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Re: New AP Physics Convention in Thermodynamics



Before 1960 the chemistry books defined work as work done by the system, so
dU = q - w.

Around 1960 this began to change. When I was in college (late 1960s)
chemistry books were about half one way and half the other way.

By the time I was in grad school (1970s) the trend toward work = work done
on the system so dU = q + w was clearly the trend (in chemistry books).

By 1980 the change was pretty complete, and I believe virtually all
chemistry books today define work as work done on the system. I have taught
it this way for virtually my whole teaching career which started in 1978.

To deal with the idea that dV, as it appears in equations, is unsigned, the
chemists simply include the minus sign explicitly in the equation for PV
work. Chemists define:

(PV work) = (negative)(integral)(P)(dV)

Some books call this expansion work, but most just call it PV work.

My degree is chemical-physics and I teach both chemistry and physics.
Therefore I have been aware of differences between definitions in chemistry
and physics for quite some time. When they differ, I often prefer physics
definitions, but on this issue I prefer the chemistry definition. We raise
the internal energy either by adding heat to the system or by doing work on
the system. That has always struck me as the right way to view it. In
general, when any kind of work is done on any system, some form of energy of
that system increases. That makes sense to me and eventually to students.
The old convention (that PV work done on the system is negative) just seems
confusing.

I think this also means the physics textbook authors need to examine their
definition of the work-energy theorem. Currently we most often see (delta
energy) = (negative)(work). For this to be true we ought to be more
explicit by writing (delta energy of system) = (negative)(work
system-done-to-surroundings). But I would be in favor of writing the
work-energy theorem as (delta energy of system) = (work done on system).

Another way to express what I am saying would be this... I find it less
confusing, and therefore I would prefer, that when different ideas are on
opposite sides of an equation, the ideas would pertain to the same object or
system. Therefore, rather than write (delta energy of system) =
(negative)(work done by system)... I would prefer to state this in two
equations... (1) (delta energy of system) = (work done to system) and (2)
(work done to system) = (negative)(work done by system).

I realize some will disagree with this, and that's what started this thread.
It is not worth arguing on the basis of science because either method is
fine. I argue simply on the basis of what seems less confusing when I teach
this to students.

Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817