[Phys-L] "the" system versus "the" environment ... or not

[John Denker <jsd@av8n.com>]

On 10/30/2013 02:33 PM, Bruce Sherwood wrote:
> We also make a big investment in the notion of "system" and how the choice
> of system affects the form of the energy equation. Just last week I
> reviewed a paper that was quite garbled, specifically due to the author
> never using the word "system" anywhere in the paper.

That's tricky.  That falls into the catagory one might call
pedagogical moral hazard.  That is, there is a temptation to
do things in the short term that we will regret in the longer
term.

Specifically:  In *simple* systems the *simplest* way to 
analyze things is in terms of quote "the" quote "system"
interacting with quote "the" quote "environment".  Folks
often choose that as a pedagogical starting point.

However ... as is so often the case, where you start out
is not where you want to end up.  As soon as we consider
problems that are even slightly more complicated than the
proverbial spherical cow in the ivory tower, the dichotomy
of "system" versus "environment" fails miserably.  Consider
something as simple as an air gun:  propellant + piston +
ambient atmosphere.  There are now three "zones" to consider.
You can call them three systems, or subsystems, or zones, or
regions, or parcels, or whatever.

Taking another step down that road, in fluid dynamics there
might well be thousands of parcels.  Each one is a "system"
unto itself ... and also serves as part of the "environment"
for other parcels.

I don't want to make your life harder.  But please realize
that when you teach the "system" versus "environment" dichotomy
it makes life harder for the students -- and for me -- down
the road, when they have to unlearn it.  I've seen chemistry
professors analyze the air gun from the dichotomous point of
view, get the wrong answer, /realize/ it's the wrong answer,
and then still take more than a week to climb down to a point
where they will listen to the right answer.

I'm not 100% sure I know the best way forward, but here is 
a hypothesis that seems plausible and worth testing:  Rather
than "investing" in the "system" versus "environment" dichotomy, 
use terms like "Region 1" versus "Region 2".  That allows you
to do things like writing the enthalpy as

          H1 = E1 + P1 V1

with no ambiguity.  You are free to leave off the subscript "1"
if it's obvious from context that you're talking about Region
1 ... but you still retain the option of putting the subscripts
back when necessary, and this is priceless.

Also, even the dimmest student will suspect that the formalism
can be extended to "Region 3" if necessary.  (In contrast, the
idea of generalizing beyond the "system" / "environment" dichotomy
is very slow to dawn on some people.)

So, that's my pitch.  If anybody thinks the "Region 1" versus
"Region 2" approach is too much of a burden on the introductory 
course, please let me know;  maybe we can work out something
better.  At the very least, please keep in mind that the "system" 
versus "environment" formalism has a serious downside.