🚀 go-pugleaf

In article <4bat3v$r3n@hg.oro.net>, tessien@oro.net (Ross Tessien) wrote:

> In article <jjl-1912952339480001@mac-5.knoware.nl>, jjl@knoware.nl 
> says...
> >
> >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 

snip

> >> >component to drive the vanes.
> >>         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. 
> 
> OK, then help me out.  If it is not the hotter temperature of the black 
> side as you seem to be implying, then are you saying that both sides are 
> at the same temperature?  If so, then where is the differential energy 
> source.  The only thing driving the gas based radiometer is kinetic 
> energy from gas molecule recoils.  It is precisly the pressure gradient 
> that forces the vane forward.  The pressure on the Black side is higher 
> than is the pressure on the white side.  If this were not so, then the 
> vane would sit still.  Yes, this higher pressure will want, and indeed 
> will in fact, dissipate.  That is what high pressures do.  This is a 
> dynamic situation though, and there is a constant conduction of energy 
> off of the black side.  This is possible precisly because it is black and 
> continues to absorb more energy.
> 
> As already Maxwell suspected, thermal creep
> >of the gas is the explanation.
> 
> This I have never heard of in a gas unless you consider the gas to be 
> static somehow.  Thermal creep I have worked with is a flow of energy 
> through a solid.  Please explain what you mean by this at the atomic 
> level so that I can understand.  Perhaps you are describing gas that is 
> creeping in slowly from the rear and then kicked off at a higher 
> velocity.  From where do you get any energy????
> 
> 
> >
> >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.
snip

Do look at Kennard or any other kinetic gas theory book discussing the effect.
Of course a temperature difference is needed. I meant to say three things:
1) Any explanation in terms of single molecule collisions is incomplete
and/or wrong.
Mean free path << dimensions at 1 mm Hg.
Even when Mean free path > dimensions you would still
need to consider temp distribution on the enclosure.
2) The radiometer effect is secondary, caused by the gas flows.
There is pressure equilibrium, the flow is driven by the temperature gradient.
This is called thermal creep flow, not to be confused with thermal creep
in a solid.
3) A correct quantitative explanation is possible but complicated.
The whole flow and temp distribution must be considered.
Already difficult when the vanes are held stationary.


BW

Jan