Crooke's Radiometer: How Does It Work Without Motors or Electronics?

[pephilg]

I have a question regarding a cool toy I saw and I’m not sure how it works.
It was a glass sphere, enclosed in the sphere was a rotating shaft with four plates. The plates had a shinny surface on one side and a dark surface on the other. The rotating shaft was free to move on each end and contained no motors or other electronics. The shaft rotates in the presence of strong sun light or the heat produced by the sunlight. I noticed when the sphere was shaded it did slow down. Shining an artificial light on it did cause it to rotate but very slowly. Can anyone explain to me the principle of operation. Thanks.

 

[Doc Al]

Crooke's Radiometer

Welcome to the forums, pephilg!

That toy is called a Crooke's Radiometer. The dark surface is a better absorber of light energy than the shiny surface, so it heats up more. In turn, the air next to the dark surface gets a little hotter, which increases its pressure. It's the pressure difference across the plates that makes it rotate.

 

[Adrian Baker]

Isn't also true to say that if the pressure inside is reduced to a low enough value, it will rotate in the OPPoSITE direction due to the momentum change of the reflected particles?

Has this ever been done?

 

[Doc Al]

Yes and yes. With the air gone, the dominant effect will be photon pressure: greatest for the shiny side. You need a good vacuum and low-friction suspension of the vanes.