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Coulomb's Law

  1. Dec 10, 2005 #1
    I'm stuck on the following problem: Point charges of q_1=-12nC are placed at (-4,0) cm and (4,0) cm. Determine the magnitude of the electrostatic force on a third charge of q_2=20nC placed at (0,8) cm. So because of symmetry I figured that the x-components of the forces will cancel each other and the total force will be 2 times the y-component. So I did 2*k*q_1*q_2*sin63.4/(0.04^2+0.08^2) and got 4.8e-4 N but the answer is supposed to be 3.1e-8 N. Any help would be appreciated, thanks.
  2. jcsd
  3. Dec 10, 2005 #2


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    What about the distance (r) ?
  4. Dec 10, 2005 #3


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    Just check all your units. That's usually the problem when your off by several orders of magnitude.
    Last edited: Dec 10, 2005
  5. Dec 10, 2005 #4
    my r^2 is (0.08^2+0.04^2). I'm pretty sure my units are right, I did 2(8.99e9)(12e-9)(20e-9)(sin63.43)/(0.08^2+0.04^2) = 4.8e-4 N
  6. Dec 10, 2005 #5


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    It looks like your answer is correct. If you copied the problem correctly, the solution must be wrong. This is clear if you do an order of magnitude estimate: the charges are of order 10^-8 C, and they are seperated by about 10^-1 m, so the force should be of order 10^10*10^-8*10^-8/(10^-1)^2, or about 10^-4 N.
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