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Homework Help: 3 Equal Charges in Equilateral Triangle - Find KE @ Inf

  1. Apr 10, 2013 #1
    1. The problem statement, all variables and given/known data

    Question is attached.

    I know there's a few ways to solve this, but I'm wondering specifically why my integral of F ds isn't working.

    2. Relevant equations

    F = k * q^2 / r^2
    U = ∫ F ds
    cos 30 = √3/2
    s= rcos30

    3. The attempt at a solution

    U = 2 * cos 30 * k * q^2 ∫ 1/r^2 ds
    U = √3 * k * q^2 ∫ 1/r^2 * √3/2 dr - (a to inf)
    U = 3/2 * k * q^2 * 1/a

    Attached Files:

    Last edited: Apr 10, 2013
  2. jcsd
  3. Apr 10, 2013 #2
    Any help would be appreciated. No one seems to be able to help me.
  4. Apr 11, 2013 #3


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    Science Advisor
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    Gold Member

    I assume your model is to let all three move apart some small amount, symmetrically. dr will be the increase in distance between any pair, while ds is the distance each moves. If so, I get dr = ds √3, which does not seem to be what you have used.
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