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Thermal Transpiration

  1. Oct 1, 2006 #1
    Crookes's Radiometer

    Hi,

    I am doing a project regarding Crookes's Radiometer. Research on wikipedia and a few other not-so-credible internet sources has stated that the physics behind the radiometer can be explained not only through Reynold's thermal transpiration, but also through the once refuted explanation that the molecules on the black side are faster/silver side are slower so there is a net force pushing on the black side. These articles state that Einstein found the forces resulting from differences in temperature actually do not cancel out near the edges of the radiometer vanes. However, none of the articles give any source for this.

    Does anyone here know of the original paper by Einstein on this? Or even a more detailed explanation?

    Also, if anyone knows of any relatively modern (post 70s or even 50s) articles regarding the radiometer, I'd really appreciate it if you'd post it here. All my research has found is a whole bunch of 1870s-1900s articles by Maxwell, Reynolds, Crookes, etc. that are horribly difficult to comprehend, full of incorrect explanations and just plain time-consuming. The only sources I have found not from that time period are A.E. Woodruff's 1966 "William Crookes and the Radiometer" as well as various books titled Kinetic theory of gases (and the stuff on the internet of course).

    Does anyone know of any good modern articles on the radiometer?

    Thanks,

    ~sphoenixee~
     
  2. jcsd
  3. Nov 18, 2006 #2
    Can someone please explain why thermal transpiration (aka thermal creeping) works? I am especially interested as to how this thermal transpiration works with Crookes's Radiometer.

    Here's what I know so far: thermal transpiration occurs in rarified gases. There are two compartments, one side warmer than the other. These compartments are then connected by a porous plate with very small openings. Due to the rarification of the gas, the molecules bounce more onto the walls of these pores than onto each other, and thus this somehow leads to a dynamic equilibrium at P1/P2=sqrt[T1/T2] instead of the normal P1/P2=T1/T2. In effect, this causes the molecules from the cooler compartment to flow into the warmer compartment. In Crookes's Radiometer, the space near the edges behaves like the porous plate, and the gas thus flows from the silver side to the black side causing a presssure difference which then causes the radiometer to spin.

    I believe this is the general idea of what happens (correct me if I'm wrong), but I have no idea why transpiration occurs, and am also confused about why it causes the Crookes's Radiometer to turn.

    Please assist.

    Thanks,

    sphoenixee
     
  4. Nov 18, 2006 #3

    Gokul43201

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    Last edited: Nov 18, 2006
  5. Nov 19, 2006 #4
    Very helpful article. Thanks Gokul!
     
  6. Dec 13, 2006 #5
    Not to rain on the parade....

    I think the Radiometer might be an excellent subject to bring up for a project, I just wouldn't try to go answering any questions.

    I recently read an article by Mike Ivsin and believe that he's correct (well at least his questions seem to lead down the right path).

    In short, he thinks that the latest theory regarding thermal transpiration happening within the Radiometer is indeed a myth.
    You can read more here:
    http://www.hyperflight.com/mainleft.htm#aboutlight

    I still don't fully understand how the mill works, but if you think about it, the mill should NEVER rotate in reverse since either the black side would be pushing (heat pressure) or the white side pulling (lack of heat pressure).

    Additionally, Scandurra has a patent which works on the theory that thermal creep happens around the 'edges' and that increasing the surface area of the edges (i.e. punching extra holes in the vanes) should speed up the mill.

    I had a unit build for me (but I can't swear to it's accuracy). It moves significantly SLOWER than my identical reference mill, so at present I'm inclines to agree with Mike as far as thermal transpiration goes.

    I'm no scientist so I'd love to hear more on the subject from people who can explain more than I can.
     
  7. Dec 16, 2006 #6
    The radiometer does rotate reverse if you put it in a refrigerator. This is because the black sides cool off faster than the silver sides.

    I don't know what you mean by the latest theory of thermal transpiration, but thermal transpiration has been used to explain Crookes Radiometer ever since 1879. That's a few hundred years back ;)

    Somebody has patented that? Interesting. Hope there's more to it than that, as increasing edge area has been known to increase efficiency for a long time (a direct implication of thermal transpiration). Actually, this was the idea I was going to do my research project on (I wanted to see what the optimum configuration of holes was). :rolleyes:

    How is it that you're having a unit built for you? If you're looking for a simple radiometer, you can find one in most science catalogs as a novelty item.
     
  8. Jan 29, 2007 #7
    Crookes's Radiometer (experimentation)

    How is it that you're having a unit built for you? If you're looking for a simple radiometer, you can find one in most science catalogs as a novelty item.

    To answer the easy question: if you have a light-mill built for you, perhaps it can be made so you can experiment with vanes with holes and/or castellations around the edges, to provide an enhanced edge effect.

    But you would also need a good air pump. I must check prices for pumps good enough for the vacuum quality involved. (It's decades since I did physics research with mechanical and diffusion pumps for high vacua!)

    Cheers, Mike.
     
  9. Jan 29, 2007 #8
    Yes, that was exactly why I had the unit built. In the end, it looks like you can't have someone else build something for you because you'll never trust the results. My vane with many holes runs much slower so either the two units I have are not evacuated to the same level or the Thermal transpiration theory is not sounds. All in all, virtually any text you read will tell you that even Einstein did not believe that Thermal Transpiration accounted for a significant % of the push factor.
     
  10. Feb 17, 2011 #9
    Re: Not to rain on the parade....

    ...
    I recently read an article by Mike Ivsin and believe that he's correct (well at least his questions seem to lead down the right path).

    In short, he thinks that the latest theory regarding thermal transpiration happening within the Radiometer is indeed a myth.
    You can read more here:
    http://www.hyperflight.com/mainleft.htm#aboutlight

    I still don't fully understand how the mill works, but if you think about it, the mill should NEVER rotate in reverse since either the black side would be pushing (heat pressure) or the white side pulling (lack of heat pressure).
    ...
    Hi,
    The basic entanglement here is that of reflection vs. absorption. During reflection there is no heat generated and the wave reflects without imparting momentum. (If photons could impart mo you could build a perpetual motion machine by having the beam bouncing between parallel mirrors.) The photonic absorption gets the heat into the gas molecules at the surface of the mill paddles and through gas expansion we get movement -- pressure increases at the dark surface. Drilling holes in the paddle will degrade the light mill performance.
    There is more stuff on this, including the instructions for light mill reversal at
    www.Hyperflight.com/oh-teacher.htm#mill_reversal
    The fact the beam of light imparts no mo at reflection is no classical physics but perhaps it could become classical in time.
     
  11. Mar 2, 2011 #10
    I have a different explanation for Crookes Radiometer. It relies on the fact that when there is a temperature difference between the gas and it's boundary, gas molecules will bounce with increased speed (that is how heat is transferred from boundary to gas). This increase in speed will exert a pressure on the boundary. In the case of Crookes Radiometer, normally this pressure will same at both side of the blade. However when there is a temperature difference between both sides of the blades the pressure will not be same. Pressure will more at the side where temperature is more (black side).
    Please see my blog post for details: http://rejeev.blogspot.com/2011/03/crookes-radiometer.html
     
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