🚀 go-pugleaf

Ross Tessien (tessien@oro.net) wrote:
: In article <4b6p20$bm7@scotsman.ed.ac.uk>, ajc@reaxp01.roe.ac.uk says...
: >
: >In article <4b6nep$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
: >
: >-- 
: >           work phone/fax: 0131 668 8356, office: 0131 668 8357
: >    institute for astronomy, royal observatory, blackford hill, 
: edinburgh
: >                     http://www.roe.ac.uk/ajcwww
: 
: 
: Well, all of the previous comments are excellent in this post and the 
: others addressing the question.  Let me take the phenomena one level down 
: in detail to show where the force originates in each of the two kinds of 
: radiometer.
: 
: The first kind of radiometer is a reflective one.  It requires a very 
: good vacuum and is not normally sold as a science toy so I doubt this is 
: the kind the teacher has.  But lets consider it any way.  Normally, to 
: maximize the effect, one has a black surface on one side and a highly 
: reflective surface on the other.  White paint meerly decreases the 
: effeciency of the effect but in space would work fine.
: 
: Now the energy imparted by a photon colliding with the black side of the 
: surface and being absorbed is one times its momentum.  This is given by 
: its relativistic mass times velocity, or, U = mc

Then you agree, a photon that has mometum has mass.

That's where this whole subject of radiometers started.



VERTNER VERGON -

Who indisputably re-established the existence of radiant mass.
Who indisputably corrected two errors in the special theory of relativity.
Who produced the final solution to the twins paradox.

And who developed a successful model for QED.