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How does a dielectric increase capacitance, conceptually.

  1. Sep 20, 2008 #1
    Hi all,

    I'm a little stumped with how the dielectric increases capacitance. The general solutions all involve using the formulas, but don't really treat it conceptually. I realise that inserting a dielectric decreases field strength, and hence decreases the voltage across the capacitor. But if I have a capacitor in a DC circuit, then shouldn't the voltage across the capacitor remain the same, i.e. equal to the voltage of the cell? Then there would be no change in voltage (or charge) so no change in capacitance? This should be easy to test, unfortunately I lack a voltmeter or a capacitor.

    Any insight would be appreciated,

    Kcodon
     
    Last edited: Sep 20, 2008
  2. jcsd
  3. Sep 20, 2008 #2

    atyy

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    Yes, the equilibrium voltage should not change, that means that the amount of charge must change. C=Q/V, so capacitance is a measure of "charge storing" or "charge separating" ability. A big capacitor can store more charge for the same voltage.
     
  4. Sep 20, 2008 #3
    But I believe the reason for only a certain amount of charge being stored on a plate of the capacitor is that as charge builds up on the plate it gets to a point where all the accumulated charge repels any additional charge, with an "force" equal and opposite to that provided by the cell voltage. I don't think it is linked to the electric field across the plates So how does more charge accumulate when there isn't a greater voltage to "squeeze" it onto the plates?
     
  5. Sep 20, 2008 #4

    atyy

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    It is intimately linked to the electric field between the plates. But we can also work with your picture. Like charges accumulate on the capacitor plate because of push from the battery. But they also don't want to accumulate on the capacitor plate because they are squeezed with each other. And those two balance. So if you put a dielectric in the middle, you are providing more opposite charge near the like charges, so the like charges are not so unhappy about being squeezed, and the same voltage can push more charge onto the capacitor plate.

    Edit: I assumed you know the picture about the dielectric being polarized: http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/dielec.html.
     
  6. Sep 20, 2008 #5
    Thanks so much atyy, this is the little explanation I have been looking for all along! I love the little lightbulb moments like this, even if it is someone else that turns the bulb on!

    Thanks again,

    Kcodon
     
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