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Homework Help: Method of Images- Force on Plane

  1. Oct 10, 2012 #1
    1. The problem statement, all variables and given/known data
    What is the force on an infinite conducting plane due to two point charges: a +2q located at a height 4h above the plane and a -q charge located at a height 2h above the plane, directly beneath the other point charge.

    2. Relevant equations

    3. The attempt at a solution
    I know that due to Newton's 3rd Law, I can just calculate the force on a single charge (due to an image charge symmetrically below the plane) and the negative of this force is the force on the plane. Does this method extend to 2 charges?

    Then I would think that I place a -2q charge at -4h, and a +q charge at -2h, and then calculate the two forces on the real charges due to the 3 other charges present... then the opposite of that force is the force ON the plane?

    Can you add the forces like that?
  2. jcsd
  3. Oct 12, 2012 #2


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    Yes, I think that is correct. However, you don't need to include the force of one real charge on the other real charge, just the force of each image charge on each real charge.
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