Re: Light Mill?

[Herbert H Gottlieb <herbgottlieb@JUNO.COM>]

On Mon, 13 Sep 1999 09:22:21 -0500 Joseph Bellina
<jbellina@SAINTMARYS.EDU> writes:
> As it happens, this was all worked out originally by J. C. Maxwell.
> cheers
A few years ago, Sam Marantz had an interesting demo that he
presented at a meeting of our New York City Physics Teachers' Club,
He placed a lighted candle at the focus of a parabolic reflector
and a rotating vane radiometer at the focus of a second parabolic
reflector at the other end of the room.  As expected, the radiometer
vanes rotated.    .........   Then he substituted a chunk of dry ice
in place of the candle ..... and the radiometer vanes reversed
their direction...    He explained that the demo proves that it is
possible to transmit COLD as well as HEAT.

Herb Gottlieb from New York City
(Where our Physics Teacher club has been meeting monthly
for the past 100 years.)



 On Mon,
> 13 Sep 1999, Mojca Cepic wrote:
>
> > There are two articles in AJP (somewhere around 1984), one
> suggesting some
> > experiments and the other theoretical one.
> >
> > The experimental one shows how to rotate light-mill backwards.
> > The light mill has four blades, on one side black on the other
> side mirror.
> > Tipical wrong eplanation heard many times is, that the light-mill
> > rotates due to the differences in photon momentum. The force of the
> > reflecting light should be twice as the force of absorbing light.
> > Unfortunately, this gives the wrong sense of rotation.
> >
> > A theoretical explanation of this phenomenon is als given in APJ.
> The text
> > is nice but for my taste uses to much formulas and to little words.
> > Anyway...
> > The real reason for rotation is, that the black part of the blade
> absorb
> > more energy, its temperature is higher than the other mirror one.
> The
> > molecules from the not complete vacuum in the bulb acommodate  on
> both sides
> > of the blade but they left black side with higher average velocity
> and
> > higher momentum than the mirror side. The light-mill is therefore
> a typical
> > heat engine, which uses temperature difference to produce work.
> >
> > A nice experiment is to rotate the light-mill in the "expected"
> direction.
> > Warm the light-mill with an infrared light. When the bulb is very
> warm, stop
> > the rotation using friction (lean the bulb) and than put on it a
> peace of
> > cold cloth (the best way is from the ice-water). The mill starts
> rotating in
> > the opposite direction as before.
> >
> > Regards
> >
> > Mojca Cepic
> >
> > Dr. Mojca Èepiè                                or
> Dr. Mojca
> > Èepiè
> > Jo¾ef Stefan Institute
> Faculty
> > of Education
> > Jamova 39
> > Kardeljeva pl. 16
> > 1000 Ljubljana
>   1000
> > Ljubljana
> > Slovenia
> > Slovenia
> > e-mail: mojca.cepic@ijs.si
> > tel.: +386 61 17 73 471
> +386 61 18
> > 92 200
> > fax.: +386 61 17 73 716
> +386 61 18
> > 92 233