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Using Coulombs Law-Comparative Analysis

  1. Nov 3, 2009 #1
    1. The problem statement, all variables and given/known data
    Look back at the Example in the Using Coulombs Law-Comparative Analysis section. Calculate the magnitude of F2 if both changes occurred simultaneously-that is, if the distance between the charges doubled and the size of once tripled, while the size of the other doubled.

    This is the original example:

    If the electrostatic force between two charges is 6.0 x 10^-8 N, what effect would each of the following changes have on the magnitude of the force?
    1) the distance between the charges is doubled
    2)One charge is doubled and the other is tripled

    F2 = (6.0x10^-8 N)xr1 / (2r1)^2
    = (6.0x10^-8 N) / 4
    = 1.5 x 10^-8 N

    2) q1=2q1

    F1/F2 = q1q2/q1q2

    (6.0x10^-8 N)/F2 = q1q2 / (2q1)(3q2)

    F2 = (6.0x10^-8N)(6)
    = 3.6 x 10^-7 N

    This is the solution to the example!!

    2. Relevant equations

    3. The attempt at a solution
    r' = 2r

    q1' = 3q1

    q2' = 2q2

    First i set F1/F2 = (q1q2/r1^2) / ((3q1)(2q1)/2r1^2)

    Then after i rearranged it and cancelled out q1q2 and r1
    i ended up with the equation F2 = 6F1/4

    then i put in F1


    F2 = 6(6.0 x 10^-8N) / 4
    = 9.0 x 10^-8 N

    Therefore, the magnitude of F2 is 9.0 x 10^-8 N

    I dont think i rearranged it correctly, can someone please confirm. Thank You
  2. jcsd
  3. Nov 3, 2009 #2


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    It looks like you combined parts (1) and (2) together. The changes are made independently, so the answer to part (1) is F1/4 and the answer to part (2) is 6 F1.
  4. Jan 20, 2011 #3
    Would that mean that F2 is (3/2) of F1? if they were combined.

    Because I obtained the same answer.
  5. Jan 20, 2011 #4
    Why do you think that your answer is wrong?

    It seems correct to me.
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