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I've been away from this conversation for a few days, so I have
catching up to do. In skimming, I have noticed a concern for the need
for a molecular explanation of Bernoulli. That concern includes moving
in too big a leap from the macro to the micro. A picture of what the
molecules are doing is not always necessary, or perhaps not always
possible, especially when discussing large-scale results of large
numbers of molecules. But when we can get that picture, it certainly
helps with understanding.
If I might give a more basic example. We learn gas laws as they have
arisen historically, or simply as the ideal gas law PV=nRT or PV=NkT.
One can address the gas laws at this level, as interactions between
pressure, volume, temperature, and number of molecules or moles. But
is it possible to gain a deeper understanding, one that stays with
students (of all ages) because it provides a concrete picture of
what's going on? Absolutely. I do a simulation with teachers in which
they pretend to be gas molecules with others being their container.
They are given a few simple rules about interaction. Once they have
those rules, they can "experience" the gas laws. Increased temperature
means they move faster. What does this do to the pressure (how often
and how hard the molecules hit their container)? Does this mean the
gas as a whole expands? No, unless the container "gives" (ask the
average elementary or middle school science teacher whether a gas
expands or not when you heat it). What happens to the pressure when
you change the number of molecules (add or subtract people from the
"container") or change the size of the container? It's obvious in the
people simulation. Do molecules "need more space" when you heat them
(a common incorrect explanation in resources)? The answer is obvious
in the people simulation. As you make the simulation more
sophisticated, you can demonstrate the physical effect on a gas when
it does work on the container or vice versa.
Such a simulation is extremely helpful in understanding the ideal gas
law. In fact, if one does this prior to ever introducing the math of
the law, the law makes conceptual sense and isn't that scary for those
(many!) who freak out at the first sight of an equals sign. The major
concern in such a simulation is that we not introduce any
misconceptions regarding how gases behave.
It's in the spirit of the simulation I describe that I asked the
question regarding Bernoulli. Is it possible to understand the
Bernoulli effect in terms of molecular motion and changes in molecular
motion on a level that makes the effect "obvious" once one pictures
what the molecules are doing? Until I get through everything in this
thread, I don't yet know the answer to that question. It might very
well be my job to use what all have generously provided, come up with
what I think is a proper molecular "picture," and run it by the group.
I do hope I have explained why I asked the question in the first place.
Bill
On Nov 24, 2010, at 4:33 PM, LaMontagne, Bob wrote:
Again, there is no argument from me about the physics of the flow in
terms of pressure, density, temperature, etc., as covered by the
Bernoulli equation.
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Forum for Physics Educators
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l