Re: dW turns into dQ

[Bob Sciamanda <trebor@VELOCITY.NET>]

----- Original Message -----
From: Joel Rauber <Joel_Rauber@SDSTATE.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, November 01, 1999 5:55 PM
Subject: Re: dW turns into dQ


> Bob wrote in part:
>
> > Yes, once you have chosen a reversible path for the calculation of
> > dS=dQ/T, you must correctly (and uniquely) specify dQ vs. dW  (to
> > correctly evaluate dS this way).  In such idealized quasi-static
> > processes, however, there is (I think) never much to quibble
> > about as to
> > what is dW and what is dQ.
>
> As mentioned in a reply to Jim Green, I'm wondering if we really
disagree.
> The above is something that I think I could've written.  In particular
the
> "once you have chosen a reversible path for the calculation of dS=dQ/T,
you
> must correctly (and uniquely) specify dQ vs. dW" part.
>
> > But I really think that logically it goes the other way.
> > Once the final
> > state is determined (in part by applying the first law, with wide
> > discretion in dW vs dQ), then the change in entropy is determined and
> > known from its state function -  this then determines dQ=TdS. In a
> > reversible process it is the entropy change which defines dQ.
>
> It strikes me that there is no wide discretion; you must partition it in
the
> unique way for the path you are utilizing in calculating dS.  But this
may
> be a chicken/egg disagreement.
>
> Joel

You misunderstand (ie., I was not clear):
In accordance with Jim's query, I was envisioning a problem which involved
applying dE= dW + dQ to a real irreversible process ( to answer some
question?), and then in addition asked for the entropy change.

In applying the first law to the real process the aforementioned wide
discretion is applicable in assigning dW vs dQ and thereby determining the
final state properties.  Then, IF YOU WISH TO CALCULATE dS directly from
the relation dS=dQ/T, then you must invent a new, reversible process
connecting the same two end states - in this calculation, dQ is unique to
your chosen reversible (unreal) process.

I added that ,since the end states are now known, dS could alternately be
calculated from its (presumably known) state function.

I also noted that for the (any) REVERSIBLE process dE=dW+TdS applies and
dQ=TdS.   I caution that this TdS is not equal to the dQ you may have used
in applying the first law to the real irreversible process (where you
enjoyed the "wide discretion").

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor