Talk:Crookes radiometer
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The "actual explanation" given is the one given in Britannica and is itself wrong.
In fact, it has always been wrong. Crookes said it was radiation pressure. Maxwell was pleased that it demonstrated a prediction of his theory of electromagnatism. Maxwell was embarassed to discover the explanation was incorrect. The theory given here was put forward, and, wary of making another mistake Maxwell checked it and found it to be incorrect also. This is easy to see: Imagine the glass to be shrunk so that it is only just big enough to allow the vanes to move, with no gap between vanes and glass (and a frictionless surface). Hold the vanes from moving. Shine light on the vanes. Hotter at black side, cooler on white side. Pressure equalises between the vanes (or the gas would move). After allowing the device to reach equilibrium there is no overall force. Let go: nothing happens. The correct solution was discovered by Osbourne Reynolds and is called thermal transpiration and is due to assymetric effects as molecules bounce off the *edge* of the vanes in the presence of the temperature gradient. This effect could be tested by cutting holes in the vanes. According to the incorrect theory, this should produce less heating and less rotation, thermal transpiration says it will increase the rotation. s
- Added the explaination that I found in the books listed on the page. Hopefully it makes sense. Both books give a long and involved proof of the effect, but I tried to simplify the general idea. The only problem with the simplification is that it kind of ignores comparing the number of gas molecules hitting the surface. Jrincayc 14:07, 5 Sep 2003 (UTC)
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[edit] Removed Text
The actual explanation has to do with temperature differentials between the two sides of the vanes. The blackened side, absorbing radiation, is slightly hotter than the silvered side.
The reason for the motion of the radiometer was determined by James Clerk Maxwell and Osborne Reynolds in the later portions of the 1800s. The effect occurs at the edges of the vanes. On the hot side, the gas molecules are moving with higher average speed than the gases on the cold side. When the hot molecules hit the edge of the vane, on average they will produce a force on the vane that is towards the cool side. When the cool molecules that are passing in the other direction hit the vane, they will on average produce a force that is towards the hot side. Since the average speed of the hot molecules is greater than the average speed of the cold molecules, there will be a force on the vane towards the cool side. See the diagram below for an illustration of the effect.
This effect, called thermal transpiration, gives the vanes their force away from the hot side and thus is the cause of the motion of the radiometer.
- This text was removed by 69.157.79.102. Jrincayc 13:08, 23 Sep 2004 (UTC)
[edit] Reynolds' force must be the stronger.
Whether the "bumps in the surface" provide the proper explanation isn't clear to me. However, the article concludes that it isn't clear whether the Einstein effect or the Reynolds' force is the stronger, whereas the external link to the explanation by Phillip Gibbs claims that the explanation by Reynolds is the correct one. The reference to Einstein must be to the photoelectric effect. This must be smaller, I think, since otherwise the radiometer would not reverse direction upon cooling. Only problem is that the explanation by Gibbs is still confusing (at least to this reader) about how the edge effect attributed to Reynolds and Maxwell actually works. (Comment by PhysicsObserver, 5 June 2005)
[edit] Practical Application
Has any means of extracting power from this device ever been found? Has the design of the vanes ever been optimised to give the greatest effect? Surely this would then give information on which effect is the primary one driving the device. If the edge effect is dominant then a vane paddle that maximises the length of the edge by making it convoluted would increase the efficiency of the machine. Whilst a design which minimised the edge length ( a circle) would minimise it. Lumos3 13:09, 26 Jun 2005 (UTC)
I second this request for clarification. Does the machine become more efficient with a more convoluted edge? Is their an optimal ratio or shape? Does the face need enough spatial extent to heat the rarefied gas, at which point it can start exerting force on the edge? What if the faces were porous? What is the optimal gas pressure inside the bulb? and how does it relate to the other parameters like the size of the faces?
[edit] dissatisfied with explanation and want reverse during cooling explained
I find it difficult to believe we have to refer to "bumps" to provide an explanation. Where's a reference to an article that shows a different texture of surface provides a faster or slower spin? The article should mention that a temp difference is needed for any engine to turn. The end of the current article is wrong because it says the dark side has the lower pressure which would turn it in the wrong direction. Obviously, there's more pressure of some sort on the black side. Section 3 discounts the higher velocity of molecules that have touched the black side, but I believe the effects "explained" in section 4 could not occur without this.
As the article says, cooling it makes it go in the opposite direction (when there's no light turning it from an external light source). I applied ice to check mine, and yes it went backwards. How does the current explanation explain that? My only explanation is that without a light source, black body radiation is making the black side cooler than the white side, so it turns backwards. Black body radiation is after all the reverse of black body absorption. But in order for this to work, the surrounding partial vacuum needs to be cooler than the vanes. So, you have to continually lower the surrounding temp in order to extract the remaining heat energy out of the vanes. Engines require a difference in temp. So the white vanes in this reverse situation are doing the same as the black vanes in the forward situation simply because the black side is cooler (when ice is applied to the external glass). So the key ingriedents for turning appear to be simply two things: 1) temperature difference between the opposite sides of each vane, and 2) that the hotter side be hotter than ambient temperature (so that it can impart more thermal energy to the surrounding molecules than they already have from ambient temp).
Similarly, the lighted situation makes it turn only as long as there is a net outflow of heat, otherwise temperature equalibrium would be reached and the engine would not turn. Heat escaping slows the molecules down relative to the hotter side of the vanes.
I wonder if a mirrored surface would work better than white.
[edit] electrons
So it has nothing to do with thermionic emission or the photoelectric effect? — Omegatron 20:34, 4 December 2005 (UTC)
[edit] redid Reynolds force based on "How does a light-mill work?" ref
As previous discussion remarked, our previous explanation of the Reynolds force had the radiometer spinning in the wrong direction. I rewrote it not from any expertise, just my summary of the "How does a light-mill work?" FAQ we refer to.
Deleted text follows. If the business about microscopic bumps is valid and should be re-introduced while still having the thing spinning the right way round, please do.
In addition to the temperature differential at the edges that Einstein considered, there is an additional factor due to how the gas interacts with the surface. Reynolds original work was on how a porous plate could be used to pump gas by heating one side and cooling the other. Reynolds found that the fast moving molecules could more easily move through a pore in a plate, or more easily move over a surface. Maxwell showed how this would cause an air current within a Crookes radiometer. On the black side, the slower moving molecules at the edges get caught in the microscopic bumps on the surface, while the faster moving molecules in the center can skip over the bumps. This causes a slight outward air flow, and creates a lower pressure in the center of the black face. Similarly on the white face, there is a slight inward air flow and a corresponding higher pressure.
One question comes up when you read my new text: essentially, why doesn't the gas momentum towards the black side collide head-on with the next vane, and stop it? My guess at the answer is that some of the momentum goes into the shell of the radiometer. I don't remember whether I actually read this anywhere, though, so am reluctant to put it in.
Eub 08:40, 11 December 2005 (UTC)
[edit] Strange Behavior
Moved this here as its more of a question than an encyclopedia entry. Lumos3 18:16, 9 October 2006 (UTC))
- "When a radiometer is placed near a microwave radiation source, such as inside a household microwave oven, eratic behavior can be observed, such as the reverse motion of vanes and plasma inside the vacuum chamber." (Needs verification and explanation Lumos3)
- "Reverse motion of vanes can also be induced by extracting heat from the device by pouring ice water over the radiometer." ( Already in article with explanation Lumos3)
