How fast will a radiometer spin in a frictionless enviroment?

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    Frictionless Spin
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Discussion Overview

The discussion revolves around the behavior of a radiometer in a frictionless environment, particularly focusing on how fast it could spin and the underlying mechanisms of its motion. Participants explore theoretical aspects, including the role of temperature differentials and photon momentum, as well as the implications of a vacuum versus a rarefied gas environment.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that in a frictionless environment, the radiometer could theoretically spin indefinitely due to constant torque, but material limits would eventually impose a maximum speed.
  • Others argue that the radiometer's motion is primarily due to temperature differentials caused by the black and white sides, leading to molecular interactions rather than photon momentum.
  • A participant questions the presence of molecules in a vacuum, suggesting that a perfect vacuum would prevent the radiometer from functioning, while others clarify that a rarefied gas is necessary for its operation.
  • There is a discussion about the direction of spin, with some asserting that the black side trails due to momentum transfer from air molecules, contradicting initial assumptions about photon momentum.
  • One participant reflects on the confusion regarding the mechanics of the radiometer, admitting uncertainty about the source of their earlier claims regarding molecular impacts.
  • Another participant emphasizes the need for a complete evacuation of the tube for the radiometer to operate on radiation pressure alone.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the mechanisms behind the radiometer's motion, particularly concerning the roles of temperature differentials and photon momentum. The discussion remains unresolved, with differing opinions on the implications of a vacuum versus a rarefied gas.

Contextual Notes

Some participants note limitations in their understanding of the underlying physics, particularly regarding the conditions necessary for the radiometer's operation and the implications of different environmental setups.

carl fischbach
If you place a radiometer in a frictionless
environment and eliminate mechanical failure how
fast will it spin?
Since the frequency of c is very high, the
doppler shift of light reaching the white and
black side is virtually negilable, even if the
radiometer is spinning at high speeds,this also
applies to infrared radiation leaving the black
side.Therefore torque acting on the radiometer
doesn't decrease greatly with increasing speed
of the radiometer.Could the radiometer reach
high speeds? Which begs another question is the
the kenetic energy contained in the rotating
radiometer plus the the energy radiated from it
greater than the light energy radiated on to
the radiometer in the first place?
 
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Originally posted by carl fischbach
If you place a radiometer in a frictionless
environment and eliminate mechanical failure how
fast will it spin?
Since the frequency of c is very high, the
doppler shift of light reaching the white and
black side is virtually negilable, even if the
radiometer is spinning at high speeds,this also
applies to infrared radiation leaving the black
side.Therefore torque acting on the radiometer
doesn't decrease greatly with increasing speed
of the radiometer.Could the radiometer reach
high speeds? Which begs another question is the
the kenetic energy contained in the rotating
radiometer plus the the energy radiated from it
greater than the light energy radiated on to
the radiometer in the first place?

Since you're assuming that there are no dissipative forces then the constant torque will keep increasing the angular momentum and thus make it spin faster and faster until the material can't handle the mechanical stresses and it will then fall apart. So it really depends of the material. Otherwise it will spin at a rate at which the speed of all particles is just less than the speed of light. But it would fall apart way before then.

Pete
 
Radiometers don't spin by the momentum of photons. At some point someone proved that it is a result of the temperature differential in the rarified gas caused by the different colors of the paddles. The black side gets a tiny bit hotter than the white/silver side and the temperature differential causes the molecules to hit at a slightly more oblique angle on one of the sides. However, I do think that if we ignore mechanical failures, and after an infinite amount of time, for all intents and purposes its instantaneous tangential velocity will be the speed of light. That'd be one hell of a radiometer. Btw, carl fischbach, I've PM'd you and you haven't responded to that or to the one (and now two) thread(s) of yours that I've posted in, what's that about? I'm interested in what you have to say about angular momentum and vorticies! I notice a recurring theme here with all your threads, what's with it? You post several threads with topics relating to rotational anything and you never really say anything in them.
 
The black side gets a tiny bit hotter than the white/silver side and the temperature differential causes the molecules to hit at a slightly more oblique angle on one of the sides.

What does this mean? What molecules and how does the temperature of the surface affect the angle they hit at? I thought this was in a vacuum, so what molecules are you talking about?

If the cause of motion is a temperature difference then some form of gas must be present to make anything move.

Edit: Rereading the original post I see that he does not specify a vacuum but says frictionless. Wouldn't that imply a vacuum? I would expect air resistance to be the main friction acting.
 
Last edited:
Yes, if one could make a perfect vacuum in a radiometer, it would quit working. I was assuming that these air molecules where perfectly frictionless. The thing about the angle that I said: I was just repeating from what I remember. I have no idea what that was referring to when I heard it, it didn't make any sense to me now or then. It's too bad I don't know the name of the paper or the guy who wrote it or anything. I am pretty sure this is right, it has to be rarefied or there'd be too much friction, and one can't easily get such a good vacuum that you couldn't get it to work, so maybe this is bunk? I really was sure of it a minute ago.
 
IIRC, part of the proof was that a radiometer spins in the opposite direction than you'd think. You'd think that there'd be two impulses (hit, rebound) on the white/silver side and one (hit, absorb {no rebound}) on the other. But that would make it spin black side forward, which I remember them (I don't know who) saying that they(radiometers) didn't. I've never gotten around to checking though.
 
Yes, the spin direction is right on, black side would lead if a radiometer worked off photon-momentum transfer, but if you look on the internet there are a couple places you can find a movie of a radiometer, plus I just took Thermal Physics last semester and the professor had one so I've seen one in real life, and can assure you that the black side trails in a radiometer. The reason is simply momentum transfer by air molecules. A radiometer is simply an enclosed environment and as was said the black side gets hotter than the white side because the white side reflects more radiation, as a result, air molecules striking the black surface pick up more thermal energy and hence "kick off" with more momentum, and by Newton's third law the black side "rebounds" with more momentum. So a radiometer, ignoring mechanical hinderances, and with a very strong heat source (yes, you can spin a radiometer in the dark by putting your hands around it) would at best spin at the terminal velocity of the blades in the interior atmosphere, which would probably be related to the air density and the surface area, but I'm not an aerospace engineer.
 
Oh good, I'm right!
 
Originally posted by Jonathan
Radiometers don't spin by the momentum of photons.

Correct

One has to completely evacuate the tube instead of leaving a poor vacuum for it to work on radiation pressure alone

Pmb
 
  • #10
Just want to add...

In which case it would spin the other way.
 

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