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How fast will a radiometer spin in a frictionless enviroment?

  1. Oct 11, 2003 #1
    If you place a radiometer in a frictionless
    enviroment 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?
  2. jcsd
  3. Oct 11, 2003 #2


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    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.

  4. Oct 11, 2003 #3
    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.
  5. Oct 11, 2003 #4


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    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: Oct 11, 2003
  6. Oct 11, 2003 #5
    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 refering 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.
  7. Oct 11, 2003 #6
    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.
  8. Oct 12, 2003 #7
    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.
  9. Oct 12, 2003 #8
    Oh good, I'm right!
  10. Oct 12, 2003 #9


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

  11. Oct 13, 2003 #10
    Just want to add...

    In which case it would spin the other way.
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