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

• From: John Denker <jsd@av8n.com>
• Date: Thu, 25 Jan 2018 15:31:04 -0700

On 01/25/2018 01:16 PM, Robert Cohen gave a summary.

I agree with what he said, but I might say it in slightly
different terms, with slightly different emphasis:

1) It is super-important to distinguish between energy
and temperature. Obviously energy is extensive, but
/density/, which is intensive, as is temperature.
A pot of boiling water is the poster child for a
process that has a changing energy per unit mass
at constant temperature.

Let's postpone the discussion of what the author should
have said, and focus on what he actually said. He used
the language of temperature, namely cooling and warming.
The tail of the question is:

[...] evaporation would be a
A. cooling process.
B. warming process.
C. Neither of these.

You don't even need to read the first part of the question
to realize that A might be true, but not usually, and B
might be true, but not usually. Therefore C is by far
more likely to contain the right answer, whatever that
may be, and is not itself excluded in any obvious way.
In particular, it covers the poster-child case, i.e. a
pot of boiling water.

Under this interpretation, the first small part of the
given answer, i.e. the qualitative part, is correct.
However, the main part of the answer is wrong. The
point he is trying to make is wrong, and the path he
takes to get there is fallacious.

2) Working backwards from the answer, we surmise that
the author may have wanted to talk about energy or
energy density rather than temperature. So to be
charitable, let's consider the emended question:

[...] the process of evaporation would be:
A'. absorbs energy.
B'. gives off energy.
C'. neither of the above.

Still working backwards, we see that C' is the desired

The answer cannot possibly be correct, because there
are no such liquids. If it had zero latent heat, it
it would have no binding energy, so it would be a gas,
not a liquid.

This is a fallacy so old that there is a Latin term

In other words, if you imagine something that does
not exist, you are free to ascribe to it whatever
properties you like. You must not, however, imagine
that it has anything to do with the real world. In
particular, it tells you precisely nothing about the
behavior of real liquids.

This time you don't need to read /any/ of the
question to know that the answer is wrong. The
evident intent of the question was to create a
situation that does not exist, and the details
of how that was done aren't really important.

3) There is a prevalent and pernicious misconception
that "thermal energy" (whatever that means) is a
form of microscopic kinetic energy, to the exclusion
of potential energy.

The author didn't say anything about PE, and a lawyer
might argue that if you didn't say anything you didn't
say anything wrong ... but students aren't lawyers.
If you talk about KE and nothing else, students will
assume that what matters is KE and nothing else.

In fact, during evaporation, you do not have high-
speed particles departing from deep in the bulk.
Instead you have a multi-step process whereby a
particle makes its way into the surface layer,
gradually (but not monotonically) reducing the
number of bonds it has, i.e. reducing its PE,
until it eventually breaks free. In contrast,
the alleged model in terms of KE only:
a) paints a wildly wrong picture of liquids in
particular, and
b) tends to spread and exacerbate misconceptions