Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] thermodynamics of open systems



John D wrote:
"Constructive suggestion: In that document and everywhere else:
Every occurrence of "dQ" should be replaced by "T dS". "

and, again:

"There is no advantage to introducing dQ. If you are ever tempted
to write dQ, write T dS instead. "
*********************************
But John, the whole point of Carl's paper is that TdS is NOT ALWAYS equal to Q (or dQ).

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

-----Original Message----- From: John Denker
Sent: Tuesday, July 17, 2012 3:54 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] thermodynamics of open systems

On 07/15/2012 02:37 PM, Carl Mungan wrote:
http://usna.edu/Users/physics/mungan/Scholarship/OpenSystems.pdf

I would be delighted by comments on it.

Constructive suggestion: In that document and everywhere else:
Every occurrence of "dQ" should be replaced by "T dS".

Rationale: As Schroeder puts it, dQ is a "crime" against the
laws of mathematics. Except in trivial cases, there is no Q
such that d(Q) is equal to T dS.

There is no advantage to introducing dQ. If you are ever tempted
to write dQ, write T dS instead. It is pronounced tee-dee-ess.
T dS is perfectly well behaved; it is just not equal to the
derivative of any function of state.

(Try hard not to get hung up on
terminology, unless that *really* impedes understanding in your opinion.)

IMHO this is a conceptual issue, not a terminology issue. If
somebody writes "Parris is in the south of Frannce" I am not going
to quibble about the spelling, because even if you fix the spelling,
the statement is conceptually wrong.

Based on a lifetime of experience with "calorimetry" students arrive
believing in caloric, i.e. believing that heat behaves like a conserved
fluid. Bad teaching reinforces this misconception again and again.

Writing dQ is a symptom, not the root cause of the misconception.
Replacing dQ with T dS is necessary but not sufficient, because the
misconception is very widespread and deeply held. Students are not
the only ones to suffer from this misconception; I seen Ivy League
professors make serious mistakes based on this misconception.

I emphasize that the cost of getting rid of dQ is practically zero.
Schroeder wrote an entire thermo book without mentioning dQ at all
(except once or twice, in scare quotes, disapprovingly).

For a fuller explanation of the fundamental issue here, see:
http://www.av8n.com/physics/non-grady.htm
and for the next level of detail:
http://www.av8n.com/physics/thermo-forms.htm


Bottom line: Same as the top line: Break the habit of writing dQ.
You have nothing to lose and much to gain by writing T dS instead
of dQ.
_______________________________________________
Forum for Physics Educators
Phys-l@mail.phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l

_______________________________________
No infections found in this incoming message
Scanned by iolo System Shield®
http://www.iolo.com