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2 suspended balls with charge q. find q

  1. Sep 18, 2009 #1
    1. The problem statement, all variables and given/known data
    Two Styrofoam balls with the same mass m = 7E-8 kg and the same positive charge q are suspended from the same point by insulating threads of length L = 0.92 m. The separation of the balls is d = 0.024 m. What is the charge q?

    I am unable to add a graphic at this time. But i understand this is a common electrophysics problem, i think the problem describes it pretty well.

    2. Relevant equations

    F = (k*q1*q2)/r^2

    3. The attempt at a solution

    One thing I will do first is find the value "h", the vertical distance between 1/2d and the ceiling (essentially completing the triangle)

    Well I wasn't exactly sure where to start, and that may be part of the problem, so here is the thought process, I hope you can follow.

    I know Fnet = 0 Fy = 0 Fx = 0

    I know to get q i need to find F from each charge.

    (note: FTy represents the Tension Force from the string in the y direction.)

    So Fy = -mg + FTy = 0 so mg = FTy

    How do find FTy? I understand it is a component of FT. would it be FTy = FT x (h/L) ??

    So Fy = -mg + FT x (h/L) and from this i could get FT???? is that correct?

    If this is correct, I can also find FTx.

    FTx = FT x (1/2d)/L

    And Fx should = (FT x (1/2d)/L) - (k*q1*q2)/d^2

    Since Fx = 0 i can set the above equation to 0 and solve for q??

    This is what I tried to do anyways, does this make sense?

    Any input is appreciated!
  2. jcsd
  3. Sep 18, 2009 #2


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    Homework Helper

    Your approach is correct.
    FTy = mg..............(1)
    FTx = k*q1*q2/d^2......(2)
    Substitute the values of FTx and FTy, then divide eq.(1) by eq.(2). From that you get the value of q.
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