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Re: CONSERVATION OF ENERGY


On Mon, 14 Jul 97 00:03:09 EDT LUDWIK KOWALSKI
<kowalskil@alpha.montclair.edu> writes:
On Sun, 13 Jul 1997 Leigh Palmer <palmer@sfu.ca> wrote:

As you state, it should properly be called heat, since a process,
though
not a uniquely specified one, is implicit. It is certainly not an
energy.
Use of the symbol dU (by which I infer you mean the change in U) is
most
improper; heat is not the change in anything. You should use Q
instead:

Q = m c dT

I did not mean to say that Q is a state function and something better
than
"thermal energy" would be appropriate because energy is a state
variable.
I did not invent "thermal energy"; many authors are now using this
term
where older textbooks used to say "heat".

It is my understanding that heat, in the first law, is defined as that

part of internal energy which is transfered (driven by?) a difference
in temetratures. The dT, in the formula above, is a difference between
the initial and final temperature of a body (in a process). In the
example I used dQ is due to friction. If I am correct (that c*m*dT is
not
heat, as defined in thermodynamics) then what name should be given to
it?
We need distinct names for distinct physical quantities.

Treating heat, work, and energy as though they are all the same
thing
is [wrong].

I agree. And that is why I was asking how a common statemet "energy =
ability to do work" should be interpreted. Or what does it mean that
"work by friction is done at the expense of kinetic energy"? If these
questions can be clearly answerd (for a specific situation invented
to discuss the issue) then some progress will be made.

1) Can somebody describe the process under consideration using the
terms
on which we can agree?

I forgot what the process was, but I believe that heat is a boundary
phenomenon only, therefore such difficulties as did arise may have come
from not drawing the control volumes and/or not dividing the process into
sub-processes when the system (object under investigation) or point of
view had changed perhaps without mention or notice. (Now, Ludwik, I can
draw you some very nice commutative diagrams; so, from a topological
perspective .. Oh, never mind.)

2) Can somebody explain the meaning of (a) "work at the expense of
energy"
and (b) "energy is an ability to do work"? Are these phrases
acceptable?
Why yes? why no?

Dave Bowman wrote (6-24-95): "Energy: That quantity in a dynamical
system [that] generates an infinitesimal displacement of that system's
microstate in time."

Presumably, under certain conditions, an infinitesimal displacement in
the microstate could change the macroscopic bddy, say, which would do
work upon the environment. So, I would say, "Yes to b", because I
trust Dave. But, I would draw a schematic for a power plant and feel
very comfortable with the affirmative answer, except the energy is not
lost. It is degraded. A better term would be availability, but that
wasn't the question was it? So, my guess is that energy, itself, is not
an ability to do work. In Dave's case, presumably, the infinitesimal
change that did work will be cancelled by some other infinitesimal change
that absorbs work. Isn't the ergodic theorem something about that?
Well, I changed my mind to "No" on Point (b).

On Point (a), let's agree that the degradation of energy is at the
expense of the energy itself and not something else. The energy, after
all, becomes 'poorer'.

That's all for me. I may have lost Ludwik's confidence anyway, although
perhaps that's not my fault. Ludwik, I can tell you a funny story about
a mathematician friend of mine who thought calculations were not math.

Regards / Tom

I an not against generalizations but I suggest we begin with the
concrete
situation at hand. This will help us to understand each other better.

Ludwik Kowalski