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[Phys-L] Figuring Physics solution Jan 2018

I am curious as to whether anyone can confirm the explanation given in the Jan 2018 "Figuring Physics" solution provided in the Physics Teacher:

Basically, the question is whether cooling would occur if molecules of every speed in a liquid had an equal chance of escape from the surface. The answer given in the column is no, with a rationale that the cooling occurs because the faster ones are the ones that are leaving and the slower ones are left behind.

The reason I ask is because I used to use somewhat similar logic but stopped, and now I'm not so sure.

I stopped for two reasons. First, the "slow molecules left behind" implies that the remaining liquid becomes cooler than the vapor that is produced. Second, I feel it obscures the fact that the process of leaving (which is a sort of bond breaking) requires an extraction of energy from the surroundings, regardless of whether the molecules involved were initially going faster or slower.

Robert Cohen Department of Physics East Stroudsburg University
570.422.3428 East Stroudsburg, PA 18301