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Homework Help: Help with with finding the magnitude of a charge

  1. Jan 19, 2009 #1
    1. The problem statement, all variables and given/known data
    The weight of a 1.0 kg object of charge Q is just balanced by another object of equal but opposite charge fixed to a support 74 cm above it. What is the magnitude of the charge Q (in μC)?

    2. Relevant equations
    F=ke|q1||q2| / r^2

    3. The attempt at a solution
    F=(1kg)(9.8m/s^2) = 9.8

    9.8=8.99e9(q^2) / .74^2 = 24.4 uC

    I think that's the right way, I'm not too sure.

    Last edited: Jan 19, 2009
  2. jcsd
  3. Jan 19, 2009 #2

    Yes equating the attrative force of electromagnetism to it's weight is the right way to go about the problem

    0 = (kQq / r2) - mg

    mg = (kQ2 / r2)

    mgr2 = (kQ2

    mgr / k = Q2

    sqrt{ (mgr / k) } = Q

    If that is what you've done then good on you! Numerically my answer comes out at: 2.84*10-5 C

    In μC , 24μC I think...

    Hope this helps!
  4. Jan 19, 2009 #3
    Yes, 24.4 is the answer, thank you for the help.
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