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Re: CONSERVATION OF ENERGY



Date: Thu, 24 Jul 1997 01:27:57 -0700
From: Leigh Palmer <palmer@sfu.ca>

.... Too much meaning is being heaped upon the word "internal". Some
discussants want this to be something which remains within the physical
boundaries of the system. ... Some want only to count "thermal" energy,
a fallacy well treated in John's note. The truth is that in classical
equilibrium thermodynamics there is only one kind of energy, the internal
energy of the system. .....

We a discussing a physical situation, and terminology which can help us to
understand it. This naturally leads to thermodynamics. But why should we
be prevented from using concepts, such as kinetic energy or atom, which
are not part of the classical formalism of that discipline?

Don't you know, Leigh, that good children should be in bed at 01:27:57 ?

**************************************************************************
Many days ago somebody objected, very categorically, to the use of the
term thermal energy.

This term is not in my dictionary, nor should it be in that of any
physics teacher. It is a source of confusion ... and a barrier to
conceptual grasp."

Following numerous precedents I did use this word and tried to justify
its usefulnes in subsequent messages. A possibility of confusion escaped
me till I read what John Mallinckrodt had to say about this.

... consider a ... situation in which a large chunk of matter is struck
and then vibrates in a complex and macroscopically observable manner.
Let's think about how we would apportion its internal energy between
mechanical and thermal components. We can (in principle) do a spectral
analysis to determine the amount of vibrational energy in each vibrational
mode. But where do we cut off the sum? ... I simply don't see any rational
way of deciding how much of the internal energy here is "mechanical" and
how much is "thermal." ... My conclusion from considering such examples
has been that we should not deceive ourselves into thinking that internal
energy can be divided into thermal and mechanical portions.

In many situations, such as an engine with a piston and a flywheel, the
distinction is clear and thinking in terms of two kinds of internal energy
may help to understand phenomena. In other situations identifications of
thermal energies may not be possible (or useful) and the all-inclusive term
"internal energy" would be used. What is wrong with this? Everything can
lead to a confusion. Would you object to the v=d/t formula because somebody
may possibly try to use it when v/c is not very small? We know that there
are many shades of gray but black and white are part of our vocabulary.

External <--- the energy outside a chosen boundary. KISS (super-simple)
Internal <--- Thermal + non-thermal (macroscopic) inside the boundary.

I desagree that thermal energy is always "a barrier to conceptual
understanding".
Ludwik Kowalski