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Re: [Phys-l] question about Bernoulli



Here's a Just-so explanation, offered without much corroboration.

The current theory relating pressure and temperature in gases, due to Clausius,
has it that the kinetic energy of molecules impacting container walls
determines the mean pressure on those surfaces.

Visualize a spherical surface plotting the (probable, mean, rms??)
speed direction vectors of the cumulative air molecule, located at its center.

If the molecules are all moved in one direction, the spherical surface is displaced
forward by that displacement rate vector, with the effect that the mean molecule is
now off centered in its sphere, the mean molecule trailing the center of its speed
vector sphere.

The transverse distance from the molecule to the sphere's speed vector surface is now
reduced from a radius, to some lesser value. It was this transverse set of trajectories
that contributed the usual pressure, and it is this set of slower transverse trajectories
that now provide a reduced side wall pressure.
The pressure being proportional to kinetic energy, this pressure drops with the
drop in the square of this transverse speed.

This is a fairly concrete vivid image, no doubt. However - no guarantees.


Brian W


On 11/17/2010 3:04 PM, William Robertson wrote:
I have for some time strived to explain the Bernoulli effect in terms
of what's happening at the molecular level. We all know the
mathematical explanation that leads to higher velocities being
associated with lower pressures, but I want something that does not
rely on the mathematics. In other words, what are the molecules doing
that leads to the Bernoulli effect? /snip/
Bill


William C. Robertson, Ph.D.