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

Equilibrium thermodynamics handles non-equilibrium (irreversible) processes, if they connect well defined initial and final equilibrium states, by noting that:
1) The initial and final states are equilibrium states in which state variables are well defined. and collectively define the system state.
2) One can then construct an alternate, reversible process consisting of a series of equilibrium states which evolve from the initial to the final state according to the "laws" of reversible, equilibrium thermodynamic processes. Every state in this reversible process is an equilibrium state and has well defined state variables.
3) One then uses the physics of equilibrium, reversible processes to calculate, along the chosen, reversible path of equilibrium states, whatever system state changes are of interest in the transition from the initial to the final states.
4) Since "a state is a state is a state", these system changes are identical to the system changes undergone by the originally considered irreversible process.

Some specifics:
**************************************
I said:
The equations of state apply only to
definable EQUILIBRIUM STATES,

JD replied:
I do not accept the restriction to "EQUILIBRIUM" states;

I continued:
where the state variables are definable.

JD replied:
We agree that the needed variables should be definable.

My specific comment:
If the state variables are all well defined, you have an equilibrium state.
*******************************************************
JD wrote:
Even something as central and basic as the Carnot heat engine
requires two heat baths, at temperature T1 and temperature T2.
Each heat bath is in equilibrium with itself ... but not with
the other.
We all know how to handle this situation. I'm just saying
we shouldn't pretend it's an equilibrium situation. Any
nontrivial thermodynamics is non-equilibrium thermodynamics.

My specific comment:
During the Carnot cycle the system under consideration is always in a well defined equilibrium state. This in no way requires the two heat baths to be in equilibrium with each other. What are you thinking of?

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
treborsci@verizon.net
http://mysite.verizon.net/res12merh/

--------------------------------------------------
From: "John Denker" <jsd@av8n.com>
Sent: Tuesday, January 12, 2010 7:01 PM
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Subject: Re: [Phys-l] T dS versus dQ

On 01/12/2010 04:28 PM, Bob Sciamanda wrote:
then E is not a function of state during the time that
the piston is provoking sound waves, shocks, et cetera
in the fluid. The analysis crashes and burns at this
point. We cannot proceed in this direction

Certainly (Duuuh!)!

OK.

The equations of state apply only to
definable.EQUILIBRIUM STATES,

I do not accept the restriction to "EQUILIBRIUM" states;
see below.

where the state variables are definable.

We agree that the needed variables should be definable.

If
you want to apply equilibrium thermodynamics

But what if I don't want that? What if I don't want to
be restricted to equilibrium situations only?

to the PROCESS, then choose a
reversible process which connects the initial and final states with an
infinite series of equilibrium states.

If you restrict attention strictly to equilibrium situations
and reversible processes, then all of thermodynamics becomes

I do not accept such a restriction.

Even something as central and basic as the Carnot heat engine
requires two heat baths, at temperature T1 and temperature T2.
Each heat bath is in equilibrium with itself ... but not with
the other.

We all know how to handle this situation. I'm just saying
we shouldn't pretend it's an equilibrium situation. Any
nontrivial thermodynamics is non-equilibrium thermodynamics.

Also: In recent days several people have put forth perfectly
well-posed scenarios involving irreversible processes. Any
serious theory of thermodynamics must be able to handle
these scenarios. In fact we know perfectly well what
happens in these situations; we can quantify the energy
and entropy and temperature. There's nothing wrong with
the physics; there is only a problem with the archaic
terminology and misbegotten concepts that we use to _model_
the physics.

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