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Homework Help: What is the charge on the second sphere?

  1. Jan 7, 2010 #1
    1. The problem statement, all variables and given/known data

    Two small plastic spheres are given positive electrical charges. When they are a distance of 14.2cm apart, the repulsive force between them has a magnitude of 0.220 N.

    A. What is the charge on each sphere if the two charges are equal? Ans: 7.02X10^-7
    B. What is the charge on first sphere if it has four times the charge of the other?
    C. What is the charge on the second sphere?

    2. Relevant equations
    Coulomb's Law --> F=kQ1Q2/r^2

    3. The attempt at a solution

    Already solved part A. Having trouble with part b. Both have a charge of 7.02X10^-7 C. The second question says if one sphere is 4 times the other, what is the charge. Wouldn't that just be 4 multiplied by 7.02X10^-7? Thats what I did, and its wrong.
  2. jcsd
  3. Jan 7, 2010 #2


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    Homework Helper

    No, that charge is on the spheres if both charges are equal. The distance and force between the spheres is given, and the charges asked if one charge is four times the other one.

  4. Jan 7, 2010 #3
    Nevermind, got it figured out.
  5. May 5, 2011 #4
    Could someone please help me with problem B, except 17cm apart and the Magnitude 0.300N, looking for Q1, Q2. Any help would be e great, I already got problem A answered which is 0.98*10^-6C.
  6. May 5, 2011 #5


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    If Q2 is four times Q1 then solve the equation

    0.3N =k*4Q1*Q1/0.17^2.

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