Naked Science Forum

General Science => General Science => Topic started by: katieHaylor on 24/05/2018 11:28:34

Title: How big could a Crookes Radiometer be and still function?
Post by: katieHaylor on 24/05/2018 11:28:34

Mervyn asks:

How big could a Crookes Radiometer (https://en.m.wikipedia.org/wiki/Crookes_radiometer) be which would still function?
I think it would be amazing to see a large one at a public building like a museum or a library.

What do you think?

Title: Re: How big could a Crookes Radiometer be and still function?
Post by: Bored chemist on 26/05/2018 21:03:17

I can't think of a reason why there would be a theoretical limit. It might be rather expensive, but there's no reason why you couldn't make one as big as an old fashioned CRT type television tube.

Title: Re: How big could a Crookes Radiometer be and still function?
Post by: Jeffersonz on 25/06/2018 09:57:31

All the stories in the forum are most important.

Title: Re: How big could a Crookes Radiometer be and still function?
Post by: RD on 25/06/2018 11:50:24

Quote from: Bored chemist on 26/05/2018 21:03:17

I can't think of a reason why there would be a theoretical limit ...

The square-cube law (https://en.wikipedia.org/wiki/Square%E2%80%93cube_law#Engineering) is going to limit its size: the force driving it increases with the area, but the mass of it increases with the volume. As you scale it up, at some point the frictional forces on the axle, (proportional to the mass), will become greater than the torque driving it, and it will seize-up.

(https://upload.wikimedia.org/wikipedia/commons/2/28/Radiometer_9965_Nevit.gif)

Title: Re: How big could a Crookes Radiometer be and still function?
Post by: Bored chemist on 25/06/2018 18:52:14

I suspect that the torque rises with the cube of the size and so it will compensate the increased losses at the bearing.