Re: System boundary

PD <TheDraperFamily@xxxxxxxxx> writes:

On Jun 6, 3:43 pm, Haines Brown <bro...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:
PD <TheDraperFam...@xxxxxxxxx> writes:
Well, your babble-generator certainly starts easily with a light pull,
doesn't it? You've read too much and learned too little. It might be
useful to talk with a *physicist* to understand the difference between
a system boundary and where a boundary condition applies. Those are
two different contexts that are sometimes but not often connected.

OK, since you are making a pronoucement on elementary physics, and you
feel that only physicists are in a position to do so, I can only infer
you are a physicist. As such you are in a position to define boundary
and boundary condition. Please do so.


Before I even embark on an attempt here, I'd like to ask you why you
haven't called up a physicist at your local university physics
department and asked him or her to describe the difference. That
conversation would be much shorter and more efficient than attempting
an education on a newsgroup.

PD

Apparently I sent my reply to PD personally rather than to the
newsgroup.

In it I gave reasons why I did not consult with local academics,
although the suggestion is certainly a valid one.

I pointed out that I was not making pronouncements, but only offered
the "propositions" as a way to obtain enlightenment concerning them.

Although not in school, I am a student anxious to learn. Most teachers
find student questions, no matter how naive or misguided,
welcome. Most academics and experts feel they have learned much from
their students.

I believe I am correct to assume that the notion of boundary
conditions is an ambivalent one. In different sciences it is
understood in different ways.

Let me offer these quotations from a classic account:

A _specific_ boundary condition is simply a postulated event in
space and time expressed by the statement that a symbol,
representing a certain physical quantity, shall have a definite
value or set of values throughout a specified region of space within
a specified interval of time.

[re. _general_ boundary conditions]... when we come to physical
problems which involve the flow of energy across the interface of
two media ... it is necessary to postulate at such an interface a
certain continuity for the functions representing the disturbance
which is being propagated. ... We have to set up certain relations
among the physical quantities descriptive of the physical
quantities descriptive of the disturbance in the two media at the
boundary.

The first paragraph seems to refer to what we call an initial
condition, and asserts that its state can't be described without
defining its boundary. The term "postulated" seems to imply that there
cannot be a description of any state without a subjective
component. Since the first paragraph refers to isolated systems, it
may imply that any description of the state of an isolated system is
in part subjective because any observation entails a causal relation
with something in the system's environment and a reference system.

The second paragraph refers to closed systems that can exchange heat
with their environment and can involve work (I assume this means
perturbations affecting a system rather than free energy within the
system doing work on its elements). Here the existence of an
interface, still does not support a purely objective definition of
system, for the function representing conditions on either side of the
interface must have an element of continuity. For example, one might
wish to define a system as having lower entropy than its environment,
but the measurement of entropy is inherently comparative.

Not mentioned by the author quoted above is the open system. This is
due at least because the context is boundary conditions, and such an
open system has none. If there is an exchange of matter with the
system's environment, can it appear as a system? We can, for example,
identity a causal nexus and call it a system. This in fact is an old
definition. However, I suspect that this ends up merely a description,
and intuitively I suspect mere description does not amount to a system
in any meaningful sense.

Apparently we must conclude that there's no entirely objective
definition of system. However, the presence of a subjective component
does not necessarily mean there is no objective truth value in a
definition of system. My "propositions" were aimed at unpacking
(salvaging) this truth value. I did not elaborate the point, but it
seems to me that we can't define system unless it involves events
(involves causal relations), and has boundary conditions.

An isolated system is constructed, either in thought (a temporary
hypothetically isolated system) or in fact (the laboratory). There is
obviously truth value arising from the laboratory (general laws),
although arguably that value is one-sided in that the outcomes are
unequivocally determined by initial conditions, which generally is not
true of the world we know.

A closed system is also constructed by the observer. Is it true that
the only definition of such a system is its entropy relative to its
environment? What functional characteristics does the interface have
to have such that entropy is exported?

In the absence of an interface (boundary conditions), we have an open
system.

--

Haines Brown, KB1GRM



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