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Homework Help: Rearranging Coulomb's Law

  1. Aug 9, 2009 #1
    1. The problem statement, all variables and given/known data
    When a -9.0 micro-coulomb charge is placed 0.12 cm from a charge q in a vacuum, the force between the 2 charges is 850 N. What is the value of q?

    2. Relevant equations
    F = kq1q2/r2

    3. The attempt at a solution
    q2 = Fr2/kq1
    q2 = (850 N)(0.0012 m2)/(9.00 x 109 Nm2/C2)(-9.00 x 10-6 C)
    q2 = -1.22 x 10-18 C

    My question is am I doing this right? The answer that my book gives is 1.5 x 10-8 C.
    Last edited: Aug 9, 2009
  2. jcsd
  3. Aug 9, 2009 #2
    It's radius squared right?
  4. Aug 9, 2009 #3
    Yes it is sorry I corrected my work above! Thanks!
  5. Aug 9, 2009 #4
    Well since you now (correctly) squared the radius, what do you get as an answer?
  6. Aug 9, 2009 #5
    The answer that I get is -1.22 x 10-18 C. When I worked the question I had squared the radius.
    Last edited: Aug 10, 2009
  7. Aug 10, 2009 #6


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    Homework Helper

    Well, the answer I get is the same one your book gives, so you're probably doing something wrong with the math. I'm not sure where that 10-18 is coming from...
  8. Aug 10, 2009 #7
    Thanks for all your help! It was me trying to put too much info into my calculator at once. If I break it up figure out the top then figure the bottom and divide I get the right answer except mine is negative.

    Thanks again!
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