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Re: [Phys-l] question about Bernoulli



Careful - if the inside cylinder wall is smooth, there will not be a pressure difference (although there may still be entrainment). To get a drop you would need a small nipple (a bump really) inside the cylider attached to the wall and a hole going through the nipple and the wall. There was a great AJP article showing this a few years ago (sorry - I don't have the time right now to get the reference - I will try to find it tomorrow morning.)

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of William Robertson
Sent: Thursday, November 18, 2010 5:55 PM
To: betwys1@sbcglobal.net; Forum for Physics Educators
Subject: Re: [Phys-l] question about Bernoulli

Yes, I believe you can omit the hole. Doesn't Bernoulli say that the
pressure on the inside of the syringe wall will be less than the
pressure outside due to the increased velocity inside?

Bill


William C. Robertson, Ph.D.


On Nov 17, 2010, at 6:11 PM, brian whatcott wrote:

Probably better to wait, as you say. :-)
The reduction in air pressure in a duct due to its motion requires
no
side hole and external molecular collisions!

B

On 11/17/2010 6:57 PM, Bernard Cleyet wrote:
Not drag, but push.

The thermal motion outside the cylinder near and at the hole
results in diffusion into the cylinder. The net motion of the gas
inside is in the direction of the piston, so those molecules
collide w/ those formerly outside resulting in a partial vacuum
(relatively) at the hole, etc.

bc waiting for JD. and thinks it's similar to the operation of a
diffusion pump.

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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l