🚀 go-pugleaf

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 
>> >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. 

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

This is a neat idea and a neat result will occur I agree.  Those of you 
with radiometers should check it out.


>
>Best Wishes,
>
>Jan

Merry Christmas, Ross