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Re: [Phys-l] App. for Was: Re: T dS versus dQ



There's nobody holier than the reformed sinner. In my case - I want the term adiabiatic to mean
"with no change in heating" and I visualize rapid compressions as wonderful trompe l'oeuil
to balance energy losses where excess work is thermalized to excess temperature in a so-called
adiabatic compression. That begs the question, "what is heating?" and I translate that
to be the increase of internal energy due to thermalized work along with the expected
temperature change due to pressure change.

Brian W

LaMontagne, Bob wrote:
It's certainly adiabatic under any definitions I have seen, it's just not isentropic - which has been shown satisfactorily in the past discussions. But it would be nice for the smiling professor to be able to give a ballpark estimate of how large the temperature difference is between slow and rapid adiabatic compression for specific instances (say mach 0.0001 versus mach 0.9).

The new generation of demo equipment that BC gave a reference to are able to track P and T very well. The response times of the thermometers are around a half second to a second and the seals on the pistons are remarkably tight. The decrease in pressure past its peak value after compression is due almost entirely to thermal conduction through the cylinder walls. We are getting pretty close to the point where fast compression can actually be shown to not be isentropic - even if adiabatic.

Since commercial programs can now model turbulence in fluids fairly well, it seems that we should be close to specifying and solving this mundane but still complex problem of a simple circular cylinder and flat piston.

Bob at PC

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Brian Whatcott [betwys1@sbcglobal.net]
Sent: Tuesday, February 09, 2010 8:46 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] App. for Was: Re: T dS versus dQ

John Mallinckrodt wrote:
..If one envisions a
cylinder that is absolutely impervious to any energy transfer
through its walls, fitted with a piston and containing Nitrogen
only, and a stop inside the cylinder that prevents the piston from
compressing the gas to less than half its original volume - and
then one applies a force with one's hand to push the piston to the
stop in 10 seconds - and then repeat with identical apparatus but
do the compression in 0.001 seconds, and then let the two pieces of
apparatus sit for a while until all waves. etc., dissipate, will
there be any difference in their final states

Yes.


- and specifically what?


The final volumes are identical, will the final temperatures and
pressure be higher?

Both will be higher than they would have been had the process been
quasistatic.
See above.

JM

Interesting that John M is once again faced with an incredulous audience,
an audience accustomed to lab demos from a smiling professor figure
that show a step compression as "adiabatic".
It's as though people could not grasp the difference between
laminar flow and turbulent flow.
What's the difference?
Entropy! Temperature! Turbulence!

Brian W
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_______________________________________________
Forum for Physics Educators
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