From: schinder@leprss.gsfc.nasa.gov (Paul J. Schinder)
Newsgroups: talk.origins
Subject: Re: Thermodynamics FAQ
Date: 30 Jul 92 00:21:09 GMT
Reply-To: schinder@leprss.gsfc.nasa.gov
Organization: Goddard Space Flight Center

In <henley.920729170804@wilbur.eng.auburn.edu> henley@eng.auburn.edu (James Paul Henley) writes:


>Point taken.  You can't ignore the microscopic considerations, but likewise,
>you can't ignore the macroscopic considerations.  I will talk about them
>independently.   Let's take a macroscopic view:

Which, of course, I don't.  When an approximation is warranted, I take it,
but I recognize it for an approximation.

>If the rate of mass transfer is negligible, then we can eliminate change
>in overall mass from the energy balance and call the mass of the earth
>constant.  This is what is called a closed, diabatic system.

>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
screw it up by dumping fossil carbon back in the biosphere, but that,
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.
Cosmic rays are energetic, but their total energy flux is probably
small (but depending on the question asked, they may be crucially
important, since they are by far the most individually energetic
particles around.)  Heat due to radioactivity keeps the core warm but
the energy flux at the surface of the earth is probably small (but
again, it's the answer to why the core is still plastic).  Neutrino
flow is probably energetically substantial, but ignorable because
virtually none of them interact with the earth (they pass right
through).  If you want to call the earth a "closed" (chemists
definition) system, I'd be willing to use that as an approximation.
That tells me nothing about the thermodynamics, though, so I don't see
why it's so important.  It's not an *isolated* system, and that's the
only significant statement thermodynamically.

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).

>>
>>>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.

>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.


>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