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Re: [Phys-L] Figuring Physics solution Jan 2018

I appreciate all of the posts in regard to the Figuring Physics puzzle.

So this is where I stand today...

If molecules of every speed in a liquid had an equal chance of escape from the surface (as proposed by the Figuring Physics question) then...

#1: ...that would mean that there was no attraction between the molecules, and thus the liquid was actually a gas, in which case there would be no cooling because there would be no attractive force that molecules would have to "overcome" to leave.


#2: ...that would mean the liquid was configured such that either...

#2a: ...all the molecules happen to have exactly the same speed, in which case the likelihood of escape would depend upon where a particular molecule is on the surface (i.e., those with less "connections" to other molecules to break will leave first).

#2b:...all the molecules just happen to have the speed necessary to escape, regardless of their connections with the other molecules, in which case all molecules on the surface would leave at the same time.

In both #2a and #2b, the molecules involved in the separation (i.e., those on the surface that are leaving and those right underneath the surface that are staying behind) would slow down as they move apart, opposite the attractive force, leading to a cooling. Either way, the cooling that occurs is *not* because we are removing the high energetic molecules and "leaving behind" the less energetic molecules, since there is no difference within the liquid prior to the evaporation..

Based on this, my feeling now is that either the answer in Physics Today is correct (no cooling) but the rationale is misleading, or both the answer and rationale are both wrong.

Do I have this right?

P.S. Another reason I am uncomfortable with the "left behind" argument is because I don't see how we can use the "left behind" argument to make any quantitative prediction of the amount of cooling (unlike the "cooling by breaking bonds" argument). If someone can provide a quantitative prediction based on the "left behind" argument, I'd consider it.

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