How does a Crookes Radiometer work?
Asked by: Darcy Fawcett


This depends on whether or not you have a vacuum in the radiometer:

The photons hitting the black side of the vanes will be absorbed transferring their momentum to the vane. Those hitting the white surface will be reflected transferring up to TWICE their momentum to the vanes.

1) In a vacuum: The above concept dominates and the white vanes trail the black vanes.

2) In a poor vacuum: the air on the black side of the vane gets heated and the air molecules give an extra "kick" to the black vane side overriding the photon momentum transfer causing the black vanes to trail. I.e. the air molecules transfer more momentum to the vane than the photons do.
Answered by: Pete Karpius, Physics Grad Student, UNH, Durham

Support US

Our server costs have gone up and our advertising revenue has gone down. You do the math! If you find our site useful, consider donating to keep us going. Thanks!

Science Quote

'All of us, are truly and literally a little bit of stardust.'

William Fowler

All rights reserved. © Copyright '1995-'2018   Privacy Statement | Cookie Policy