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Energy of an uniformly charged bar of length L

  1. Sep 21, 2009 #1
    Hi All

    I would like to know why is it so difficult to calculate by integration the electric potential energy of an uniformily charged bar of length L and total charge Q. I have tried hard, thinking it would be as easy as the case of a uniformly charged sphere but my efforts failed.

    Why these two systems are so different ?

    Best Regards

  2. jcsd
  3. Sep 21, 2009 #2


    Staff: Mentor

    Because there is less symmetry in the bar case than in the sphere case.
  4. Sep 21, 2009 #3
    Ok with respect to the simmetry, but how this argument relates to divergence in the computation of the energy's integral?


  5. Sep 21, 2009 #4


    User Avatar
    Science Advisor

    The radius of your line charge is zero. This means an infinite E field at the wire.
    The energy calculation is infinite because of this. The energy of a point charge is also infinite. You need finite fields to get finite energies.
  6. Sep 22, 2009 #5
    ok. good.


  7. Oct 1, 2009 #6
    Let me just provoke a little further. If my bar (or rod) of length L were a cylinder surface uniformly charged, then no infinite would appear in the energy calculation. Is it?

    best regards,

  8. Oct 2, 2009 #7


    User Avatar
    Science Advisor

    That's right.
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