Thermal creep

[Itoero]

If I understand it correctly then in thermal creep, gas can move from a cold to a warm side...it doesn't need a pressure difference.

But what causes gas to behave like that?

And how can that drive the vanes in Crooks Radiometer?

[imatfaal]

thermal transpiration or thermal effusion are the terms I have heard used. And yes this is the accepted explanation for the Crooke's radiometer. It is an effect driven by the difference in energy and momentum (and thus force when undergoing an acceleration change when colliding) in gas molecules approaching the edge from the hotter black side and those approaching the edge from the cooler white side - on average the (tangential component) of the force exerted on the edge will be greater from those molecules on the hotter side. As a tiniest bit more force is exerted on the edge by molecules from the black/hotter side than the white/colder side then there is a net force which causes rotation - as the resultant force is directed from the black side it rotates in with the white side leading. I am struggling to find a nice reference for the general effect

[Itoero]

But what makes warmer or colder molecules to move to the edges?

Temperature increases pressure. So normally speaking, warm molecules would be moved faster to the silver side of a vane,which would cause the rotation.

Why is this wrong?

[imatfaal]

But what makes warmer or colder molecules to move to the edges?

 

Random chance - there are molecule moving in every direction. Some hit the edge

 

 

Temperature increases pressure. So normally speaking, warm molecules would be moved faster to the silver side of a vane,which would cause the rotation.

Why is this wrong?

 

The pressure explanation was actually debunked by no less a physics superstar than James Clark Maxwell - who loved the idea; but having had egg on his face from already supporting the erroneous radiation pressure explanation (if it were radiation pressure then the white side which is reflecting photons is under higher force than the black side which is absorbing - remember momentum is conserved) went through the maths with a fine toothed comb and found that the increased pressure on the black / hotter side did not cause a motive effect.

 

Reynold's heat transpiration - in the presence of a porous membrane gas will flow from the colder side to the hotter side and create a pressure gradient. The vanes of a crooke's radiometer are not porous - but thermal transpiration works because of the edges of the pores (ie those surfaces at right angles to the main face) and the vanes do have edges and these are enough. You can think of it in two ways - in the manner I outlined above with different tangential component forces from molecules from hot and cold side or in the following way which is not better but fits better with the thermal transpiration idea. The gas molecules tend to flow from cold to hot near the edges - so there is a net movement at the edge from the cold white side to the hot black side, thus the pressure increases locally at the hot black side, thus the vane experiences more force on the hot black side than the cold white, the the vane moves, the pressure equalised and it all starts again.

[Itoero]

The gas molecules tend to flow from cold to hot near the edges

Why do molecules do that?

Can I compare it with adverse current at the river banks of a river with a strong current?

Edited January 15, 2016 by Itoero

[imatfaal]

Why do molecules do that?

 

I am not sure there is a great and complete explanation - the root cause is as I suggest above; if you think about it when the gas exerts a force on the surface then they must be a reactive force as a newtons third law pair. If the surface is mobile the surface moves - but if the surface is fixed all we can observe is the gas moving under the reactive force. This is basically Maxwell's explanation as far as I can tell from scraps.

 

This paper seems to suggest that as heuristics go this is as goods as it presently gets

Journal of the Physical Society of Japan. Vol. 67, No.7, July, 1998, pp. 2277-2280.

Maxwell's Thermal Creep in Two Space Dimensions. Koichiro SHIDA

williamhoover.info/Scans1990s/1998-8.pdf

 

 

Can I compare it with adverse current at the river banks of a river with a strong current?

 

Possibly - but I don't know what causes the counter-current. I would say no as the situation is not similar - you only have one flow in thermal transpiration ; ie not a strong flow and a weaker counter-flow.

 

Thermal creep happens in a good vacuum where a molecule is about as likely to cross from one side of container to the other unhindered as it is to hit another molecule - this does not sound like a good parallel to a fast flowing river