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

I have run into this issue in other contexts. You try to make a point by
raising a counterfactual -- what would happen if the universe were
different than it is? Sometimes this works, but as a teaching method it
frequently runs into trouble. Your students (or your colleagues on the
list serve) start thinking about the implications of what you have
suggested and they start finding all kinds of ways that your little
counterfactual leads to weird results. It is tempting to try to prop up
your original questions: "No, no, no -- I just want you to consider what
would happen if the universe were different in the one way and no other!"
But it is no use...your audience persists in their reasoning and your
counterfactual leads to not just the limited conclusion you hoped for but a
lot of other conflicts with the laws of physics.

But maybe that is not the worst thing that can happen. It shows that the
threads of the tapestry are woven together.

In case you are curious, here are two counterfactual-type questions that I
tried (and failed with) in the past:

What would happen if an object exerted forces on other objects but
experienced no reactions?

What would happen if induced current flowed in the opposite direction of
the one indicated by Lenz's law? (Discussions here on this list helped me
with this one.)

On Mon, Jan 22, 2018 at 11:26 AM, Paul Nord <paul.nord@valpo.edu> wrote:

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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Forum for Physics Educators
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http://www.phys-l.org/mailman/listinfo/phys-l