I believe breakdown resistance across 1" air gap is 10,000 volts. How about a 1" gap of a perfect vacuum?
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Posted by Gary Helfert on December 15, 2004, 11:20 am
Please log in for more thread options infinite, however I believe at some voltage the electrons will jump the gap. Any ideas? | ||||||||||||||||
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Posted by Fritz Schlunder on December 15, 2004, 12:11 pm
Please log in for more thread options > Any ideas?
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. | ||||||||||||||||
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Posted by John Larkin on December 15, 2004, 12:33 pm
Please log in for more thread options On Wed, 15 Dec 2004 08:20:06 GMT, "Gary Helfert"
>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? > 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 | ||||||||||||||||
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Posted by John Popelish on December 15, 2004, 2:14 pm
Please log in for more thread options Gary Helfert wrote:
>
> 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? 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. -- John Popelish | ||||||||||||||||
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Posted by peterken on December 16, 2004, 2:15 am
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> This is an academic question among my coworkers. Some believe it is
gap.
> infinite, however I believe at some voltage the electrons will jump the > Any ideas?
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) | ||||||||||||||||
> infinite, however I believe at some voltage the electrons will jump the