Re: [Phys-L] mean free path for electrons vs. air molecules

[John Denker <jsd@av8n.com>]

On 08/29/2013 04:29 PM, Robert Cohen wrote:
> In my copy of Knight Physics, it states that the "average distance an
> electron travels between collisions" in air is 2.0 microns.  It then
> uses this to determine the breakdown field strength.

There are people who now the right answer, namely the people who 
build Geiger–Müller tubes.  For the MFP of an electron in air, they
consistently use a value that is 5.6 times larger than the value for 
an air molecule in air ... using the same factor across the whole
range of densities.  At STP, this comes out to nowhere near 2 microns.
  http://en.wikipedia.org/wiki/Paschen%27s_law#Physical_mechanism
  http://mysite.du.edu/~jcalvert/phys/dischg.htm

The factor of 5.6 is a little bit weird, but not crazy weird.  A 
factor of 4 would be super-easy to explain:  Just say that the
electron has zero size compared to a molecule, so the cross-sectional
area for a collision goes down by a factor of 4.  That is:
  (radius+radius)^2 versus (radius+0)^2

The 2 micron number is waaaay too large.  I decline to speculate
about its provenance or significance.