Thermodynamics, Statistical Physics, and Quantum Mechanics

88

The forces due to the striking of the molecules on the upper and lower

surfaces are, respectively, and (see Figure S.4.5):

where is the concentration of the air molecules, and we have used the

fact that the number of molecules colliding with 1 of the surface per

second is approximately (the exact number is see Problem

4.14). The net resulting force is

Substituting for we have

Unfortunately,this estimate istotally wrongsince it assumesthat thecon-
centration ofmolecules is the sameabove andbelow thepanel, whereas it
would be higher near the cold surface and lower near the hot surface (see
Problem 4.4) to ensure the same pressure above and below. That’s why
irons don’t fly.

SOLUTIONS