Re: Bernoulli Principle, 2nd try

[pvalev <pvalev@BAS.BG>]

--- Tucker Hiatt <thiatt@USFCA.EDU> wrote:
> At 3:57 PM -0400 6/25/03, Wolfgang Rueckner wrote:
> > My question is this -- can one make an argument about what the
> > pressure difference ought to be from a molecular motion point of
> > view?  And I'm not talking about a detailed kinetic theory of gases
> > derivation but rather a plausible argument that could be used in an
> > introductory physics course.
>
> I appreciate your question very much, Wolfgang.  I am disappointed
> that the answer seems, so far, to be "No; no one (on PHYS-L) *can*
> explain Bernoulli's Principle from a molecular point of view."
>
> I certainly wish I could do it.  Many of my introductory students
> enjoy learning about the Bernoulli effect, and then are disappointed
> when I am unable to explain it.  I am able to take a molecular point
> of view in explaining other fluid phenomena (e.g., static pressure,
> buoyancy, capillarity, and Pascal's Principle), but not Bernoulli's
> Principle.
>
> So, forgive me PHYS-Lers, but I must re-issue Wolfgang's question:
> FROM A MOLECULAR POINT OF VIEW, WHY DOES FLUID PRESSURE VARY WITH
> FLUID SPEED?

I certainly don't have the answer but the problem seems
quite general - from a molecular point of view, the pressure
in a liquid is obscure. Let me give an example where I am
somewhat more competent. A membrane separates a solution and
a pure solvent, and the membrane is permeable only to solvent
molecules. This traditional osmotic picture has only one
peculiarity - the membrane is movable along its perpandicular.
Clearly the membrane will move in the direction of the pure
solvent, and the question is: What pushes it? This is again a
pressure problem, and it is incredibly difficult to convince
e.g. thermodynamicists that the only agent that could push
the membrane are solute molecules. All textbooks claim that
osmotic pressure is due to "the tendency of the solvent to
move from a region where the solvent's chemical potential
is high to a region where it is low". Generations of students
have been learning this explanation by rote and so simple
physical thinking, answering questions of the sort "what pushes
what", has been completely eradicated.
   The problem with Bernoulli's principle may be similar -
some simple physical explanation may have become completely
incompatible with our established conceptual framework.

Pentcho