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A question about dielectrics

  1. Jun 23, 2008 #1
    hi everyone, i've got a question.

    say i have two electrode plates placed pretty close together, with two pieces of a ceramic high dielectric constant material.. x7r class ceramics for instance, in between the two plates, with a gap in the middle of these two ceramic inserts, filled with vacuum.

    like so:
    | = electrode
    : = high dielectric constant material.

    |: :|


    now, if i ran this electrode on AC, with a much higher than 120HZ source.

    what would happen to any free electrons in the vacuum?

    from what i understand about capacitors what would happen is that the electrons in the vacuum would be moved quite quickly across the gap, given enough voltage to normally cause an arc to form.
    this processes would repeat and the electrons would move across the gap in the opposite direction, and again and again as the AC source continued. there wouldn't be that much absorption of the free electrons
    since the ceramic has such a high dielectric constant.

    is this correct, or am i missing something basic that would cause free electrons in the vacuum to move through the dielectrics?

    similarly, what would happen when air was in the gap, and there was enough voltage to ionize the air?



    thanks,

    chris.
     
    Last edited: Jun 23, 2008
  2. jcsd
  3. Jun 23, 2008 #2

    Defennder

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    How can there be free electrons in a vacuum? Is there something I'm missing here?
     
  4. Jun 23, 2008 #3
    re: Defennder

    well there could be electrons injected into the vacuum i think.. i looked up "electrons in vacuum" on google and there's lots about laser acceleration of electrons in a vacuum.

    and i mean.. wikipedia dude:

    'Electrons in an atom are bound to that atom, while electrons moving freely in vacuum, space or certain media are free electrons that can be focused into an electron beam'

    the 'electrons moving freely in vacuum' seems to have pretty much exactly the language i stated.

    pretty basic.
     
    Last edited: Jun 23, 2008
  5. Jun 24, 2008 #4

    berkeman

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    I suppose the free electrons would move back and forth given AC excitation. Sort of like moving static charge back and forth.
     
  6. Jun 24, 2008 #5

    NoTime

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    Electrons are not necessarily bound to the metallic surface.
    Surface smoothness has a lot to do with this.
    So yes free electrons are possible between capacitor plates.
    This goes into the leakage parameter associated with capacitors.
    As Berkeman mentions they tend to go back and forth with AC excitation.

    Recently a cathode optimizing this effect, using carbon nanotubes, has been developed that allows construction of a low voltage vacuum tube without a heater.

    Not entirely certain of the current status, but I think there are now some commercial devices utilizing this effect as opposed to just research only devices.
     
  7. Jun 24, 2008 #6
    re: notime

    thanks for the response.

    would increased surface smoothness make it so that there would be less free electrons, or more?
     
  8. Jun 24, 2008 #7

    NoTime

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    Less.
    Sharp pointy bits tend to emit free electrons.
     
  9. Jun 24, 2008 #8

    dlgoff

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    "Recently a cathode optimizing this effect, using carbon nanotubes, has been developed that allows construction of a low voltage vacuum tube without a heater."

    Cool. I'm lazy. Do you have a link?
     
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