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Homework Help: Energy and Capacitors of metal spheres

  1. Jun 23, 2010 #1
    1. The problem statement, all variables and given/known data

    Two uncharged metal spheres, spaced 15.0 cm apart , have a capacitance of 24.0 pF. How much work would it take to move 12.0 nC of charge from one sphere to the other?

    2. Relevant equations

    V = Q/[4(pi)eor]
    (delta)K + (delta)U = 0 *I think

    3. The attempt at a solution

    I'm not quite sure how to do this problem. I think I would first have to find out the potential difference between the spheres?

    V = Q/[4(pi)eo*r]
    =(12.0 x 10^-9)/[4(pi)(8.85 x 10^-12)(1.5 x 10^-1)
    =719 V

    Can somebody please tell me if I'm going in the right direction and, if not, could you please steer me in the right direction?
     
  2. jcsd
  3. Jun 23, 2010 #2
    From the law of energy conservation, the work needed = the change in the energy of the capacitor. So all you need to find is the initial energy and the final energy.
     
  4. Jun 23, 2010 #3
    So, U = Q^2/2C would be the equation to use?
     
  5. Jun 23, 2010 #4
  6. Jun 23, 2010 #5
    Would the initial energy be 0 because they are originally uncharged?
     
  7. Jun 23, 2010 #6
    What do you think? :)
     
  8. Jun 23, 2010 #7
    I think so because the electric field depends on the charge of the spheres. Therefore, if there is no charge, there can't be any electric field.
     
  9. Jun 23, 2010 #8
    There you go :)
     
  10. Jun 23, 2010 #9
    Why is the distance given?
     
  11. Jun 23, 2010 #10
    It's redundant.
     
  12. Jun 23, 2010 #11
    Oh, ok. Thanks for your time.
     
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