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Re: [Phys-L] thermodynamics of open systems

Some of the pedantry associated with Q and dQ is becoming increasingly tiresome. The historic association of dQ is "an infinitely small amount of Q", There is a well-established notion of "infinitesimal" associated with the name of Robinson. If one can relate measurements related to temperature to notions of transfer of something called "Q" that is involved in a difusive process, then Q can be given an operational meaning, on an appropriate time scale. Since most infants learn the meanings of "hot" and "cold", expeerientially, at an early age, it is good pedagogy to use these terms.
In physics, the equations are used to make unambiguous predictions. Words associated with the equations are appropriate when they guide the user to making correct predictions.
Regards,
Jack

"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley




On Tue, 17 Jul 2012, John Denker wrote:

On 07/17/2012 01:51 PM, Bob Sciamanda wrote:

But John, the whole point of Carl's paper is that TdS is NOT ALWAYS equal to Q (or dQ).

I'll let Carl speak for himself ... but if that was the point, it
would have sufficed to write a far shorter and simpler paper.

On 07/15/2012 02:37 PM, Carl Mungan wrote:> (Try hard not to get hung up on
terminology, unless that *really* impedes understanding in your opinion.)

Maybe we are (despite our best efforts) getting hung up on terminology.

AFAICT when people write "dQ" there are only two things it could mean:
a) dQ = the derivative of Q
b) dQ = abusive shorthand for T dS

If there are additional possibilities, I have no idea what meaning was
intended.

In case (a), please explain why you think Q exists (except in trivial
cases).

In case (b), please explain why it would not be better in every way
to write T dS instead of dQ.

If there are other possibilities, I have no clue what they might be, so
please explain.
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