Re: ENERGY WITH Q

["Carl E. Mungan" <mungan@USNA.EDU>]

Ludwik K wrote:

> Carl Mungan wrote: "Either heat is energy IN A SYSTEM or
> it is energy IN TRANSIT from one system to another.

I slightly oversimplified the latter definition. I would partly agree
with Jim G that heat is not really "energy in transit" anymore than
work is. Rather it is a mechanism for changing the energy of one
system at the expense (or gain) of a second system's energy. But
since the gain by one always equals the loss of the other, I think
it's helpful to start with the above simplified wording. I disagree
with Jim G that we cannot talk about energy transfers and think John
D has made a cogent case for this.

> To respect traditional terminology the Q of calorimetry should
> be called heat

This comment mystifies me. I only recognize one kind of heat. If
calorimetry is a "non-thermodynamic" example of heat, please provide
a "thermodynamic" example. I think I will be able to convince you
that any thermodynamic example you can come up with can be reduced to
an equivalent calorimetric process.

> Please suggest a better pair of short words for my draft.

You often like to substitute Martian names. I partly understand the
point you're trying to get across (that it is the concept not the
name that matters). But usually I find it more helpful to instead
clarify what the familiar name means by way of examples or
counter-examples. We currently have the following familiar terms on
the table:
- heat
- work
- pseudowork
- internal energy
- mechanical energy
- entropy
and some people wish to add:
-thermal energy
which I prefer to avoid, but not to the point of being close-minded.
In cases where thermal energy is the best term to use, I'll use it!
The above list, in my opinion, covers all of the basic issues and
examples I've seen discussed and I see little need to add new words
to it. Certainly I can't imagine what "pseudoheat" might mean.

> Personally, I prefer call F*ds work and reserve the word
> pseudowork for the concept to be introduced later. [Those
> who call F*ds pseudowork are confusing students. This
> kind of confusion is totally unnecessary.]

At this point, I'm just repeating. But one last time:
F*ds = work
F*ds = pseudowork
Both are these are correct. Just incomplete. Specify what F you mean
and what ds you mean. Then both are clear and useful concepts. Why
there is any mystery to this baffles me. I understand some people
don't want to teach both. Fine, there is limited time in an intro
course, I understand choices have to be made. But what really amazes
me is some people seem to imply or feel that one of these definitions
is not useful or is actually wrong or confusing or something. Often
we discuss things on this list that we might not actually teach.
Fine. But why restrict our personal toolboxes? This is why I'm
willing to toy with thermal energy and why I don't like to teach in a
style where I force students to use "perfect" terminology before
they're allowed to open their mouth in class or I feel I have to
constantly "guard" what I say against the possibility that maybe at
some advanced level it might be wrong. Let's all lighten up and get
back to having fun analyzing physics problems rather than vocabulary
nuances.

But I will say that if you choose to teach pseudowork, Ludwik, it
should *definitely* come before (in fact, way before) you discuss
(thermodynamic) work. Pseudowork (in my opinion) is the much more
natural and simple concept both from the context (immediately
following Newton's laws in texts) and from students' experience
(working with mechanics problems). In I'd guess 80% of problems,
pseudowork and work either mean the same thing or pseudowork was what
you really want even if you jumped through some hoops by beginning
with the first law of thermo. Generally, that 20% of problems where
the difference really matters involve subtle issues best left out in
a first pass or until the thermo sections of the textbook are
reached. I would simply drop a few words to the class that other
aspects of this particular problem will be revisited later when we
have a few more concepts in our toolbox.

Finally, Tom W wrote:

> It seems obvious to me that heat and
> work are quite distinct.

I would say this is an overstatement. They're distinct, but if the
distinction was obvious in all cases, the discussion would have ended
long ago.

Example: a block slides across a rough stationary table. How much of
the mechanical energy of the block was lost via work and how much via
heat?

I think it is possible to answer this question. But it is certainly
not "obvious" or "quite distinct" without considerable modeling,
analysis, and thought.
--
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/