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Something about parallel capacitors

  1. Nov 23, 2009 #1
    If two parallel plates are facing one another and one of them are moved without opening the switch, what's the difference with moving the plates after opening the switch?
     
  2. jcsd
  3. Nov 23, 2009 #2
    Sorry, You'll have to be more detailed than that.
     
  4. Nov 23, 2009 #3

    clem

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    If they are moved before opening, V is constant. If after, Q is constant.
     
  5. Nov 26, 2009 #4
    So my textbook says that if the distance between the parallel plates are increased from d to d' without opening the switch first(so that means that electric current is flowing), then only V is constant and Q, E is not constant. But if d is increased to d' after opening the switch(stopping the current), then Q and E is constant.
     
  6. Nov 26, 2009 #5

    sophiecentaur

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    Moving the plates involves doing work. That work will be transfered to potential energy or Kinetic Energy (a flow of charge), depending on whether there is a conducting path or not.
     
  7. Nov 26, 2009 #6
    When the switch is closed then V is the value of the power supply. So if you then increase d you put some energy back in the power supply. Increasing d means lowering the value of the capacitor. Since Q=C*V then Q will decrease. Also since E=V/d it means that E decreases. So your text reads: only V is constant, and Q and E are not constant. I think your book is a bit confusing (but correct).
    The second part is OK.
     
  8. Nov 26, 2009 #7

    crx

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    -dielectric constant will decrease, capacitance will decrease, voltage between the plates will increase, and some of the charges will move back to the power supply. The energy to move the electrons back to the source will come from the work of the force that is used to depart the plates.
     
    Last edited: Nov 26, 2009
  9. Nov 26, 2009 #8

    Born2bwire

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    Dielectric constant is a material property, it would not change here unless one were to change the dielectric between the plates.
     
  10. Nov 26, 2009 #9

    crx

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    Of course it doesn't! I just wanted to see if you pay attention...:redface:
     
    Last edited: Nov 26, 2009
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