Re: The Crookes Radiometer

Wortman <W@xxxxxxxxxx> wrote:

In article
<g6n5l3$9u9$3@xxxxxxxxxxxx>baez@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx (John
Baez) wrote:

It seems there's still a lot of mystery about how this gadget
works. If you have thought deeply about this issue, try:

Gee, the explanation I heard is that the black sides of the veins run
hotter than the gas in the bulb while the white sides are closer to the
temperature of the gas. When a molecule strikes the black vein it on
average increases in energy and leaves at a higher speed imparting more
momentum on average to the vein than a molecule striking the cooler
white side. It may also be critical that the bulb is run at a reduced
gas pressure. This will increase the mean free pathof the gas molecules.
If the pressure is low enough the mean free path would be comparable to
the bulb radius. This would allow gas molecules carrying heat from the
veins to be cooled by collision with the glass bulb rather than heating
the rest of the gas. This explanationhas always worked for me. Is there
something obviously wrong with it?

Yes. No explanation based on collisions of single molecules
with the vanes can be correct.
In addition you have to consider the distribution function
f(r,v) of all the molecules.
(you can simplify by holding the vane stationary
and calculating the net force on it. Else you get an f(r,v,t)
It becomes difficult because of phenomena like gas flow or creep.

Jan

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