🚀 go-pugleaf

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?

	andrew

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