set the 0 of the potential at the surface of the sphere (which I can do because it is at an equipotential because it is a conductor), then I don't see how the analysis is different than Example 3.8?
    Last edited: Mar 10, 2008
  2. jcsd
  3. Mar 10, 2008 #2


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    If the sphere is at V=0, then infinity is at -Q/R.
    To be consistent with other potentials, you should keep phi(infinity)=0.
    Then the analysis is the same, but phi=Q/R+phi(grounded sphere).
  4. Mar 10, 2008 #3
    How did you get that? Did you do that in your head?
  5. Mar 10, 2008 #4
    Err -- given the uniform E field, the potential at infinity is always infinite. I think ehrenfest had the right idea -- make the conductor of potential zero, and do the necessary algebra. I don't have the book, so I don't know if it reduces to some other problem in there.
  6. Mar 10, 2008 #5


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    I meant the potential due only to the sphere, which is added to the -zE_0.
    If the conducting sphere has charge Q, there will be the additional potential,
    Q/R. Yes, done in my head.
    But, do it your way if you want.
  7. Mar 11, 2008 #6
    rethink where you set V = 0 and consider example 3.8
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