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Charge on a Capacitor

  1. Aug 17, 2012 #1
    Can someone explain what exactly happens when a capacitor is charged?
    At the positive plate, are loosely bound electrons pulled onto the plate from the dielectric or are whole molecules in the dielectric rotated and orientated so that the negative part of the molecule is close to the plate? Why don't electrons return to their rest position after the charging voltage is removed just like a spring returns to a zero displacement state when a force is removed? Also how can a capacitor work when the dielectric is a vacuum? If the dielectric is removed, does the charge disappear?
     
    Last edited: Aug 17, 2012
  2. jcsd
  3. Aug 17, 2012 #2

    mfb

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    Staff: Mentor

    Which molecules/electrons do you mean, and where? In the dielectric between the plates?

    You should begin with this case, it is required to understand the situation with material inside.
    The opposite charges on the plates create an electric field in between, this leads to a potential difference. You can exploit this difference in a circuit, for example - you give the charges (here: electrons on the negative side) a way to go to the other, positive, side.

    Now, a material between the plates can have internal dipoles or other methods of charge separation. Those dipoles will align their orientation in a charged capacitor. This gives an effective (smaller) negative charge at the positive side and an effective (smaller) positive charge at the negative side. This reduces the electric field strength and therefore the voltage, compared to vacuum. In other words, you need more charges for the same potential difference, the capacity increased.
     
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