From: schinder@leprss.gsfc.nasa.gov (Paul J. Schinder)
Newsgroups: talk.origins
Subject: Re: Thermodynamics FAQ
Date: 29 Jul 92 15:12:46 GMT
Reply-To: schinder@leprss.gsfc.nasa.gov
Organization: Goddard Space Flight Center

In <henley.920728071653@wilbur.eng.auburn.edu> henley@eng.auburn.edu (James Paul Henley) writes:

>o What is Thermodynamics?

[better, but I still have a few comments]


>For any system that you want to apply a thermodynamic analysis to, you have
>to use the appropriate means of studying the system - microscopic or
>macroscopic.  When dealing with a very large system such as the earth, a
>macroscopic analysis is the only one that makes sense.

Not true.  You run the risk of making an error similar to Kelvin's
(about the time it would take for the earth to cool), and ignoring
internal degrees of freedom (on the microscopic level) that are
important for the question you are asking.  That's why it's always
wise to keep the "little picture" in mind.  You can go from
Boltzmann's equation to hydrodynamics, but you can't do the reverse.

>Also the border between mass and energy.  An electron in molecular orbit
>is considered as part of the matter of the molecule, but let that electron
>escape at a high velocity, and it acts more like energy.  So at the
>molecular level there are only two types of systems - open and closed - 
>because when you lose matter from an atom, you can't always distinguish
>whether the escaping matter is mass or energy.  At the macroscopic level,
>it is easy to distinguish.  Just define matter to be made of low velocity
>atoms and molecules and pure energy to be made of high velocity sub-atomic 
>particles.  We can draw a pretty clear line - a low velocity hydrogen ion
>is mass.  Anything smaller, or a proton moving faster is energy.  At the 
>macroscopic level it is very simple to make the disctinction.

As I said in e-mail, energy is an measurable attribute of a particle
or collection of particles, and it's value depends on the observer; it
is not an invariant (Galilean or Lorentz), and not some mysterious
"property" that floats around in space somewhere waiting to be
measured.  I would *not* call an high energy electron "energy" and a
low energy electron "mass".  Electrons have a (rest) mass, and
electrons have energy, and that's as far as I would go.  For some
purposes, for example, (and there are many other similar examples) I'd
make the distinction between a relativisitic (p >> m) and
non-relativistic (p << m) electron, and I'd approximate the
relationship between the energy and momentum of the electron (E = p
for relativistic, E = p^2/(2m) for non-relativisitic), but there's no
fundamental difference (the correct realation E^2 = p^2 + m^2 gives
both approximations in the appropriate limit).  Different observers
will disagree on which of two electrons are relativistic and
non-relativistic.

>I will wait until we come to a consensus on question 1 before furthur
>discussion of the other questions.
>------------------------------------------------------------------------------

>Entropy is a measure of the disorder of the energy of a system.  Ordered
>energy is available to do work, disordered energy is not. So, mathematically

Missed this one the first time around.  I don't know exactly what you
mean by "disordered energy", but if you mean heat energy (random
kinetic energy of molecules), this is wrong as stated, as anyone who's
played with a toy steam engine knows.  If you take a hot gas and let
it expand, useful work can be done.  The second law says that not
*all* of the heat energy can be extracted.  I don't know how you
distinguish between "ordered" and "disordered" energy.

>James P. Henley Jr.
>Chemical Engineering Dept.
>Auburn University
--
--------
Paul J. Schinder
NASA Goddard Space Flight Center
schinder@leprss.gsfc.nasa.gov
Newsgroups: talk.origins