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Homework Help: Electrical potential energy: 3 charges in a line

  1. Feb 11, 2009 #1
    1. The problem statement, all variables and given/known data

    A positive charge, q1 is at (-20cm,0). An equal and opposite charge q2 is at (20cm, 0). A positive charge q3, equal to q1, is placed at (60cm,0). What is the potential energy of the three charges??


    2. Relevant equations

    EPE= kq / r


    3. The attempt at a solution
    EPE = k( (5 X 10^-9)/.2) + (5 X 10^-6)/.6) - (5 X 10^-6)/.2))
    = wrong answer
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Feb 11, 2009 #2
    It looks like you have additional information to provide such as the charges of q1,q2, q3 also your equation for electric potential energy is incorrect should be:
    [tex] U = k \frac{q_a q_b}{r_{ab}}[/tex]
    Finally I need to note that r is not the position but rather the relative distance between charges a and b. With these adjustments the answer should clear up a bit
     
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