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Re: radiometer paradox

Brad Shue wrote:

On Sat, 14 Jun 1997, Bob Sciamanda wrote:
Addendum: If you refrigerate the Crookes radiometer sufficiently, the
thermal effect can be reduced so that the radiation pressure efect
dominates and the thing turns "backwards" (ie., in accordance with
Joseph's expectations) under a strong light.

At what temperature is this effect seen? How do you illuminate it?
==============================================================================
James Bradford Shue jshue@comp.uark.edu
University of Arkansas Voice phone:(501) 575-6059
Physics Lab/Demo Curator FAX Number: (501) 575-4580
WWW Page http://comp.uark.edu/~jshue
==============================================================================
I don't know where I heard that this could be done; I have never tried
it. So this morning I went out to a local "World of Science" store and
bought a Crookes Radiometer to play with ($7.95). I could not get it to
run backwards while shining a strong light on it in the refrigerator (or
freezer). I then tried various things and have come to the following
"understanding":

Nothing that I was able to do could make the radiation pressure effect
dominant; however, the thermal effect can make it run "backwards". When
the radiometer is rising in temperature (energy exchange with warmer
surroundings), the black vanes rise faster than the white vanes and it
goes (say) clockwise. But when the radiometer is falling in temperature
(energy exchange with cooler surroundings), the black vanes fall faster
(in temperature)than the white vanes and it goes counter-clockwise (now
the air next to the black surfaces is cooler!).

You don't need a light beam, a hot air dryer will do, or even the
proximity to a hot stove heating element. Conversely, if the radiometer
at room temperature (or, better, warmed up) is placed in the freezer it
will run backwards - with no light beam.

The first clue came when I put the radiometer in a microwave oven (!)
for a 1 second blast. The whole bulb glowed nicely with a nitrogen
discharge and the vanes spun like mad in the forward (heating up)
direction. When the oven stopped, the vane rotation coasted to a halt
and immediately sped up significantly IN THE OPPOSITE (cooling down)
direction before coming to a dead stop. I have since verified that this
momentary reversal can easily be provoked after ordinary (non microwave
oven!) forward motion, if the radiometer has been warmed above room temp
in the process.

So it seems that under all these "ordinary" circumstances at least,
radiation pressure plays no discernible role in the behavior of this
Crookes radiometer. I wonder if "high vacuum" versions are available
and whether these can be driven by radiation pressure as easily as mine
is driven by thermal effects.

Great Phun!!!
-
Bob Sciamanda sciamanda@edinboro.edu
Dept of Physics sciamanda@worldnet.att.net
Edinboro Univ of PA http://www.edinboro.edu/~sciamanda/home.html
Edinboro, PA (814)838-7185