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

[Paul Nord <paul.nord@valpo.edu>]

John,

It seems that the physics concept Hewitt is trying to show is simply that
'liquids cool by evaporation because the faster molecules leave the
surface.'  That concept seems fundamentally sound.

We might like to add a few caveats about surface tension, energy
distribution, and equilibrium.  But isn't it basically true that if such a
strange liquid existed, it wouldn't cool by evaporation?

Paul

(P.S. I had a bigger gripe with December's Figuring Physics when it
referred to the momentum of the windshield and not the momentum of the
entire car.  That was an odd mistake for Hewitt.)


On Sat, Jan 20, 2018 at 10:18 PM, John Denker via Phys-l <
phys-l@mail.phys-l.org> wrote:

> On 01/20/2018 08:59 PM, Jeffrey Schnick wrote:
>
> > (I think the explanation given in the Physics Teacher is conceptually
> correct.)
>
> It's not correct, conceptually or otherwise.
>
> As Robert Cohen pointed out at the beginning of this thread,
> when the molecule leaves the liquid, it loses its binding
> energy (van der Waals or whatever).  If it loses energy
> but there is no cooling, what happens to conservation of
> energy??????
>
> Also, if the explanation were correct it would apply
> equally to evaporation (sublimation) from a solid.
> It's even more obviously wrong in that case.
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