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Coulombs Law Question

  1. Oct 19, 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

    Solution:
    1)
    F2 = (6.0x10^-8 N)xr1 / (2r1)^2
    = (6.0x10^-8 N) / 4
    = 1.5 x 10^-8 N

    2) q1=2q1
    q2=3q1

    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

    I can do the actually solution to this problem, but my only question is what are the values of q1 and q2 and r? Since they're literally exactly the same in both questions, does it mean that they all just double and triple from the original? Meaning r = 4r1, q1 = 6q1, q2 = 6q2

    Does anyone have any suggestions?

    Thank You
     
  2. jcsd
  3. Oct 19, 2009 #2

    cepheid

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    Staff Emeritus
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    Gold Member

    I don't understand where you get the factors of 4 and 6 from.

    Double = multiply by a factor of 2.

    Triple = multiply by a factor of 3.

    The only difference is that, instead of making each of these changes separately, ALL three changes are now to be made at the same time so that (primed symbols represent the "new" quantities after the changes):

    r' = 2r

    q1' = 3q1

    q2' = 2q2
     
  4. Nov 3, 2009 #3
    ohh okay, sorry i thought it mean't something else.

    Do i ended up setting F1/F2 = (q1q2/r1^2) / ((3q1)(2q1)/2r1^2)

    I rearranged it to be F2 = 6F1/4r1 (after cancellations and rearranging)

    Im not sure if i rearranged it correctly...
     
  5. Nov 3, 2009 #4
    sorry i forgot to cancel r1, its actually F2 = 6F1/4
     
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