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Homework Help: Applying Coulomb's Law

  1. Oct 19, 2012 #1
    1. The problem statement, all variables and given/known data
    What is the distance between two charges of -5.00 C each if the force of electrostatic repulsion acting on them is 4.00 x 10^3 N?

    q1 = -5.00 C
    q2 = -5.00 C
    Fe = 4.00 x 10^3
    k = 8.99 x 10^9 Nm^2/C^2

    2. Relevant equations

    Fe = kq1q2/r^2

    3. The attempt at a solution

    I rearranged the equation above and got:

    r = (sqrt) kq1q2/Fe

    r = (sqrt) (8.99 x 10^9 Nm^2/C^2)(5.00 C)(5.00 C) / (4.00 x 10^3 N)
    = 7495.832175 m

    To three significant digits my answer would be 7.50 x 10^ 3 m
  2. jcsd
  3. Oct 19, 2012 #2


    User Avatar
    Homework Helper
    Gold Member

    Hi Kennedy111,

    Your math and approach look fine to me. :approve:

    But just for the sake of practicality, you might wish to double check that you're using the correct units from the beginning. Five Coulombs (whether negative or positive) is a lot of charge. Most charges in electrostatics problems I'm used to use units of μC.

    I'm just sayin', in the system you're modeling, as you've expressed it, there is still a four kilo-newton force between the charges, even after separating them by seven and a half kilometers (several miles). That's a lot of charge!
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