Re: Sparks (and ions)

[brian whatcott <inet@intellisys.net>]

At 13:42 9/5/98 -0700, Bill Beaty wrote:
> ...charge cannot leave a
> conductor until the e-field at the surface of the conductor is strong
> enough to defeat the "work-function" of that material.  At very high
> values of e-field, charges do start spewing from a conductor.  This is
> called "field emission", and is usually achived by using incredibly sharp
> metal points in a vacuum.  If you placed your hand in a vacuum, and
> cranked the voltage up (to megavolts? teravolts?), the charges would
> finally start to leap from your skin because of their own repulsion.  But
> the tens of kilovolts from rug-scuffing are too low to cause this, unless
> you happen to have some very sharp dust-motes clinging to your fingertip.

> William J. Beaty 

In Bill's interesting piece, he associates high electric field with high
 voltage (megavolts? teravolts?)
 In fact the field is a measure of the product of potential and distance,
so a few volts at very close quarters can do the trick.

 I'm sure he is aware of how easily electrostatic voltage can break down
junctions on field effect transistors. The voltage is not so high, but the
dimensions are very small. 

On the topic of gas conductivity, because it is almost entirely an
extrinsic phenomenon, one needs either Ludwik's cosmic rays, or flame
ionization, or photoelectric effect from ultra-violet emissions, or X-rays,
or electron 
beams, or radioactive radiations to provide the conductive mechanism.
 (Around here, we have granite hills.)

Paschen found ("Paschen's Law") that there is some critical pressure for a 
given gap which minimizes the voltage needed to initiate a spark.
Above this low pressure, the product of pressure and gap length is 
constant for a given gas at the limiting/breakdown voltage 
(340 volts for air at a pressure in mm Hg times gap in mm. of 5.71)

This is another way of saying the limiting resistivity of air or any other
 gas is proportional to its pressure ( when above the critical pressure)

  Above the critical pressure at low potential differences less than 80
volts, the conductivity effect is Ohmic, varying current is proportioned to
varying voltage. 
> From there to the beginning of the corona/spark/arc regime the current 
remains relatively flat (non-ohmic) with increasing volts.

Brian Whatcott    Altus OK

References:
Standard Handbook for Engineers 9th Ed. A.E. Knowlton   McGraw-Hill
sec 4 para 578 et seq.

This is a topic that engaged the attention of a famous physicist: see
Conduction of Electricity through Gases
 J.J. & G.P Thomson  Cambridge U.Press
brian whatcott <inet@intellisys.net>
Altus OK