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Some Thoughts



The current Conservation of Energy thread is now well over eighty posts. I
wonder what that indicates about our understanding of the subject. Here are
some thoughts:

1) I suggest that if there is a discrepancy in the use of a phrase ("heat",
"thermal energy", etc) try expressing what you have to say *without* the
phrase. If you can not do this, then you might want to wonder if you really
understand what it is you are trying to say.

2) Words *do* matter. Of course one can use *any* word for anything, but
the object is to be understood not to argue over the word. BUT if the same
word is used for multiple things, understanding is hampered -- if not destroyed.

3) For example if the word "heat" is used for what is *inside* of a system
*and* for what is done to the system, no one will understand -- not even the
speaker. If when discussing F=ma, a student were to talk about the "force
acting upon an object" and in the same paragraph, "the force *of* the
object, wouldn't the student be corrected? "F" is the force; "ma" is NOT a
force. Why then do we talk about "heat of the system" and "heat as an
action done to the system"?

4) The First Law is a good guide to this usage: If W+Q=dU and a speaker
wants to maintain consistent usage in a discussion, then just what meaning
should be assigned to each term? Well, there is the Second Law to deal with
as well: if dS=Q/T, then those things assigned to Q should change S. And a
=name for Q needs to be agreed upon. Usually "Q" is named "heat" -- this is
just fine, but then "heat" is what is done *to* the system -- and in the
process changes the S of the system. U is *inside* the system -- now what
should U be called -- there is not much point in using *two* terms:
"internal energy" *and* "thermal energy" -- at least without further reason
to do so -- and that doesn't seem manifest. I guess W should be called
"work" but is that useful???

5) There should be a sharp distinctions between "Q" and "W"; that
distinction should be that "Q" changes S and "W" does not. Some examples
might help here: a Bunsen burner does "Q", pdV does not, it therefore is
"W". An egg beater *does* give rise to "Q" ie it *does* change S, but there
is no "dT" between the system and the universe. Are we willing to call what
the egg beater does "heat". If we are being consistent with our First Law
and Second Law criteria, we must. If we do not want to be consistent, we
just don't understand thermodynamics and our students will be mightily confused.

6) If the above is not carefully crafted in our minds, we will say things
like mcdT is "heat" -- this is not very good usage; the mcdt is *inside*
the system therefor it is "internal energy". When we write dU=mcdT, we are
just saying this. But when we write Q=mcdT, we are saying that some
*outside* influence, "Q", acted upon the system and has changed its
"internal energy". If we insist on using the four-letter word, we can say
that "heat" has been *done* to the system -- in parallel with saying (in
other circumstances) that "work" has been done to the system.

7) In the equation W+Q=dU, we should mean that "W" and/or "Q" might be done
*to* (or by) the system and thereby the "internal energy" of the system is
changed. Just as we say that "F" is applied *to* an object and the
*resultant* is that the "ma" *of* the object is changed. If "Q" is done to
the system the "S" will change; if "W" then "S" does not. If at this point
the word "heat" is used, that should be done with *great* care. But if one
wants to internalize the lessons of thermo, try not to use the word for
awhile until the concepts sink in.

6) And for the sake of students never say that "heat flows" -- unless you
are talking to Carnot.



Jim Green
JMGreen@sisna.com