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Homework Help: Potential across capacitance

  1. Feb 28, 2014 #1
    A first capacitor is initially connected to potential source ?. The charged capacitor is then removed
    from the source and connected to a second initially uncharged capacitor. Determine the final potential
    across the first capacitor long after the switch is closed.

    1. The problem statement, all variables and given/known data
    Q1 charged by capacitor C1

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 28, 2014 #2
    You're going to have to give some effort to the problem. What do you think is going to happen once the the switch is closed, in terms of changes in charges and potentials, since those are the only quantities that can vary?
  4. Feb 28, 2014 #3
    So if it let charges at C1 when connected to voltage source to be Q' then after let charges at C1 and C2 to be Q1 and Q2. It should be Q1+Q2=Q' ?
  5. Feb 28, 2014 #4
    Yes, from conservation of charge. If you use the equation you listed, you can substitute capacitance times potential for each charge, and then that should open up the potential on C1
  6. Feb 28, 2014 #5
    So V(C1+C2)=C1ε , and solving for V=(C1ε)/(C1+C2) is this correct?
  7. Feb 28, 2014 #6
    Yes, that's right. V is the same across both capacitors because they become equipotential surfaces.
  8. Feb 28, 2014 #7
    Alright Thank you.
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