🚀 go-pugleaf

In article <4b6p20$bm7@scotsman.ed.ac.uk>, A.Cooke@roe.ac.uk wrote:

> In article <4b6nep$gu3@ixnews2.ix.netcom.com>,
> Alan \"Uncle Al\" Schwartz  <uncleal0@ix.netcom.com> wrote:
> >The radiometer is under a light vacuum.  Heating of the black side of 
> >each vane launches mass convection currents with sufficient orthogonal 
> >component to drive the vanes.
> 
>         what is the fine detail behind the phrase `convection currents'?
> 
>         i guess that the dark side of the vane is heated by radiation
>         and so any air molecule that collides with the vane is heated
>         slightly and then leaves with more energy that it arrived (on 
>         average).  this gives a net loss of momentum to leaving molecules
>         and so the vane moves in the opposite direction.
> 
>         (you could think of the process as molecules being held on the
>         surface, heated, and released like the gas flying out of a
>         rocket.)
> 
>         alternatively, if the mean free path of molecules is less than
>         the size of the vane, then these heated molecules remain to the
>         dark side of the vane for a few collisions, giving a locally
>         increased pressure.  (i'm automatically suspicious about this because
>         it's talking about gas that's not in thermal equilibrium).
> 
>         i think these are two separate processes and i don't know which
>         dominates, or if `convection currents' implies some other process -
>         any comments?
> 

Yes, comment:

The effect is strongest at 1 mm Hg, not in a vacuum, with mean free path
still short compared to vane size.
It is much stronger than the radiation pressure of light, predicted by
Maxwell, first measured by Lebedev, early 20th.
The explanation based on stronger recoil from the hot side is wrong.
The recoil would drive the gas away and establish a counteracting pressure
gradient, which is impossible. As already Maxwell suspected, thermal creep
of the gas is the explanation.

A discussion may be found in
Kennard, Kinetic Theory of gasses, McGrawHill, 1938.
And also in other books on the kinetic theory of gasses.

Kennard refers also to
Epstein, Zeits. Physik, _54_, 537, (1929),  (prob. in German)
for a quantitative derivation of the strenght of the force.
and to the experimental quantitative confirmation.

Nice DIY exp. for Xmas: Take radiometer, spinning in front of the
fireplace, outside when it is freezing and see what happens.

Best Wishes,

Jan