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Capacitors, Electric Fields, and Dielectrics

  1. Oct 14, 2005 #1
    I have a question that is confusing me perhaps one of you can help me.
    If I hook up a constant potential difference to a capacitor and place a dielectric inside of it, will the electric field decrease even if the plate separation remains constant?
    I think that the capacitance will increase as well as the charge on the plates, meaning that the electric field would have to remain the same, since the potential is constant? or am I wrong? if so why?
    Thanks for the help
     
  2. jcsd
  3. Oct 14, 2005 #2

    Doc Al

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    Staff: Mentor

    You are correct.
     
  4. Oct 14, 2005 #3
    Do you know of anyplace that offers credible proof like in a book or an article?

    This was a test question and I argued with my physics teacher about it. He thinks the electric field will decrease.. where I do not and he said I am wrong.

    I would like to show him that the electric field doesn't decrease when a contstant potential difference is applied.
     
  5. Oct 14, 2005 #4

    Doc Al

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    The electric field in a parallel plate capacitor, as you apparently realize, depends only on the potential difference and the distance between the plates:
    [tex]E = \frac{\Delta V}{d}[/itex]

    This kind of thing should be in any textbook; here's a web site that discusses it: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/dielec.html#c2
     
  6. Oct 14, 2005 #5
    That is the formula I explained to him. I don't see why he doesn't get it
     
  7. Oct 15, 2005 #6
    Is it because he thought that the electric field is decreased due to the presence of the opposite electric field set up by the dielectric? But he forget that the voltage is still applied after the insertion of dielectric, which will bring the electric field back.

    Pls correct me if I'm wrong.
     
  8. Oct 15, 2005 #7
    That must be what he is thinking.

    DocAl, would this also apply to non parallel plate capacitors?
     
  9. Oct 16, 2005 #8

    Doc Al

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    Sure. The electric field is determined by the applied voltage and the geometry of the conductors, not by the presence of a dielectric.

    As Alpha2005 notes, your instructor is probably thinking of this sequence:
    (1) Apply a voltage to the (empty) capacitor
    (2) Remove voltage source
    (3) Insert dielectric​
    In this case, she would be correct. The induced polarization charge within the dielectric will reduce the effective electric field.
     
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