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Homework Help: Electrostatic force question with coulombs law

  1. Jan 11, 2010 #1
    1. The problem statement, all variables and given/known data

    A particle of charge of +4.20 x 10^-6 C is 12.0 cm distant from a second particle of charge of -1.70 x 10^-6 C. Calculate the magnitude of the electrostatic force between the particles.

    2. Relevant equations

    Coulombs law is F = (K*Q1*Q2) / R^2 where k = 8.99*10^9, q1 and q2 are the charges, and R is the distance in meters

    3. The attempt at a solution

    Im hoping im just making a simple mistake and not seeing it for this question cause this really seems like plug and chug.

    K Q1 Q2 R
    so I did (8.99 x 10^9)(4.20x10^-6)(-1.70x10^-6) / (.12)^2

    = -4.457 N

    that gave me a wrong answer so i tried different variations of putting the decimal like -4457.541, 44.57 and -44.57. What am i doing wrong, i might have just fudged up since I'm doing this by hand but I'm not seeing it. Either that or I have done something fundamentally wrong and don't know any better. It's been a while since I've done a class like this so any help is appreciated.
  2. jcsd
  3. Jan 11, 2010 #2


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    Homework Helper

    The problem asks the magnitude of the force, which is positive. The number itself is good.

  4. Jan 12, 2010 #3
    heh well like i said it's been awhile, I kept thinking since they attracted the force would be negative. Thanks a lot though, guess i just needed a second pair of eyes
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