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Colums law problem

  1. May 3, 2007 #1
    1. The problem statement, all variables and given/known data

    Take three charged objects and place them on the x-axis. The first charge is + 11 microC, and is placed at +0.85 m. The second charge is -4.6 microC, and is placed at -0.22 m. The third charge is placed at the origin. If the net force on the third charge has a magnitude of 6.0(10-4) N, what is the magnitude of the charge?


    2. Relevant equations

    F=k (q1q2)/r^2

    3. The attempt at a solution


    i can find each charge magnitude in respect to the 3rd charge with the equation, but don't no where to go from there.
    Thanks
     
  2. jcsd
  3. May 3, 2007 #2

    Doc Al

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    Staff: Mentor

    Show exactly what you did. Hint: Call the unknown charge Q. Use Coulomb's law to solve for the force exerted on that charge by each of the other two charges.
     
  4. May 3, 2007 #3
    ok i got my answer but im not sure if its right.
    Basicly i added 2 of coulombs equations 2gether.
    6E-4= K(4.6xq2)/(0.22^2)+ K(11xq2)/(0.85^2)
    after some algebra
    6E-4=8.5E11q2+1.37E11q2
    Factor out q2
    q2= 6E-4/9.87E11
    i got q2 to be around 6E-16 Micro C which = 6E-10 C
    the answer is suppose to be "6.1E-10C". It might rounding error or i might be wayyyy off.
    Can someone please check this?
    Thanks
     
  5. May 3, 2007 #4

    Doc Al

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    Staff: Mentor

    The charges are in microC, so they should be 4.6E-6 & 11E-6 C in this equation. That should explain why your answer is so far off.
     
  6. May 3, 2007 #5
    yes, but is my train of thought even close to what your suppose to do?
     
  7. May 3, 2007 #6

    Doc Al

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    Staff: Mentor

    Your method is perfectly correct. Fix those charge values and you should get the right answer.
     
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