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Homework Help: Energy density of a capacitor

  1. Jan 30, 2014 #1
    1. The problem statement, all variables and given/known data

    A parallel-plate capacitor with C = 16 mu or micro FF is charged so as to contain 1.7 J of energy. If the capacitor has a vacuum between plates that are spaced by 0.35 mm, what is the energy density (the energy per unit volume)?

    I am a little lost of this problem and have no idea if I'm doing it correctly. Am I ?

    2. Relevant equations
    v = A*d

    3. The attempt at a solution
    E=1.7/45.71=.0371 J/m^3
  2. jcsd
  3. Jan 30, 2014 #2


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    Is the capacitance 16 μF (microfarad)?

    How do you calculate the volume?

  4. Jan 31, 2014 #3
    Yes, I believe that is (microfarad); it doesn't say specifically.
    I was very unsure on that step and wasn't quite sure how to compute the volume. Do I even to find the volume?
  5. Jan 31, 2014 #4


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    As yo have written, energy density is energy divided by volume. So you need it.

  6. Jan 31, 2014 #5
    Okay so how do I find the area in order to find volume.
  7. Jan 31, 2014 #6


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    What is the formula for the capacitance?

  8. Jan 31, 2014 #7
    C=(k* epsilon_0*A)/d
  9. Jan 31, 2014 #8
    The volume between the plates is given by Ad where 'A' is area of the plates and 'd' is separation between them . 'd' is given whereas 'A' is unknown .

    Using C= ε0A/d ,find the value of A.

    Now,energy density = Energy stored/volume .
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