From: skroger@zeus.calpoly.edu (Seth L. Kroger)
Newsgroups: talk.origins
Subject: Re: Thermodynamics FAQ
Date: 25 Jul 92 01:03:21 GMT
Organization: California Polytechnic State University, San Luis Obispo


My Responce to Henley's Thermo: Part 2


henley@eng.auburn.edu (James Paul Henley) writes:
>-----------------------------------------------------------------------------
>o What are the First and Second Laws of Thermodynamics?
>
>The first law is generally stated in terms of a closed system, also called
>a control mass.  So an auxiliary law is the conservation of mass:
>
>               THE LAW OF THE CONSERVATION OF MASS
>
>    "The mass of a control mass never changes."  [1, p.120]

A law?  Bzzzt.  A control mass system is DEFINED as having a constant
mass.  There are situations where Conservation of Mass is an
appropriate principle (and others where it's definitely not), but here
your're working with an arbitrarily defined system, not a basic
principle.

A closed system?  No, because as the 1st Law shows, you have energy
(work/heat) going in/out of the system.  "Closed system" is not a
proper thermodynamic term; it's a common term for an "isolated
system." It looks like Henley is taking the liberty to change the
definition into something that looks like it will support his
position.

>               THE FIRST LAW OF THERMODYNAMICS
>
>For a Closed system (control mass)
>
>  "A change of the total energy (kinetic, potential, and internal) is equal
>   to the work done on the control mass plus the heat transfer to the control
>   mass."  [1, p.121]
>
>  "Although energy assumes many forms, the total quantity of energy is
>   constant, and when energy dissapears in one form, it appears simultaneously
>   in other forms."  [2, p.22]
>
>E2 - E1 = 1Q2 + 1W2
>
>E2 : energy of the system at state 1
>E1 : energy of the system at state 2
>1Q2: the net heat transfer into the system in going from state 1 to state 2
>1W2: the net work done on the system in going from state 1 to state 2

See, energy input and output in the system.  It certainly not closed
in the conventional sense, though it may be in Henleyspeak.

>[1, p.121]
>
>  "Equililibrium is a word denoting a static condition, the absence of change.
>   In thermodynamics it is taken to mean not only the absence of change, but
>   the absence of any tendency toward change on a macroscopic scale.  Thus
>   a system at equilibrium is one which exists under such conditions that
>   there is no tendency for a change in state to occur."  [2, p.37]
>
>For an Open system:  (control volume)
>
>    rate of change of energy = energy flow rate in - energy flow rate out
>
>Control Volume : A system fixed in space which permits mass to cross the
>system boundaries. [1, p.130]

A system with a specific volume or shape, not necessarily "fixed" in
space.

>            .     .      .     .
>d~Ecv/d~t = Ein - Eout + Qcv + W
>
>d~ : partial derivative
>
>Ecv : Total energy of the control volume
>.    .
>Ein, Eout : Energy flow rate in and out, across crossing boundaries (Energy
>.           flux at crossing boundaries)
>Qcv : Net rate of heat transfer into system across inside boudaries.  (heat
>.      flux)
>W  : Net rate of work done on system by surroundings  (power)
>
>[1, p.133]
>
>Steady State: Implies that conditions at all points in the [system]* are
>              constant with time.  For this to be the case, all rates must
>              be constant, and there must be no accumulation of material
>              or energy within the [system] over the period of time
>              considered. [2, 30]
>
>* lit. apparatus.  For a control volume analysis, each component of the
>                   apparatus is considered an open system (control volume).
>                   The apparatus as a whole is actually a closed system.
>                   A point in the system would be represented by a
>                   particular location in the apparatus.
>
>                   For a cyclical process in a closed system, a point
>                   in the system would be represented by a periodic time
>                   in the cycle.

Sorry but your attempt to make a control volume apparatus into a
closed system doesn't work, even with your redefinition of a closed
system.  Take, for example, a jet engine.  It produces thrust by
taking in air, burning fuel in it, and exhausting it out the back.
Hardly the type of apparatus that has no mass flow into or out of it.

>Pseudo steady state is a condition in which it is convenient to assume
>steady state for portions of a non-steady state system.  
>
>THE SECOND LAW OF THERMODYNAMICS:
>
>    "The entropy S, an extensive* equilibrium property, must always increase
>     or remain constant for an isolated system**.  [1, p.187]
>
>dSi >= 0
>
>dSi : change in entropy of an isolated system**

Good, at least you have the proper term here.

>* The units of extensive entropy are energy divided by temperature.  The units
>  of the intensive property would be energy divided by temperature and mass,
>  or divided by temperature and moles.

Huh?  Extensive?  What's that?

(An extensive property is something that is dependent on the amount or
size of the system you're measuring.  Eg. mass or volume.  On the
other hand, intensive properties aren't dependent on amount.  For
example, the temperature and density of some water will be the same
whether there's 1 liter of 2 liters of water.)

>  In SI units:
>
>     S (the extensive property) has units:    J/K
>     s (the intensive property) has units:   J/(kg K)  or J/(kg mole  K)
>
>**An isolated system is one in which there is no mass or energy transfer
>  across system boundaries.  (see How do the Laws...Apply to Various Systems)
>
>In terms of a non-isolated system:
>
>dSsys + dSsur >= 0  or alternately: dSu >= 0
>
>dSsys : change in entropy of system
>dSsur : change in entropy of surroundings
>dSu   : change in entropy of universe
>
>Entropy generation within a system is due to friction.  In the absence of any
>friction, then the net entropy change is 0.  Friction here includes things like
>electical resistance, resistance to heat transfer, resistance to chemical
>reactions (inverse of rate constant), mechanical friction, air resistance, etc.
>For any real process, there is friction.  Entropy is transferred with heat
>transfer, and the direction of entropy transfer is the same as that of heat
>transfer. So, any time heat is transferred into the system, the entropy of the
>system increases.  Any time heat is transferred out of the system,
>the entropy of the system decreases.  Heat transfer out is the only
>means of decreasing system entropy of a closed system.  For an open system,
>entropy can be transferred out with energy transfer out, and with mass transfer
>out.
>
>So, any real process increases the entropy of the system + surroundings,
>but whether the entropy of the system itself increases or decreases is
>dependent on the heat transfer.  
>
>Since for an isolated system, there is no heat transfer, there is no
>means of reducing the entropy of the system. Which means that for any 
>real process in an isolated system, the entropy of the system increases, 
>and the energy available to do work decreases.
>
>Equilibrium, the state in which all properties stop changing, is defined
>by the second law as is the state of maximum entropy, that is, there is no 
>more energy available to do work, and no capacity for change.
>
>[1] _Fundamentals_of_Engineering_Thermodynamics_, Howell and Buckius,
>McGraw-Hill, 1987
>[2] _Introduction_to_Chemical_Engineering_Thermodynamics_, _Fourth_Edition_,
>Smith and van Ness, McGraw-Hill, 1987
>[3] _Classical_Thermodynamics_of_Nonelectrolyte_Solutions_, van Ness and
>Abbott,  McGraw-Hill, 1982
>
>
>James P. Henley Jr.
>Chemical Engineering Dept.
>Auburn University


|======================================================================|
| Seth Kroger                                                          |
| skroger@pan.calpoly.edu                                              |
| Cal Poly, San Luis Obispo                                            |