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Finding potential difference across capacitor plates?

  1. Apr 2, 2012 #1
    1. The problem statement, all variables and given/known data
    Two capacitors are identical, except that one is empty and the other is filled with a dielectric (k=3.6). The empty capacitor is connected to a -11V battery. What must be the potential difference across the plates of the capacitor filled with a dielectric so that it stores the same amount of electrical energy as the empty capacity?


    2. Relevant equations
    C=q/V C=kε°0)A/d E=q/(ε°)A
    Estored=1/2qv = 1/2 cv^2


    3. The attempt at a solution
    I don't really know what to do. I'm guessing that there's a lack of information but however the part about the empty capacitor, that would mean Estored = 0 right? But even though I plug in something with zero and solve for C, C would be zero which would mean 0=q/V or the other equation and if I solve for a variable it would be just zero.

    I'm guessing my logic is probably wrong but I'm really lost. I basically have only 2 known variables from the question.
    Any help would be awesome!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Apr 2, 2012 #2

    ehild

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    Homework Helper
    Gold Member

    Empty capacitor means that the space between the plates is not filled with dielectric.

    ehild
     
  4. Apr 2, 2012 #3
    So does that mean I let C=kε°A/d equal to each other so kε°A/d=kε°A/d?
    And then from that plug in 3.6 for one of the k, and 1 for the other?
    And from that, the k's will cancel, the A's will cancel b/c identical, and I'm not sure about the d's. Well is that the right method or am I still doing something wrong?
     
  5. Apr 2, 2012 #4

    gneill

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    Staff: Mentor

    How would the k's cancel if they are not the same?
     
  6. Apr 2, 2012 #5
    Oops sorry. Well I meant that there will be a ratio when you divide one by the other; when transferring k over to solve for the unknown variable, there will be an actual number
    Anyways I'm still lost
     
  7. Apr 3, 2012 #6

    gneill

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    Staff: Mentor

    Start by working out how the values of the two capacitors are related (write an expression).
     
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