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Question about capacitors with dielectrics

  1. Sep 20, 2009 #1
    Say i have a capacitor and i supply it with a voltage V and after that it has charge density sigmainitial on each plate.

    Now i remove the voltage source and then introduce a dilectric with area A that it equal to the area of one of the plates in the capacitor to the capacitor.

    But this dielectric is only touching one plate.

    My question is: is the net charge density on each plate(sigmainitial - sigmainduced) the same in both plates or different?

    please explain...
  2. jcsd
  3. Sep 20, 2009 #2


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    I think the answer is that the same charge is there but the molecules in the dielectric have all polarised a bit. This has increased the capacity ov the capacitor. Equivalent to moving the plates closer together. Now
    so the voltage will have dropped. As for the energy difference, some work has been done in moving the dielectric into place- nothing violated either way, I think.
  4. Sep 20, 2009 #3
    Hi Anon-
    Sophie is correct. Charge is conserved, and the charge distribution is unchanged. The stored energy is

    E = Q2/2C

    So when the dielectric is pulled (not pushed) in, the stored energy drops, and work is done.

    Bob S
  5. Sep 20, 2009 #4


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    Yes- the same work that the plates would do if they were allowed to come together and have the equivalent capacitance.
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