Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A few quick conceptual questions about capacitators and dielectrics

  1. Feb 28, 2012 #1
    1. The problem statement, all variables and given/known data
    So my question is conceptual. I am trying to figure out in what direction an electron will move if it starts between two plates of a parallel plate capacitator. One of the plates is at 0V and the other is at 50V.

    Also, would placing a dielectric in a charged, isolated capacitator change its voltage but not charge?

    3. The attempt at a solution
    I know the 100 V plate would repel a negative charge. I would think the electron would move towards the 0V plate just because that is the way a capacitator is supposed to work (moving across an electric potential). Is this correct?

    For the second question, I think charge wouldn't change when adding a dielectric in an isolated capacitator because there would be nowhere for the charge to go. But then the voltage would drop? Can someone verify that I am understanding this correctly?
  2. jcsd
  3. Feb 28, 2012 #2
    If you insert a dialectic into the capacitor, the dialectic will produce an induced electric field called the "displacement" field. This field acts to reduce the net field in the volume of the dialectic. If the capacitor voltage is not held constant by a battery or some other source, then yes the voltage will drop, but the charge will remain constant.

    If the voltage is supplied by some battery, then the voltage will remain constant, but the energy stored in the capacitor will increase. This is because the displacement field has energy, and maintaining the potential forces the battery to do extra work by adding more charges to the plate.
  4. Feb 28, 2012 #3
    recall that because of history, we are stuck with positive test charges

    a positive test charge will want to move to the decreasing potential, this would be from 50 to 0

    therefore, an electron would be the opposite, right?
  5. Feb 28, 2012 #4
    Hmm okay, thanks for the responses!

    My teacher wasn't too clear with wording the question, so I'm not sure if isolated means no voltage is being supplied... So if a battery was supplying voltage, the voltage would stay the same but the charge would increase to make potential stay the same?

    Right, I forgot that only positive charges move across decreasing potential. That makes perfect sense now.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook