Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] Sparks in vacuum



I only skimmed JD's reference, so I may be repeating that an early FE microscope was the first to directly* reveal crystal structure with a W tip.


* as opposed to electron and neutron diffraction. W, probably the easiest to make a very sharp tip.

bc remembers photos if W in Sci. Am. and possibly even Life.



On 2009, Mar 27, , at 08:04, Joseph Bellina wrote:

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.

/************************************
Down with categorical imperative!
flutzpah@yahoo.com
************************************/




________________________________
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



-
-
-
-
-
-
-
-



AVISO LEGAL:
Esta información es privada y confidencial y está dirigida
únicamente a su destinatario. Si usted no es el destinatario
original de este mensaje y por este medio pudo acceder a dicha
información por favor elimine el mensaje. La distribución o copia
de este mensaje está estrictamente prohibida. Esta comunicación es
sólo para propósitos de información y no debe ser considerada como
propuesta, aceptación ni como una declaración de voluntad oficial
de YPF S.A. y/o subsidiarias y/o afiliadas. La transmisión de e-
mails no garantiza que el correo electrónico sea seguro o libre de
error. Por consiguiente, no manifestamos que esta información sea
completa o precisa. Toda información está sujeta a alterarse sin
previo aviso.

This information is private and confidential and intended for the
recipient only. If you are not the intended recipient of this
message you are hereby notified that any review, dissemination,
distribution or copying of this message is strictly prohibited.
This communication is for information purposes only and shall not
be regarded neither as a proposal, acceptance nor as a statement of
will or official statement from YPF S.A. and/or subsidiaries and/or
affiliates. Email transmission cannot be guaranteed to be secure or
error-free. Therefore, we do not represent that this information is
complete or accurate and it should not be relied upon as such. All
information is subject to change without notice.

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




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

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