[Tann San (OP)]

Hi, around christmas time in town there is a market.  They always have these little toys for around the house.  It appears to be two little metal plates, one black and one grey/silver.  They are suspended in a glass cylinder and spin constantly.  The stall owners always say that the two plates are attracted to each other (or just one to the other...) and require no battery etc to spin.  I was wondering if this was a well known effect.  They also say that it only works in a total vacuum which is why they are in a sealed glass container.  Coulnt this be used on a larger scale to produce clean energy?

[gsmollin]

The device you describe is known as a Crooke's Radiometer. The description of operation you got from the store clerk is nonsense. The radiometer is powered by thermal radiation, as its name suggests. The paddle wheel is suspended in a partial vacuum. The black side of the paddle wheel gets warm from absorbing thermal radiation. The air molecules adjacent to the black surface also become warmer, and exert a small amount of pressure on the black surface. The wheel turns away from from this higher pressure region.

Crooke originally expected the pressure of solar radiation, bouncing off of the silvered surface, to push the radiometer in a direction away from the silvered side. Instead, the black surface gets pushed by the thermal kinetic energy of air molecules impacting against it (e. g. "pressure"), and the radiometer turns away from the black surface.

This is a solar-powered device, and falls under the general heading of solar thermodynamic. Yes, similar devices are used for energy conversion. I think the solar dynamic power conversion device nearest to Crooke's Radiometer would be the closed, Brayton-cycle system.

[Tann San (OP)]

Hi, wow thanks for the reply.  You answered everything I could of wanted and more. Thanks again.

[gsmollin]

For those readers who may not be familiar with this toy, here is a link: http://scientificsonline.com/product.asp_Q_pn_E_3060082 . While you're there, check out the drinking bird, and the Stirling engine.

[gsmollin]

The device you describe is known as a Crooke's Radiometer. The description of operation you got from the store clerk is nonsense. The radiometer is powered by thermal radiation, as its name suggests. The paddle wheel is suspended in a partial vacuum. The black side of the paddle wheel gets warm from absorbing thermal radiation. The air molecules adjacent to the black surface also become warmer, and exert a small amount of pressure on the black surface. The wheel turns away from from this higher pressure region.

Crooke originally expected the pressure of solar radiation, bouncing off of the silvered surface, to push the radiometer in a direction away from the silvered side. Instead, the black surface gets pushed by the thermal kinetic energy of air molecules impacting against it (e. g. "pressure"), and the radiometer turns away from the black surface.

This is a solar-powered device, and falls under the general heading of solar thermodynamic. Yes, similar devices are used for energy conversion. I think the solar dynamic power conversion device nearest to Crooke's Radiometer would be the closed, Brayton-cycle system.

[Tann San (OP)]

Hi, wow thanks for the reply.  You answered everything I could of wanted and more. Thanks again.

[gsmollin]

For those readers who may not be familiar with this toy, here is a link: http://scientificsonline.com/product.asp_Q_pn_E_3060082 . While you're there, check out the drinking bird, and the Stirling engine.