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I believe breakdown resistance across 1" air gap is 10,000 volts. How about a 1" gap of a perfect va

Discussion in 'Electronic Basics' started by Gary Helfert, Dec 15, 2004.

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  1. Gary Helfert

    Gary Helfert Guest

    This is an academic question among my coworkers. Some believe it is
    infinite, however I believe at some voltage the electrons will jump the gap.
    Any ideas?
     

  2. Vacuum doesn't have infinite dielectric strength, however it can be fairly
    large or really quite pitiful. I understand the shape of the electrodes
    (needle points will concentrate electric field lines and decrease breakdown
    strength), the temperature of the electrodes (high temperature leading to
    thermionic emission which takes minimal voltage), and the specific electrode
    material used (different vacuum work functions) will significantly alter the
    effective dielectric strength of vacuum. Presumably cathode ray tube (CRT)
    monitors and TVs use electron guns which work in vacuum but only require a
    few tens of kV (but they use electrode heating) or less.
     
  3. Since vacuum contains no electrons, it cannot break down and have its
    electrons start to travel under the influence of an electric field, as
    matter filled dielectrics can.
    http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Capacitors/Diaelectric.html

    Put a pair of surfaces in a vacuum and apply a potential difference
    across them, and the breakdown involves the release of electrons from
    the more negative surface, a completely different process than
    dielectric breakdown. But once those electrons are released, they
    travel, unimpeded through vacuum.
     
  4. John Larkin

    John Larkin Guest

    A vacuum can support a huge electric field, infinite except for
    obscure quantum effects at incredible field levels. But real
    electrodes will have surface emission effects that will rip ions out
    of the metals or whatever; once a few ions get loose and whack the
    opposite electrode, all hell breaks loose. It takes something like
    1e8v/m to rip ions out of metal, but you can get that sort of field at
    the tip of a rough spikey bit using only kilovolts. Clean high-vacuum
    systems can work in the tens of megavolts per meter range, as I
    recall.

    As in interesting aside, look up the Farnsworth Multipactor effect.
    It's cause some serious grief in satellite RF systems.

    John
     
  5. peterken

    peterken Guest

    Don't really know for vacuum, but I assume it's a better insulator as
    "normal" air...
    My (practical) experience is that 10kV / cm...12kV /cm can be done in a
    "normal" environment without any breakdown (unless spikey surfaces or highly
    humid air are involved)
     
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