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Re: Stop using calorie?



On 21 Sep 1997 Herb Gottlieb wrote:

Please believe that I am sincere in asking, " How do you explain Joule's
paddle wheel experiment WITH calorimetry?" I have been explaining that
the work done (by the falling weights attached to the paddle wheel)
resulted in an increased temperature of the water of known mass in the
tank. This explanation does not require any calorimetry.

Without being too philosophical (about explanations) let me pretend of
being your unusual student. "Staying in school yesterday I was repeating
the demo and noticed that the change of temperature is smaller when more
water is added to the cylinder. Why is it so?"

Back to our teacher-to-teacher mode.
Calorimetry does not explain anything. It only makes us aware that there
is something besides the temperature. Today we call it "internal energy"
and say it is extensive rather than intensive. In teaching calorimetry,
preferably in a lab, you show students how to measure some changes in
internal energy. Joule measured Q and found that it is nearly equal to W.
You are right, Herb; the microscopic model is needed to EXPLAIN this fact.
But wouln't you agree that students who know facts, in this case Q=W, are
better prepared for explanations than those who do not?

I am NOT saying that starting with a microscopic model of water (or, more
realistically, extrapolating from the ideal gas theory) and ending with
Joule's experiment is wrong. But, so far, it is not my preference. Knowing
the experimental facts may provide motivation for learning theory, knowing
the theory may provide motivation for experimental testing. Are these two
approaches pedagogically equivalent? I am referring to introductory physics
courses only. Why yes? Why not?
Ludwik Kowalski
P.S.
Thaks for quoting Rumford who wrote "Heat generated .... or excited, as I
would rather choose to express it ..."

His "heat excited" phrase is worth emphasizing. It parallels exciting sound
oscillations (in a tuning fork or in its resonating box). Nobody things that
sound FLOWING to us from an orchestra is a material substance. OK, I am
ignoring the m*c^2 aspect of sound energy, Dan.