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Homework Help: Electric force between two charges on z-axis

  1. Oct 28, 2005 #1
    Two charges -2q and +q are located on the z-axis at distance 'd' and '3d' respectively. The x-y plane is a grounded conductor. Find the force on the charge +q.
    My work:
    Let the force be F. Distance between the charges is '2d'.
    [tex]F = \frac{1}{4\pi \epsilon_{0}}\left(\frac{(-2q)(q)}{{(2d)}^2}\right)[/tex]
    Can I apply the Coulomb's equation here the way I have done? Will the grounded x-y plane have any effect on the force?
  2. jcsd
  3. Oct 28, 2005 #2


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    A better question is: does the charge below the grounded conductor have any effect on the force?
  4. Oct 29, 2005 #3
    Umm...the charges mentioned here are above the grounded plane. The grounded plane has a zero potential. Are you talking about image charges?
  5. Oct 30, 2005 #4
    Can someone help me proceed?
  6. Oct 30, 2005 #5

    Physics Monkey

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    You know the grounded conductor will "draw up charge from infinity" so that it can maintain a constant potential over its surface. This charge will exert a force, right? How can you describe the field produced by a conducting sheet in the presence of point charges?
  7. Nov 5, 2005 #6
    Took me a while to read up on the theory. I am convinced this is an image problem. Well, the electric field is normal to the surface of the conducting sheet, right?
    [tex]\vec E = \frac{\partial V}{\partial z}\hat z[/tex]
    But what about the force?
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