Re: TdS is not dQ or d(anything)

[Jim Green <JMGreen@SISNA.COM>]

 10:48 10 05 2003 , the following was  received:
> On 05/10/2003 12:14 PM, Bob Sciamanda wrote:
> > ... the term TdS represents the heat transfer dQrev.
>
> I know practically every reference writes
>         TdS "=" dQ
> but it makes my skin crawl every time I see it.

Folks, John and I don't always agree, but we do agree on this point.  [I
emphasize that in my view there is no such thing as "heat
transfer".  "Heat" is not a substance.  It is a form of "work".]

The problem arises when France did poorly in a war with England -- partly
because of England's railway transportation system.  Carnot, a diplomat's
son, could see the advantage of improving the efficiency of steam engines
if France were to do better the next time.  [Alas they never have.  And
England has babysat them to this very day.]

So like a good amateur physicist Carnot set up a simple model --  the
ubiquitous adiabatic cylinder with a piston and an ideal gas -- and based
his thermodynamics on Aristotle -- both terribly unfortunate because due to
the awkward momentum of physicists, we still have this unhelpful model in
every science textbook today.

Now purely just by bald coincidence, the work done by the ideal piston in
this ideal system doesn't change the system entropy!  [It would be very
nice if someone could give an example where the entropy of a system could
be calculated due to any other mechanical macroscopic work where the
entropy change is zero.]

And because Carnot -- and his followers -- based on Aristotle -- thought of
"heat" as a substance that flowed into the system, they came to the
conclusion that dS=deltaQ/T [Note there is no such thing as dQ].

And here we are, a few years later, believing this uhh ---- I was going to
write "crap" but I think that may be too strong for this list.

We believe it because Carnot, Kelvin, et al taught it.  Then their students
taught it. Then their students taught it. And now we teach it to our
students.  And their students....  This will likely continue until more of
us wake up.  John's students are lucky.  Except they have to deal with
flowing energy.  Well I guess it will take more time.

Jim


Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen