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Homework Help: Electric Potential of 2 Point Charges

  1. Jun 9, 2012 #1
    1. The problem statement, all variables and given/known data
    The 2.1mm diameter spheres in the figure are released from rest. What are their speeds vc and vd at the instant they collide?

    C has 2nC of charge and a mass of 1g.
    D has -1nC of charge and a mass of 2g.
    The starting distance between the two spheres is 10mm. Each sphere as a diameter of 2.1mm.

    2. Relevant equations
    Q is the charge and r is the distance between the two spheres.

    3. The attempt at a solution
    ΔKC+ΔUC = 0 = ΔKD+ΔUD
    Ki for spheres C and D are 0 since the spheres begin at rest:

    The rest of my attempt is basically putting the numbers in and solving for KCf in terms of KDf and then using that to get KDf. I just want to know if I am heading in the right direction.
  2. jcsd
  3. Jun 9, 2012 #2
    Hi vforvirginia! welcome to pf!! :smile:

    In your energy conservation equations you are comparing the energies of two different balls and making them equal. I think that should be re-framed as, the total energy of the balls initially is equal to their total energy at the end.
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