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Homework Help: Question on 2d charge arrays

  1. Jan 31, 2012 #1
    1. The problem statement, all variables and given/known data
    Derive and simplify an expression for the electrical potential energy of the charge configuration. It's an equilateral triangle with charges +q, -q and +q.

    2. Relevant equations

    3. The attempt at a solution
    I know how to do this with squares (it's basically exploding the symmetry). For example in a square there are charges #s1,2,3 and 4. I do the equation with 1 respect to 2,3 and 4. Then with 2 respect to 3 and 4. Finally, 3 respect to 4. I see which terms cancel out, then it's all algebra.

    I did this in this triangle, getting -kq2/r as the final expression, which for some reason doesn't convince me (perhaps because it's too simple? lol).

    I don't know if in the square you are ''unconsciently'' multiplying by the sin90, therefore I should multiply by sin60 here. If so, is it just multiplying sin60 by each term?

    I do know that electrical potential energy is a vector quantity and not a scalar quantity like voltage, right?

  2. jcsd
  3. Jan 31, 2012 #2
    so would the expression be -kq2/r *sin60?
  4. Feb 1, 2012 #3


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    Homework Helper

    The potential energy is a scalar quantity. Show your work in detail, please.

    Last edited: Feb 1, 2012
  5. Feb 1, 2012 #4


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    It is correct if you mean the side of the triangle by "r".

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