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Re: ENERGY WITH Q



> Ludwik's exposition has not provided a sufficiently-clear
distinction between thermal energy and nonthermal energy.
> We were looking forward to getting one.

In many textbooks thermal energy, which is really
mechanical energy of randomly agitated molecules
and atoms, is referred to as internal energy. In my
opinion this term is not desirable because “internal"
could easily be misinterpreted as “all energy inside
a system”. This would be wrong, some energy inside a
system can be nonthermal. Think about a system
containing compressed springs and rotating wheels.

At the macroscopic level we can say that thermal
energy is the energy a system has by virtue of its
temperature. At the microscopic level we can say
that thermal energy is the mechanical (kinetic and
potential) energy of molecules. The nonthermal
energy of a system is its Etot-Eth.

The trouble I have with terminology (thermal versus
internal) is that the so-called "energy state", for
example, for a molecule, is determined by its total
energy and the idea of thermal energy does not
apply to a single molecule. Do we really need the
term "internal"? What is wrong with keeping only
thermal, non-thermal and total?

Traditionally, I have been rather unhappy with the term "thermal
energy." However, I am willing to be open minded and have read John
D's web page in that spirit. I can always count on learning a lot
from him. Here are some of my thoughts:

1. I am not satisfied with merely getting a clear distinction between
thermal and nonthermal energy unless someone can also give me a clear
distinction between the mechanisms for *changing* each of these
(either separately or simultaneously, as long as you tell me how to
calculate how big the change in each term individually is). I posed
this question the other day and I have not seen an answer yet. Here
it is again. I stop a block with a string. No change in the block's
thermal energy occurs. Now I stop the block via sliding friction. The
block's thermal energy changes. Why does friction change the thermal
energy of an object but tension does not (in these particular
situations)? Give me an algorithm for deciding how a particular force
changes the thermal energy, in other words, for partitioning the
changes in the two terms on the RHS of the First Law (which I usually
call mechanical and internal energy, but would not necessarily be
adverse to calling nonthermal and thermal energy).

2. Do I correctly understand that nonthermal energy is *always*
identifiable as *large-scale* translational, rotational, and
vibrational energy of identifiably macroscopic parts of the system?
In that case, I feel like the distinction between thermal energy and
internal energy is mostly a matter of quibbling over hairs. Consider
say Joule's experiment. I use a paddle wheel to agitate water and
find it consequently warms up. Okay, suppose my wheel actually starts
out by uniformly rotating the water. Then viscosity gradually dampens
out this uniform rotation, producing random translational motions of
the water molecules. Am I correct in supposing that you would
describe this as saying the water initially had nonthermal energy and
this gradually became thermal energy?

3. Many textbooks have long lists of energy (eg. Tipler). They
separately list thermal energy, chemical energy, nuclear energy,
radiative energy, electromagnetic energy, etc. Do I correctly
understand that you would disagree and call of these things "thermal
energy"?

If I correctly understand then I don't think I have any problems.
What you call "thermal energy" is what I usually call "internal
energy" in thermodynamics - ie. where the gross mechanical terms are
ignored or assumed to have appropriately thermalized with the
internal degrees of freedom.

I would also venture to agree with how I interpreted John M's
comments. We cannot always call a particular energy transfer 100%
work or 100% heat - eg. light can be on a continuum with ideal laser
radiation (100% work) at one end and ideal blackbody radiation (100%
heat) at the other end. Most real light sources fall somewhere
in-between. However, sometimes I *can* neatly partition an energy
transfer into 100% work or heat. We often do this in thermo and it
can be very helpful to do so. But even in cases where I cannot, I
*can* assign a percentage based on the entropy flow and it *may* be
helpful to do so. Or it may not, in which case I would agree with
John D.
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu http://physics.usna.edu/physics/faculty/mungan/