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# Re: [Phys-l] T dS versus dQ

• From: John Denker <jsd@av8n.com>
• Date: Mon, 18 Jan 2010 10:58:20 -0700

On 01/17/2010 11:33 PM, Bernard Cleyet wrote:

Could one substitute a heating element instead of the speaker? Very
easy to measure the E in.

That depends.
-- If you want to do that as a separate, *additional*
experiment, as part of a compare-and-contrast exercise,
that's a good idea.
-- If you want to do that to the exclusion of the version
based on the sound transducer, that's not good.

The experiments discussed yesterday serve multiple
purposes. Getting rid of the speaker version would
defeat some of the most interesting purposes.

For starters, one of my objectives was to demonstrate
that there are multiple mutually-inconsistent definitions
definitions of "heat". If you re-arrange things so
that only entropy that enters the gas flows across the
heater/gas boundary, then some of the inconsistencies go
away. My point is that dissipative processes are part
of the real world, and any worthwhile theory of
thermodynamics needs to be able to account for entropy
that is created from scratch within the interior of the
gas, *not* flowing in across any boundary. This means
that the definition that defines heat and work by
dividing dE into T dS and P dV (the "heat" term and the
"work" term) is grossly inconsistent with the definition
that defines heat in terms of energy flowing across a
boundary due to a difference in temperature.

Another of my objectives was to make the point that you
ought not take dE = T dS - P dV as an axiom or article
of faith or "first law", because Mother Nature does not
uphold this "law". You use calculus to expand dE in
terms of T dS and P dV *provided* that E can be expressed
as a function of S and V alone ... but this simply cannot
be done when there are sound waves running around. If
you get rid of the sound transducer, it defeats this
objective.

climbing technique, I don't want an answer that says
don't worry about it because you can always ride to
the top of the mountain in a helicopter. That doesn't
answer my question; it involves different apparatus,
different technique, different concepts, and different
terminology.

processes, I don't want an answer that says don't worry
about it, because you can always get to the same final
state using some combination of reversible processes.
That doesn't answer my question; it involves different
apparatus, different technique, different concepts, and
different terminology.

Dissipative processes are part of the real world.

p.s. one prob. students will be suspicious of using two different