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

W/O reading carefully, I understand the more important reason for cooling is the change of state, (work done to remove molecules) not what’s left. (partitioning)


bc thinks: if the above is correct, he was grossly mis-taught. More interleaved (below)

On 2018/Jan/23, at 09:18, John Denker via Phys-l <phys-l@mail.phys-l.org> wrote:

On 01/23/2018 05:04 AM, Robert Cohen wrote:

I'll start with the simple situation of a gas in a one-dimensional
container (so the particles only travel along that one dimension),
made up of particles of identical speeds under no attraction and
totally elastic collisions. That way the speeds remain identical.


The most interesting thing about particle C2 is
/not/ its speed.


The separation is more important than the speed?


It’s taken ~25 posts to get to this point.

I recommend that P. Hewitt submit his figuring physics to the editor or JD before publishing.

OTOH, the error(s) elicit much clarification.


I agree with all that.

Let me tell you /why/ I agree. It gets back to something
they teach in kindergarten: Check the work.

One useful check is to see whether the model given above
is a dead end, or whether it lays a good foundation for
thinking about things in 3D. This is not a check I expect
students to make in the introductory course, but it is
the sort of check I would make before teaching the 1D
model.

Let's split the difference and look at 2D. Here is a
snapshot of some molecules:

A B C D E

1 x x
2 x x x x x x





main points:
1) His main conclusion is not correct. It is not
only the faster ones that can break free.
2) Students are not lawyers. If you talk about
speed and nothing else, they will assume that
all that matters is speed and nothing else.


Additional lesser points:
-- Essentially /nothing/ is completely independent
of speed, so my lawyer says that Hewitt is asking
about properties of the null set. This set
contains immovable objects, irresistible forces,
and things that don't conserve energy, such as
Hewitt's liquid that evaporates without cooling.
You can make any predictions you like about such
things, confident that no experiment will ever
prove you wrong.

-- The usual example of a simple thermodynamic
system is an ideal gas. That's fine as a starting
point, but (a) that's not the only feasible
starting point, and (b) it's not a very nice
ending point, because ideal gases have a great
many peculiarities -- such as having no energy
other than kinetic energy -- that are not shared
by other thermodynamic systems, including liquids
and solids and molecules et cetera.

The most interesting thing about particle C2 is
/not/ its speed.

When you are in check-the-work mode, when you see
a dubious thermodynamic argument, the first thing
to check is whether it wrongly imputes ideal-gas
behavior to something else.

-- In my experience, all-too-often Hewitt understands
things at the starting-point level only, and generalizes
naïve ideas to more complicated situations, without
bothering to check the work.
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