From: henley@eng.auburn.edu (James Paul Henley)
Newsgroups: talk.origins
Subject: Re: Thermodynamics FAQ
Date: 30 Jul 92 14:51:17 GMT
Organization: Auburn University Engineering

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?

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.

>screw it up by dumping fossil carbon back in the biosphere, but that,

Guess what?  A model of Global CO emissions based on weather satellite
data shows that the major region of greenhouse gas concentrations is
in a band from northern South America to southern Central Africa.  Biomass
burning in the topics appears to be more significant than industrial
fossil fuel burning.  There are localized CO peaks in industrialized
areas, but they are only local.  There is a lot of other indications that
global warming and greenhouse gases are both caused by deforestation, not
fossil fuel emissions.  The director of the Pulp and Paper Institute here
told me that Israel may have altered the weather patterns in the middle east
by reforestation.  They have had a yearly increase in rainfall ever since they
began the reforestation.  Of course this is a local (microscopic) effect.  That
means that somewhere else, there is a yearly decrease in rainfall.  That appears
to be North Central Africa - the spreading Sahara, and possibly South America.

"The Contribution of Anthropogenic Emissions to the Global Distribution of
 CO in the Troposphere."  R.D. Saylor,  _The_International_Symposium_
 _on_the_Chemistry_of_the_Global_Atmosphere_, 1990, Chamrousse,
 France.

>according what I've read, takes 10's of years to become important.
>(There are secular changes that can be detected on long astronomical
>timescales; as I understand it, ice ages are now well understood in
>these terms.)  The mass flux in and out is probably negligibly small.

That is exactly the kind of thing that a macroscopic thermodynamic 
analysis would predict.

>
>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.

>>It's very simple.  You get out your steam tables, calculate the enthalpy
>>and entropy of the steam, determine what the enthalpy of the steam will
>>be at the same entropy but at the temperature that it is exhausted,
>>and delta H - difference in enthalpy, is how much work you can get out
>>of the steam.  In doing that, you are using the First Law to do an energy
>>balance, and the Second Law to do an entropy balance.  Elementary engireering
>>thermodynamics.  You are assuming an adiabatic, reversible expansion, so
>>to get the actual amount of work, you need to know how much friction was
>>involved and how much heat was lost to the surroundings.  The heat you
>>measure, and the amount of friction involved requires an entropy balance.
>>You measure the actual pressure of the exhausted steam, calculate the
>>entropy of the steam at that pressure,  and do an overall energy and
>>enthalpy balance to determine the amount of lost work. 
>
>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.

>--
>--------
>Paul J. Schinder
>NASA Goddard Space Flight Center
>schinder@leprss.gsfc.nasa.gov


James P. Henley Jr.
Chemical Engineering Dept.
Auburn University
Newsgroups: talk.origins