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Re: Physics Definition of Work -- Quis custodiet ipsos custodes?



At 04:45 PM 10/1/01 -0500, QUIST, OREN wrote:
I was at a meeting at the Regional College Board office in Chicago last
Friday where their Advanced Placement testing procedures were being
explained.

In one of their "pieces" their new AP exams were being described and it
stated that (something to the effect);

"we are now going to use the new definition of physics work where we
measure work done "on" the system rather than work done "by" the system.
The result is to have all plus signs in the First Law of Thermo -- just as
chemistry does it."

Now, I realize we have had a few "threads" on this matter in the not too
distant past. But, it was my understanding that this was being considered
-- but not necessarily adopted. This tells me that it is now adopted!!!

I have no big problem with this change. But, I do object if the change was
brought about by an arbitrary change made by someone in a testing company.
Or, if it was made to bring us in alignment with chemistry. If the physics
community has debated this issue and made this decision -- great! But, if
not, what do we do now?

Thoughts?

Wow. My thoughts include the following:

1) There are many situations where sign conventions are well
established. For instance, electrical engineers take the sign of the
electron to be negative, just to be compatible with physics and with
long-standing tradition -- even though the opposite sign would be more
convenient.

2) OTOH, there are _many_ situations where there is no well-established
conventional sign. Sometimes the details of notation differ between
physics and chemistry. Sometimes they differ from subfield to subfield
within physics. Sometimes they differ from practitioner to practitioner
within a subfield.

3) Choosing a notation is like choosing a reference frame: It cannot
possibly change the final result of any physically-significant
calculation. (It may change the appearance of some intermediate results,
but that's a horse of quite different color.)

If/when you are communicating intermediate results, you have a duty to
-- explain what reference frame you are using, and, similarly
-- explain what sign conventions you have adopted, and, more generally
-- define your terminology, except possibly for aspects that are 100%
standardized, and
-- define even the 100%-standard terminology, when you are communicating
with beginners, since they can't be expected to know what's standard and
what's not.

4) There is no standard definition of "physics work" and probably never
will be. There are in fact two competing notions:
-- work done _by_ the system
-- work done _on_ the system
and the over-abbreviated phrase "physics work" is just guaranteed to be
ambiguous.

5) IMHO a test, or a text, or anything else that expects people to know
exactly what "physics work" denotes is defective. Anything that assumes a
standard definition of the "W" symbol is similarly defective.

a) Obviously the author is not fulfilling the duty mentioned in item (3)
above.

b) Perhaps the author is stuck at the rote learning level, and/or
expects the students to be stuck at the rote learning level: memorizing
doctrinaire definitions without regard to meaning, and memorizing the
appearance of formulas without regard to meaning.

c) It takes only a moment's effort to restructure questions to that they
ask for a physically-significant result, independent of reference frame and
independent of notation.

6) Testing is important. It's not as important as some people think it is,
but important nevertheless. Students and teachers put a lot of weight on
standardized tests, more than is appropriate given the quality of
present-day tests. The testers evaluate us -- but who is evaluating the
testers?