Crooks radiometer

Discussion in 'General Science & Technology' started by fishtail, May 13, 2007.

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  1. fishtail Registered Senior Member

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    http://en.wikipedia.org/wiki/Crookes_radiometer

    Over the years, there have been many attempts to explain how a Crookes radiometer works:

    Crookes incorrectly suggested that the force was due to the pressure of light. This theory was originally supported by James Clerk Maxwell who had predicted this force. This explanation is still often seen in leaflets packaged with the device. The first experiment to disprove this theory was done by Arthur Schuster in 1876, who observed that there was a force on the glass bulb of the Crookes radiometer that was in the opposite direction to the rotation of the vanes. This showed that the force turning the vanes was generated inside the radiometer. If light pressure was the cause of the rotation, then the better the vacuum in the bulb, the less air resistance to movement, and the faster the vanes should spin. In 1901, with a better vacuum pump, Pyotr Lebedev showed that in fact, the radiometer only works when there is low pressure gas in the bulb, and the vanes stay motionless in a hard vacuum. Finally, if light pressure were the motive force, the radiometer would spin in the opposite direction as the photons on the shiny side being reflected would deposit more momentum than on the black side where the photons are absorbed. The actual pressure exerted by light is far too small to move these vanes but can be measured with devices such as the Nichols radiometer.
    Another incorrect theory was that the heat on the dark side was causing the material to outgas, which pushed the radiometer around. This was effectively disproved by both Schuster's and Lebedev's experiments.
    A partial explanation is that gas molecules hitting the warmer side of the vane will pick up some of the heat i.e. will bounce off the vane with increased speed. Giving the molecule this extra boost effectively means that a minute pressure is exerted on the vane. The imbalance of this effect between the warmer black side and the cooler silver side means the net pressure on the vane is equivalent to a push on the black side, and as a result the vanes spin round with the black side trailing. The problem with this idea is that the faster moving molecules produce more force, they also do a better job of stopping other molecules from reaching the vane, so the force on the vane should be exactly the same — the greater temperature causes a decrease in local density which results in the same force on both sides. Years after this explanation was dismissed, Albert Einstein showed that the two pressures do not cancel out exactly at the edges of the vanes because of the temperature difference there. The force predicted by Einstein would be enough to move the vanes, but not fast enough.
    The final piece of the puzzle, thermal transpiration, was theorized by Osborne Reynolds, but first published by James Clerk Maxwell in the last paper before his death in 1879. Reynolds found that if a porous plate is kept hotter on one side than the other, the interactions between gas molecules and the plates are such that gas will flow through from the cooler to the hotter side. The vanes of a typical Crookes radiometer are not porous, but the space past their edges behave like the pores in Reynolds's plate. On average, the gas molecules move from the cold side toward the hot side whenever the pressure ratio is less than the square root of the (absolute) temperature ratio. The pressure difference causes the vane to move cold (white) side forward.
    Both Einstein's and Reynolds's forces appear to cause a Crookes radiometer to rotate, although it still isn't clear which one is stronger.
     
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  3. fishtail Registered Senior Member

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    As it can be shown that photon pressure is not the cause of rotation
    (it stops working if the vacuum is increased), (another dent in the photon
    has mass theory).
    And yet it is still not fully explained how it works.
     
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  5. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    What was “dismissed"? I owned a cheap one prior to moving to Brazil. I have always believed the "hot molecules leaving from the black side" theory was correct, but now I think the objection to it may be valid.

    Think about it this way: Assume a sealed, gas-filled, cubic box with two perfectly transparent glass walls on opposite sides. Laser light passing thru the glass (without heating the glass) and exactly parallel to opposite walls 1&2 heats the gas, but only in the half adjacent to metal wall 1, which is identical metal wall 2. Walls 3 and 4 are opposite each other and perfect insulators. Surely more heat escapes out of the box via wall 1 than wall 2 as the gas adjacent to wall 1 is hotter and less dense. (I magically abolished convection within the box, but even if present, it does not completely cancel the argument I am making.) If there were more pressure on wall 1 than on wall 2 the box should (on the perfectly frictionless table it sits or in the vaccuum of deep space) constantly accelerate with wall 1 leading. This is false as the laser is providing energy but no momentum it the direction box would be moving, so the pressure on walls 1 & 2 are equal and I must admit I no longer understand something I thought I did for 40 years.

    I guess I should thank you, but please do not help me again.

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    Do you have a link that supports the "there are less of them, as adjacent warmer gas is less dense etc.” objection?
     
    Last edited by a moderator: May 14, 2007
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  7. fishtail Registered Senior Member

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    Billy T, i have been looking but canot find anything yet.
     
  8. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    I think, I have modified my box arguement after you posted to make it rigiorous proof that I do not understand how it works. My box argument is neat and simple - at least I can be happy to have thought of it.
     
  9. Pete It's not rocket surgery Registered Senior Member

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    Hi fishtail,
    In the same section of the Wikipedia article from which you copied the bulk of your post, you'll find this:

    The actual pressure exerted by light is far too small to move these vanes but can be measured with devices such as the Nichols radiometer.​

    Note that light pressure doesn't mean that light has mass - it means that light has momentum.

    The explanations you copied from Wikipedia seem pretty complete. What more do you want?
     
  10. Pete It's not rocket surgery Registered Senior Member

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  11. fishtail Registered Senior Member

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    Pete, my comment only refers to the bottom sentence.

    I an sure i said that it is a proof (against) photons having mass.
     
    Last edited: May 14, 2007
  12. Pete It's not rocket surgery Registered Senior Member

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    You said:
    There are a few problems with these statements, both in premises and conclusion:

    Firstly, there is no serious "photon has mass" theory that needs denting.
    Secondly, a test for photon pressure is not a test for photon mass.
    Thirdly, the Crookes radiometer isn't sensitive enough to test for photon pressure anyway. A Nichols radiometer is, and shows that photon pressure does exist.
     
  13. D H Some other guy Valued Senior Member

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    Fishtail:

    Just because the Crookes radiometer isn't sensitive enough to measure photon momentum does not mean that photons have no momentum. Solar radiation pressure is a real effect. It is one of the more significant perturbations on geostationary satellite orbits.

    That photons have momentum does not mean they have mass. No experiments to date have been able to measure a non-zero photon mass (a statistically significant non-zero mass, that is).
     
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