From Feynman's Six easy pieces:

Then a molecule leaves it is due to an accidental, extra accumulation of a little bit more than ordinary energy, which it needs if it is to break away from the attractions of its neighbors. Therefore, since those that leave have more energy than the average, the ones that are left have less average motion than they had before. So the liquid gradually cools if it evaporates

Why do the molecules which are left have less motion? I can't understand why. Is it because there are fewer molecules for them to jiggle against and get more speed? Could someone please explain? Thank you in advance.

  • 2
    $\begingroup$ Suppose you measure the average speed of three molecules. Then you take away the molecule that has the highest speed. What will average speed of the remaining two molecules be? Higher or lower than the previous average? $\endgroup$
    – fishinear
    Feb 26 at 16:03
  • $\begingroup$ oh yeah, that makes sense. $\endgroup$
    – brund45
    Feb 26 at 16:23
  • 1
    $\begingroup$ thank you by the way $\endgroup$
    – brund45
    Feb 26 at 16:23
  • 1
    $\begingroup$ Feynman doesn't say the individual molecules that are left behind have less motion. He says "..the ones that are left have less average motion than they had before" $\endgroup$
    – Bob D
    Feb 26 at 17:01


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