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Homework Help: Electrostatic force of Repulsion

  1. Sep 3, 2010 #1
    1. The problem statement, all variables and given/known data
    Identical isolated conducting spheres 1 and 2 have equal charges and are separated by a distance that is large compared with their diameters (Fig. 21-22a). The magnitude of the electrostatic force acting on sphere 2 due to sphere 1 is F = 6.5 N. Suppose now that a third identical sphere 3, having an insulating handle and initially neutral, is touched first to sphere 1 (Fig. 21-22b), then to sphere 2 (Fig. 21-22c), and finally removed (Fig. 21-22d). What is the magnitude of the electrostatic force F' that now acts on sphere 2?


    2. Relevant equations

    F = (Q1*Q2)K/r^2

    3. The attempt at a solution

    so each touch takes away half the total charge in the beginning right?
    first i thought it was half taken away from each sphere
    so the final would be Q1 = Q1/ 2 and Q2 = Q2/2 and since theyre the same
    i just combined them and made it Q1 *Q2 = Q/4
    k is constant
    r^2 is constant
    so i though the answer would be 6.5/ 4
    ......but it's not
    am i thinking the right way? what am i doing wrong?
     
  2. jcsd
  3. Sep 3, 2010 #2

    Doc Al

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    Staff: Mentor

    When the conducting spheres touch, charges rearrange so that each sphere ends up with half the total charge.

    If the original charge on the first two spheres is Q:

    What's the charge on sphere 1 and sphere 3 after they touch?

    What's the charge on sphere 2 and sphere 3 just before they touch?
    What's the charge on sphere 2 and sphere 3 after they touch?
     
  4. Sep 3, 2010 #3
    so if each sphere has charge Q
    so then charge on 1 would be 1/2 Q
    the charge on 3 would be 1/2Q as well since it was neutral and got charged
    then 2 would still be 1Q becasue it was not touched yet and 3 would still be 1/2Q
    then after they touch 2 and 3 charges would add up to 3/2Q
    then each would take away half from that?
    so then 3/4 Q qould be the final charge for 2 &3
    is that the right way of thinking?
     
  5. Sep 3, 2010 #4

    Doc Al

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    Staff: Mentor

    Exactly.
     
  6. Sep 3, 2010 #5
    Thank You!!:biggrin:
     
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