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



I guess I should add what I omitted, namely that in this model the idea is
that there is a chain reaction. If there is one free electron somewhere
(which can be due to the passage of cosmic-ray muons or to nearby
radioactivity), and the field is big enough to accelerate that electron in
one mean free path to a big enough kinetic energy to be able to knock an
electron out of a molecule, now you have two electrons that will accelerate
and collide, then 4, 8, 16, 32, 64..... a chain reaction. Now the air is
heavily ionized and is a conductor.

This model, designed to estimate the breakdown strength of air, predicts
something it wasn't specifically designed to do, namely that if you double
the air density you cut the mean free path in two and therefore need twice
the field to accelerate an electron enough in the shorter mean free path to
be able to ionize a molecule. In fact, it is observed that if you double
the air density the critical field is twice as big. I understand that
transformers are sometimes filled with high-pressure gas, serving as a good
insulator.

Bruce