Re: Thermal Energy - thermalization of rotational energy

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Thu, 7 Mar 2002, Jim Green wrote:

> John assures me that his question is sincere -- I would never suspect that
> he really does not know what is usually included in "internal energy".  And
> I would prefer to begin with his/the list's definition of "thermal energy"
> as I posed in my last post.
>
> But he says that he would include bulk rotational an translational motion
> in "internal energy" so I guess that there is in fact a basis for a
> discussion here.

Jim is referring to a behind the scenes conversation we had this
afternoon after he wrote me of his suspicion that my questions
were not seriously intended.  Unfortunately he
has--unintentionally I assume--misquoted me here so I would like
to set the record straight by copying the list on my message to
him in which I wrote:

> Jim, I understand that you may be skeptical of my request, but
> it was sincerely intended and it is not a trick question or a
> challenge.  Perhaps it will help lessen your anxiety if I tell
> you my own answer.  You can pick it apart if you like, but I
> do hope that if you disagree with my answer you will tell me
> your own rather than simply ridiculing mine.
>
> I (usually) consider bulk rotational kinetic energy and large
> amplitude macroscopic system vibrations to be "internal
> energy." There are many occasions, however, where I find that
> they must be distinguished from atomic scale vibrations and
> rotations.  The distinction (often) seems to me most clearly
> indicated by the use of two subcategories of internal energy
> that I might call "thermal internal energy" and "nonthermal
> internal energy."
>
> The hedge words are honestly intended to acknowledge that
> exceptions do exist and that I am willing to be flexible
> enough to accept a litle ambiguity.

So just to be clear, I do *not* consider bulk translational
kinetic energy of a system to be "internal."  As I said in my
message to Jim, I *do* (usually) consider bulk rotational kinetic
energy and large amplitude macroscopic system vibrational energy
to be "internal energy."  This is because these energies--*unlike*
bulk translational kinetic energy and *like* other forms of
internal energy--are invariant properties OF the system.

Jim goes on:

> Internal energy is the state variable where the first law is valid.  And I
> suppose we should include the Second Law as well.  It includes all the
> random motion KE (trans, vib, rot) and atomic PE -- usually with a bit of
> hand waving.
>
> Surely it does not include bulk rotational or translational motion.

And again, I never suggested that it should include bulk
translational motion.

> But I am left with the notion that I have missed something.

The only thing I'm missing is an answer to my followup question
which was, "If you do not consider bulk rotational kinetic energy
and large amplitude macroscopic system vibrations to be 'internal
energy' (and I think you are saying that you don't), would you
consistently put them in the same category as bulk translational
kinetic energy or would you sometimes put them in some third
category?"

I think I have already put *my* cards on the table above, but I
don't want you to feel like I am trying to trap you.  I ask the
question because *I* think that bulk translational kinetic energy
is in a class of its own (since it is *not* an invariant property
of the system.)  I'm interested in knowing whether you agree and,
if so, if you might suggest a name to distinguish between between
these different types of "noninternal energy."

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm