From: schinder@leprss.gsfc.nasa.gov (Paul J. Schinder)
Newsgroups: talk.origins
Subject: Re: Thermodynamics FAQ
Date: 30 Jul 92 16:52:00 GMT
Reply-To: schinder@leprss.gsfc.nasa.gov
Organization: Goddard Space Flight Center

In <henley.920730095117@wilbur.eng.auburn.edu> henley@eng.auburn.edu (James Paul Henley) writes:

>In article <schinder.712455669@leprss.gsfc.nasa.gov> schinder@leprss.gsfc.nasa.gov writes:
>>In <henley.920729170804@wilbur.eng.auburn.edu> henley@eng.auburn.edu (James Paul Henley) writes:
>>
>>
>>>Are there any microscopic considerations that will alter this overall
>>>energy balance?  If so, tell me what they are, and I will include them
>>>in the energy balance.  The only one that I can think of would be
>>>a short term change in cloud cover.  Enough to significantly alter the
>>>energy balance because of reflected radiation.
>>
>>I haven't done the computations, but here's what I'd guess offhand.
>>The overwhelming contribution is radiation in from the sun (1440 w/m^2
>>at the top of the atmosphere) and radiation out.  There are small
>>scale fluctuations, but the earth is in radiative equilibrium on long
>>enough timescales (>= order of a year or so).  We're doing our bit to

>Do you mean that rate of energy in = rate of energy out, or that the
>rates are not changing?

Rate in = rate out.  That's what radiative equilibrium means.  No
*significant* portion of this energy is being trapped in internal
degrees of freedom (for example, life) on any timescale, compared to
the amount that's simply radiated away.

>A very important distinction.   If rate in = rate out, then macroscopically
>the earth is at steady state.  You have to take a long enough time period
>to smooth out the random fluxuations (chaos).  Steady state does *not*
>mean equilibrium.  At equilibrium there are no dynamics, at steady state,
>the dynamics are cyclical.  This is where the difference between closed
>and isolated comes in.

Sigh.  Again we come down to definitions.  I know, then, of *no*
equilibrium situations, and the concept is of only of theoretic
interest.  In steady state, the dynamics need not be cyclical, they
can, for example, be stochastic (random).  Who's worried about
microphysics here; why is the dynamics of the internal degrees of
freedom important to you?  To me a "steady state" is when no quantity
of interest is changing on timescales of interest, and my use of the
word may change depending on the question asked.  Chaos is not random
fluctuations; in a mathematical sense, a chaotic system is one in
which very small changes in the initial conditions lead to arbitrarily
(although not necessarily unbounded) large differences in the state of
the system at later times.  Note that this does *not* mean the state
is not deterministic, simply that if we don't know the initial
conditions we're likely to be increasingly wrong about our predictions
of the state at later times.

When I see the word "equilibrium" it is usually used 1) when no
changes of interesting size are occuring to the system on interesting
timescales (this is also true of steady state systems), *and* 2) the
state of the system is sufficiently well described by thermodynamic
variables alone.  There are steady state situations which are not in
equilibrium in nature, such that you have to actually solve rate
equations to figure out what the constituents are doing rather than
using equilibrium distributions for them.  Sometimes you use a mixed
analysis (radiation hydrodynamics, for example, in which the fluid is
treated as in "local thermodynamic equilibrium" but the photon
distribution function is explicitly calculated, as is the transfer in
energy between them).  No one I know uses "equilibrium" in the extreme
sense you use above.

>>
>>Now we have to investigate the various energy flows.  Since the
>>overwhelmingly dominant one is 6000 K blackbody photons in and 300 K
>>blackbody photons out, it's trivial to see that substantial "work" may
>>be done on the earth, and the total entropy of the "closed" system
>>does not *necessarily* increase, although it might *actually* (my
>>guess would be that is constant on timescales of years, since the
>>earth is in radiative equilibrium on those timescales).

>Again, does radiative equilibrium mean energy in = energy out, or just
>that the rate of change is very small?  And yes, you definitely have to
>choose the proper timescale.  Even yearly is too short of a period.  I
>would guess that you would see cycles occurring at periods of 12 and 60
>years, and possibly even longer.  That means integrating over a time
>period of 60 years to get a mean.  That should smooth out most random
>fluxuations.

That the energy trapped is negligible (energy in = energy out).  If we
got rid of humans (foom, gee, that was much easier than that silly
flood stuff), you'd be hard pressed to see any significant changes
over periods of 10's to 100's of years, I'd guess.  Evolutionary
changes would show up on longer timescales, and of course there's
always tectonics.  The climate is more or less stable on timescales of
this magnitude, and follows subtle astronomical changes in insolation
on longer timescales, from what I've read.

>>
>>This is mostly a quibble about terminology, but since terminology in
>>physics is used (we hope) in precise ways, I find it important.  I'm
>>not claiming that you can't compute the fraction of heat energy that
>>can be turned into work.  Your wording, however, seems to indicate
>>that there is some way to say "look, this energy over here is
>>disordered, while, hey, this energy over there is ordered".  That's
>>not possible, since you can convert energy into energy into energy ...
>>at a price.

>No, I am saying that you can accurately predict, based on the entropy of
>a system, the maximum work than can be done.  How accurate?  
>You are only limited by the accuracy and precision of your measurements.
>If you go beyond 5 significant figures, then you exceed the accuracy of
>the steam tables, and need some other means of calculating the properties.
>Of course, the steam tables are only for changing the state of water,
>if you start splitting open atomic nuclei, then you have a much higher
>potential energy source.  It still obeys the laws.

I agree.  You should probably change the wording of this section of
your FAQ to reflect this accurate statement.  You cannot identify
"ordered" and "disordered" energy.  All you can say is that some
fraction is unavailable to do useful "work".


>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