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# Re: [Phys-l] Thermodynamics question

I don't think so, because T ds >= dq so dw is not restricted to just P dv. The equation T ds = du + Pdv is a standard state variable equation for a control mass. I thought it was a universal equation that was not tied to a particular process, but maybe that's where my thinking is wrong. I don't have any texts of that level to check at home. I'll verify it when I get to my office Monday.

Thanks,

Bob at PC

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Bob Sciamanda [treborsci@verizon.net]
Sent: Sunday, January 10, 2010 6:02 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Thermodynamics question

Then your third equation (T ds = du + P dv) needs another term. As it
stands, it seems to assume dw = Pdv.

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

--------------------------------------------------
From: "LaMontagne, Bob" <RLAMONT@providence.edu>
Sent: Sunday, January 10, 2010 5:46 PM
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Subject: Re: [Phys-l] Thermodynamics question

Yes, my dw, by definition, includes all energy tranfers out of the system
that are not related to temperature differences.

Bob at PC

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu
[phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Bob Sciamanda
[treborsci@verizon.net]
Sent: Sunday, January 10, 2010 5:01 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Thermodynamics question

If your system includes a battery which can interact with the outside,
your
dw must be more than just pdv - dw must include a term to account for the
energy transferred by that battery.

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

--------------------------------------------------
From: "LaMontagne, Bob" <RLAMONT@providence.edu>
Sent: Sunday, January 10, 2010 4:42 PM
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Subject: [Phys-l] Thermodynamics question

For better or worse, my engineering thermodynamics text is wedded to the

du = dq - dw

approach to the first law. Here, dq is the energy transferred to the
system due to a demperature difference and dw is energy transferred out
of
the system by all other means - both are inexact differentials.

Also, it takes the traditional approach of relating ds and dq in any
process (reversible or irreversible) as

T ds >= dq

The Gibbs relation amongst the state variables is (P is pressure and dv
is
change in specific volume)

T ds = du + P dv

one can put this together algebraically to obtain the following

P dv = T ds - du > dq - du = dw

Therefore

dw <= P dv

I am puzzled by this because I can think of a system where dv = 0 yet the
system sends energy out to its environment by a battery inside the system
attached by wires to a resistor outside in the environment. The energy,
dw, transferred out is definitely > 0.

As I stated in the beginning , disavowing du = dq - dw is not going to
help me here.

Bob at PC
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l