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Homework Help: Finding electric force between two particles

  1. Sep 11, 2010 #1
    1. The problem statement, all variables and given/known data
    Two particles with electric charges Q and −3Q are separated by a distance of 2.5 m.
    If Q = 4.9 C, what is the electric force between the two particles?


    2. Relevant equations

    Coloumb's equation: F=k*q1*q2/r^2
    k=9.0*10^9

    3. The attempt at a solution
    I just tried to plug in the variables into the above equation.
    so I got F= (9e9)*(4.9)*(3*4.9)/((2.5/2)^2)
    F= 4.15e11
    Apparently, that is not the correct answer, but I don't know what I did wrong!
    I also tried to divide that number by 2 to get F= 2.07e11 which was also not right.

    What am I doing wrong?
     
  2. jcsd
  3. Sep 11, 2010 #2

    kuruman

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    If the separation between the charges is r = 2.5 m, then r^2 = 2.5^2, not (2.5/2)^2. I am not sure what all those divisions by 2 are meant to accomplish.
     
  4. Sep 11, 2010 #3

    lewando

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    Why are you dividing the distance by 2? Consider your signs, too.

    [edit: beaten by kuruman. not happy]
     
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