From: schinder@leprss.gsfc.nasa.gov (Paul J. Schinder)
Subject: Re: Thermodynamics FAQ
Reply-To: schinder@leprss.gsfc.nasa.gov
Organization: Goddard Space Flight Center
Date: Fri, 24 Jul 1992 01:44:50 GMT

In <henley.920723165933@wilbur.eng.auburn.edu> henley@eng.auburn.edu (James Paul Henley) writes:

>o What is Thermodynamics?
>o What are the First and Second Laws of Thermodynamics?
>o How are the Laws of Thermodynamics applied to various systems?
>o Does Snowflake formation violate the Second Law of Thermodynamics?


[lots of unnecessary verbiage but basically OK although obsolete; you
should learn some statistical mechanics, which helps generalize a lot
of the work of the 19th century engineers]

>This definition does not include energy that exists apart from matter.  For
>example, radiation in space.  Pure energy, apart from matter, is not
>measureable.  This is somewhat like the uncertainty principle.  In order to

Foolish mysticism, completely unlike "the uncertainty principle".
Energy is definitely measurable.  (*Define* the term "pure energy").
High energy experimentalists spend their lives measuring it.  There's
no real distinction between matter and energy; one can be converted
into the other.

>measure energy, you must first allow it to interact with matter, and then you
>measure the effect on the matter.  But once you have done that, it is no
>longer pure energy, so you can't be certain that what you are measuring is
>accurate for pure energy.  For example, the speed of light.  We must depend
>on the interaction of light with matter to measure the speed, but in so doing,
>we are no longer measuring light *apart from* matter, but rather measuring the
>effect of light *on* matter.  By this definition of the universe, pure energy
>exists in another dimension - outside of the space-time continuum that we call
>the universe.  It is only when energy interacts with matter that it enters the
>universe.  This gives us a clue about the nature of entropy - anytime energy
>interacts with matter, some of that energy is transferred to the matter, and
>some of that energy becomes disordered.

Gack.  This dreck is actually in an engineering textbook, or is it
something you just made up to for your own (gee, I wonder what)
reasons?

>Take the earth - we know that gas molecules can occasionally escape into space,
>meteors occasionally shower down into the atmosphere, and space missions leave
>the earth.  So in the strictest sense, the earth is an open system.  But if
>we study the rate of overall energy transfer, and compare that with the rate
>of transfer due to matter entering and leaving, then for all practical
>purposes the earth is currently a closed system, because the effect on the
>overall energy balance is negligible.  If we take the upper reaches of the
>atmosphere as the system boundary, then we can also say that the system
>has a fixed boundary.  In effect, we are really saying that a balloon is a
>model of the earth - for the purpose of thermodynamic analyses.

However, you're neglecting the incoming vs. outgoing radiation.  In
this respect, the earth is *not* (and can't be so long as the sun
shines and the earth radiates) a closed system.  In any sense.  No
doubt (for obvious reasons) you want to try to ignore this "energy",
but that's too bad.  Thermodyanics has no problems with it.

>Two different forms of the Laws of Thermo are used for open and for closed
>systems.   One consideration in an open system is the fact that energy
>can be transferred across the system boundaries due to intrinsic energy
>of the mass that is transferred.

Energy can be transfered in other ways than mass flow.


>--------------------------------------------------------------------------
>o Does Snowflake formation violate the Second Law of Thermodynamics?

>A forming snowflake is an open system.  There is mass transfer across the
>boundary.  If snowflake formation causes a reduction in the entropy
>of the snowflake, then, by the second law, the entropy change of the
>surroundings must increase.

There's also the latent heat released to the environment as the ice
crystalizes (energy released as the molecules bond).

>What about the order of the snowflake?  A snowflake indeed appears to have
>a high degree of order, but remember, we are talking about ordered energy.

We're talking about ordered water molecules.  Not energy.

>James P. Henley Jr.
>Chemical Engineering Dept.
>Auburn University

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