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Homework Help: Coulombs Law Question

  1. Sep 7, 2010 #1
    1. The problem statement, all variables and given/known data

    Two negatively charged objects repel each other with a measured force of 6.3 N when they are 0.5 cm apart. If the excess charge on one of the objects is caused by 8.3 x 10^22 extra electrons, use Coulomb's Law to calculate the number of excess electrons on the second object.

    2. Relevant equations

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

    1e = 1.602 * 10^-19 c
    1c = 6.242 * 10^18 e
    k = 9.0 * 10^9 N*m^2/c^2

    3. The attempt at a solution

    I calculated the excess charge to be:

    (8.3*10^22 e) * (1.602 *10^-19) = 1.3 * 10^4 c

    I used this as q2 in the F = k*(q1)*(q2) / r^2 equation and rearranged to find that:

    q1 = (F * r^2) / k*q2
    q1 = (6.3N * 0.005m^2) / (9.0*10^9N*m^2/c^2)*(1.3*10^4c)
    q1 = 1.3 * 10^-18

    Since 1c = 6.242*10^18e, to find number of excess electrons i multiplied q1 by 6.242*10^18 = 8.1146 e

    So is the number of excess electrons on the second object 8.1 ?
  2. jcsd
  3. Sep 7, 2010 #2


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    Homework Helper

    Welcome to PF, dwach!
    Your calc looks good, and thank you for taking time to write out the method so clearly. I don't know what accuracy you need; I'm used to 3 digits so I keep 4 in intermediate steps and I got 1.318 x 10^-18 for the charge, which is 8.24 electrons.
  4. Sep 7, 2010 #3
    Thank you for the quick response Delphi51. In this question I need 2 digit accuracy so would you suggest that I carry 3 digits through the intermediate steps?
    Last edited: Sep 7, 2010
  5. Sep 7, 2010 #4


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    Homework Helper

    Yes, you must carry 3 digits to get 2 at the end. In practise, you probably will just keep all the digits your calculator produces for the first calc and only round when you write down the final answer.
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