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About Electrostatic Forces on Parallel Plate Capacitor

  1. Jul 10, 2011 #1
    I've been thinking about this for a while, and I think people of PF will be able to answer it haha

    In parallel plate capacitor, we charge the capacitor by connecting a power supply/battery. Then, after few minutes, the capacitor is fully charged with different signs, + and -. With that difference in signs, there must be forces, same magnitude, acting on both plates which draw them closer. What I want to ask is, how do people maintain the distance between the plates? I mean, there are attractive forces, so, there must be another force with different direction right? If not, then the plates will be accelerated toward each other.

    I am confused by this. Many lectures I've watched never talk about electrostatic forces again after they get to parallel plate capacitor.

    Ah, another one, how do people maintain potential difference? Doesn't the same problem apply? I just haven't fully understood the concept of voltage yet. Because voltage is generated by point charge and then suddenly it is there by itself, can someone explains to me?

    Thanks in advance.
     
    Last edited: Jul 10, 2011
  2. jcsd
  3. Jul 10, 2011 #2

    jtbell

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    Staff: Mentor

    Real capacitors usually have some kind of solid material (called a "dielectric") between the plates. Sometimes the dielectric is liquid, in which case the plates have to be rigid enough to withstand the attractive force between them.
     
  4. Jul 10, 2011 #3
    Thanks for the answer, so the dielectric is used to keep the distance so that the plates won't touch each other right?

    "the plates have to be rigid enough to withstand the attractive force between them", is there any normal forces that help this process?
     
  5. Jul 10, 2011 #4
    The force between the plates is pretty small anyway and for fixed value capacitors it is the mechanical structure,including any dielectrics etc as mentioned by jtbell,that maintains the separation.
    The potential difference is proportional to the charge build up on the plates and is maintained as long as the charge remains.If the charge leaks away the pd drops.
    There are variable capacitors where either the overlap plate area can be changed or the plate separation changed.When these are charged and disconnected from the circuit then changing the capacitance changes the pd.
     
  6. Jul 10, 2011 #5
    Ah got it now, thank you! thank you!
     
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