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I've been playing around with a Crookes radiometer for fun and I happened across something I can't figure out.

I know the fundamentals of the radiometer (e.g. partial vacuum, thermodynamics, etc.), but should it react to static electricity in such a way as to actually control the movement of the vane?

Out of curiosity, thinking the vane wouldn't move, I took a plastic comb and brushed my hair a few times (verified static with the "water" trick), I then put the comb near the radiometer and it started to move. I thought it was a fluke, that maybe I bumped the table or there was dust on the glass reacting, but after stabilizing and cleaning the radiometer, I brushed again and was able to twirl the comb a few millimeters above the radiometer and get the vane to spin in the direction I was twirling the comb around it.

I was able to even slow the radiometer by twirling the comb in one direction and letting it spin, then "charging" the comb again and twirling it in the opposite direction.

I had also stuck a neodymium magnet to the radiometer thinking the vane was metallic in some way, but could not get any movement, so I know the vane isn't reacting to any magnetic fields possibly produced.

I've been able to reproduce the results with any sort of item that can produce a statically charged field (i.e. combs, plastic pipes, etc.), but I can't find any explanations to this phenomena (or even if any experiments of the likes have been tried in the past).

One thought that occurred, but I can't find any evidence to back, is that the small amount of molecules in the vacuum are attracted to the static field, thus the attraction and movement cause a force to be exerted on the vane; again, I can't find any evidence to back this, nor do I have a spectrometer to verify what gases are indeed in the vacuum.

Thanks in advance for any insight or informational links (math formula welcome too if any).

txtechhelp
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2 Answers2

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The answer to this question is a lot like the answer to Why does the comb attract the pieces of papers if they're neutral? I'm guessing that your comb, which will be negatively charged, is not evenly charged across its width. So even though you are holding the center of the comb above the vane, the electric field between the comb and the ground (plane) is not entirely vertical at the position of the vane.

The horizontal component of the electric field induces a dipole moment in the vane. When you spin the comb, the vane tries to keep its dipole aligned with the rotating horizontal component of the electric field.

Comments:

  1. Just because your vane is not magnetic doesn't mean it is not conducting.

  2. The vane could be non-conducting and this would still work as an explanation, as long as the material is polarizable.

Community
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Gerry Harp
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  • I agree with your comments and your answer makes sense with regards to the charged comb and vane; so it is more probable the vane is reacting to the static field than say the static field is affecting the vacuum or glass or some other component of the radiometer? – txtechhelp May 28 '16 at 19:41
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The four (metal) vanes of a Crooke's are attached to an axle.
There is very little friction between the axle and its supports so a very small torque applied to the vanes would produce a noticeable change in the rotation of the vanes.
My suggestion is that charges are induced on the vanes by the charged comb.
Thus there is a net force of attraction between the induced charges and the charged comb which produces a torque on the vanes.

Farcher
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