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Homework Help: Capacitors and Dielectrics of a circuit

  1. Apr 18, 2006 #1
    Two capacitors are identical, except that one is empty and the other is filled with a dielectric ( k= 5.00). The empty capacitor is connected to a 6.0 V battery. What must be the potential difference across the plates of the capacitor filled with a dielectric such that it stores the same amount of electrical energy as the empty capacitor?

    So here are my thoughts so far.....
    V= 6 V
    Energy=.5CV^2
    So the energy of the empty capacitor is .5V^2 which is .5(18^2)=18 right????? they give no value for C so i assume it is one???
    I have no idea where k comes into the picture. please help. thanks.
     
  2. jcsd
  3. Apr 18, 2006 #2

    berkeman

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    Staff: Mentor

    What is the equation for the capacitance of a parallel plate capacitor in terms of epsilon, A (area) and d (separation distance), ignoring fringe effects at the edges? What is the relationship between epsilon-zero (vacuum value) and the epsilon inside the dielectric?
     
  4. Apr 18, 2006 #3
    Answer

    I just figured it out.
    Energy=.5CV^2
    Energy=.5kV^2

    so .5CV^2=.5kV^2
    so CV^2=kV^2
    C=1
    so V^2=kV^2
    so 6^2=5V^2
    so V= sqrt(36/5)=2.7 V
     
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