This is a question relating to a Crooke's radiometer: Assume that due to absorption, the local temperature close to the black surface is T = 50C, while the temperature close to the white surface is T = 20C. For a gas with density n = 2:5 x 10E20m-3, compute the net force on one blade of area 1cm2. What do you expect in the limit of vanishing gas density? I would appreciate any help you can give, if anyone knows what the 'limit of vanishing gas density' is or has any equations that would be useful in solving the problem. I'm sure this is incorrect but can you use the formula for Kinetic Energy- KE=3/2 kT and subtract the kinetic energy by the white surface from that of the black surface? Thanks!