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Homework Help: Point Charges simple Question

  1. Apr 21, 2008 #1
    1. The problem statement, all variables and given/known data
    Charge A and charge B are 3.00 m apart, and charge A is +2.00 C and charge B is +3.00 C. Charge C is located between them at a certain point and the force on charge C is zero. How far from charge A is charge C?

    2. Relevant equations

    3. The attempt at a solution
    I am reviewing for my final and I can't remember how to do this!!! I wanted to use...
    F = K*Q1*Q2 / r^2
    But I can't have 0 in the denominator so I know I am missing something obvious...but I am not seeing it.
  2. jcsd
  3. Apr 21, 2008 #2
    you're solving for when the NET force is zero. let A be charge of A, B be charge of B, and C for C, etc. distance d apart. so the distance from C to A is R1, and C to B is R2 then R1 + R2 = d.
    and the 2 forces need to equal eachother - i.e. F1 = kAC/(R1^2) = kBC/(R2^2) does that make sense?
  4. Apr 21, 2008 #3
    Well yea but I don't have R2? So I tried using like R2 = 3- R1 but I can't get that to work...
  5. Apr 21, 2008 #4
    thats how you have to do it, R1 and R2 are linked by the constant d (3 in this case). you'll end up with a binomial or something that you can solve numerically if nothing else.
    A/(R1^2) = B/(3-R1)^2
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