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Homework Help: Conductors and Gauss Law

  1. Feb 5, 2009 #1
    1. The problem statement, all variables and given/known data
    http://img21.imageshack.us/img21/8079/orderyy3.th.jpg [Broken]

    2. Relevant equations

    3. The attempt at a solution

    Well all I know that the smallest will be C, as it's 0 (inside the conductor), I think the next one is A, and I don't know about B and D
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Feb 5, 2009 #2


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    Inside the inner shell at A is 0, as for B and D which one is farther away from the charge source?
  4. Feb 5, 2009 #3
    so A and C are both 0? D is farther... but flux isn't associated with radius isn't it
  5. Feb 5, 2009 #4


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    Imagining a Gaussian sphere enclosing the charge as far out as A, it would have less charge than a sphere at B.
    The field at D will be smaller than B because it is spread out more in space.
    Pretty tough to compare A and D.

    I don't know how this works. What makes E = 0 in the conducting layer? It seems to me that the field due to the central charge would cause a charge separation in conducting layer C, so that (assuming central q is positive) there will be a layer of negative charge on the inner side of layer C and a positive one on the outer side. Do you know if that is correct?
  6. Feb 5, 2009 #5
    E = 0 in the conducting layer because the electric field is 0 inside the conductor
  7. Feb 5, 2009 #6


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    If there were an E-field inside the conductor ... and the electrons are free to move ... how can an E-field be maintained?

    Here's a lecture that covers this and more on electrostatics:

    (about 20 min in. Though the whole lecture is good.)
    Last edited by a moderator: Sep 25, 2014
  8. Feb 5, 2009 #7
    Therefore I was right that there is no E field inside the conductors right?
    Last edited by a moderator: Sep 25, 2014
  9. Feb 5, 2009 #8
    Yes, there should be no E-field inside a conductor, therefore the charge enclosed by a Gaussian surface drawn inside a conductor should be zero.
  10. Feb 6, 2009 #9


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    The inner shell isn't zero, because it's an insulating shell instead of conducting, so charge is distributed uniformly.
  11. Feb 6, 2009 #10


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    If the point A is inside the shell - shell being the operative word - and inside of that then, it is 0. If it were a solid sphere with uniformly distributed charge, you would be correct (if A is not the center). But they call it a shell.
  12. Feb 6, 2009 #11
    So what is the charge on A then?
  13. Feb 6, 2009 #12


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    Hmmm....this problem is certainly open to some interpretation!

    If it is indeed a shell, and if A is inside the shell; even that doesn't guarantee the field is zero inside. Charge isn't free to distribute itself on an insulator, so there is no guarantee that the charge Q is uniform over the insulating shell. If it isn't uniform, then the field will not be zero.

    I suspect that you are to assume that the charge Q is not distributed uniformly over the shell, and that the point A is very close to the charge, producing a large field at A. But that's just a guess.

    @EquinoX--- You should probably ask for clarification from your prof or a TA.
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