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Re: Light Mill?



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 Čepič or Dr. Mojca
Čepič
Jožef 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