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Re: [Phys-l] Sparks in vacuum



Just a few comments.

You are correct, if the electric field between the plates is high enough, little fingers of metal normally lying in the plane of the surface can be drawn up off the plane. Now the region is no longer planar, but rather a metal piece with a small radius of curvature where the electric field near the surface goes roughly as the inverse of the radius of curvature. As a result, there is the possibility of quantum tunnelling of electrons out of the metal, and as Dan pointed out the electron current density will depend on the strenght of the electric field, and work function of the metal, more work function less current. If the current is high enough, the sliver of metal can become hot from ohmic heating, creating the possibility for thermionic emission, that is electrons in the metal no longer need to tunnel through the barrier, but can go over it. That means even more current flows, things get hotter, the metal vaporizes and you have a small arc or spark at the surface.

As C O rightly points out, how this goes depends on the real properties of the plates. On the other hand the analogy with metal grinding I don't think is a good one.

cheers,

joe

Joseph J. Bellina, Jr. Ph.D.
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556

On Mar 27, 2009, at 10:24 AM, curtis osterhoudt wrote:

As you've posited, there's no "sparky medium" to get an ionization stream going from one parallel plate to the other. However, there is certainly the possibility that small portions (slivers) of one plate may be torn from that plate and attracted to the other one. If they're fimly attached, the energy evolved in the tearing process can easily heat the slivers to incandescence. These would constitute sparks of a form, such as are produced in metal- grinding. Plasmas are roughly the same thing, writ larger and without that solid bit of metal in the middle.

My gut feeling is that the question is ill-posed, and your immediate reaction is the right one: In a perfect vacuum "conventional" sparks will not form. The "unconventional" sparks very definitely depend on the metals used, the plates' imperfections, and any fringing fields surrounding the plates.

C.O.

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________________________________
From: "CARABAJAL PEREZ, MARCIAL ROBERTO" <mcarabajalp@ypf.com>
To: "phys-l@carnot.physics.buffalo.edu" <phys- l@carnot.physics.buffalo.edu>
Sent: Friday, March 27, 2009 8:16:24 AM
Subject: [Phys-l] Sparks in vacuum

Hello:

Students made me an interest question. In a perfect vacuum, Which is the potential (voltage) required to obtain a spark between two parallel plates at distance d ?. SInce ionization is not possible in perfect vacuum, I find it difficult to answer. Can you help me ?.

My best regards.

Roberto



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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l