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Metals and Conductivity question

  1. Jul 14, 2014 #1
    (Heads-up: these might be QM questions)

    Two questions:


    1.) What is it that gives rise to the fact that conductive metals provide a much lower potential for electrons to exist, than in free space?



    2.) If conduction in wires take place due to an E-field that "travels" through/along the wire, why do the electrons move so slow?
     
  2. jcsd
  3. Jul 14, 2014 #2

    UltrafastPED

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  4. Jul 14, 2014 #3
    I thought free space would be a pretty good conductor once you disassociate the electron from its host. It's not like it has any phonons to scatter an electron.

    One draw back of space though would be that the electrons would repel each other. This effect is mitigated by the positive nuclei in a material.
     
  5. Jul 14, 2014 #4

    UltrafastPED

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    A capacitor in a vacuum works just fine; you can build them with two metal plates.

    If there is a source of electrons (e.g., photo-electron gun) and a sufficient voltage difference, you can generate a current between the plates.
     
  6. Jul 14, 2014 #5
    Yes, a capacitor in vacuum works fine, because the voltage across the capacitor is less than the work function. While free space does have a dielectric constant, it doesn't have electric dipoles to get polarized.
     
  7. Jul 14, 2014 #6

    UltrafastPED

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    The term "work function" is normally used for the energy required to liberate an electron from a metal via the photo-electric effect.

    My research area involved designing and building photo-electron guns for use in ultrahigh vacuum systems. While every material has a breakdown field strength, I have generated 6 MV/m fields inside of an electron gun with 30,000 volts across 5 mm, and detected no electron current in a very sensitive detector rig. Others have generated field strengths of more than twice that; you just need to use good design and the right materials.

    In my system the cathode was a thin film of polycrystalline gold, 40 nm thick; the laser pulses were transmitted from the back, through a sapphire substrate. The work function for gold is about 4.5 eV, though you will see higher values given. I used a 780 nm ultrafast laser, shifted to 260 nm (deep UV), which has almost exactly 4.5 eV.

    This setup worked just fine ... laser pulse generates electron pulse; no laser, no electrons.

    PS: The dielectric constants for air and vacuum are nearly the same. This is why light travels almost as fast in air as it does in vacuum.
    See http://hyperphysics.phy-astr.gsu.edu/hbase/tables/diel.html
     
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