Re: [Phys-l] T dS versus dQ

[Brian Whatcott <betwys1@sbcglobal.net>]

Wow!
Not only a particularly clear presentation of the effects to be expected
from a given compression, in terms of temperature,  pressure,  entropy & 
energy but a plausible construction of how well-prepared people 
contributing to the thread come up with faulty mind models...

Brian W

John Mallinckrodt wrote:
> /snip/  The minor triumph of thermodynamics that I alluded to before 
> is that, for a given final volume, we know immediately that the work 
> done, the final energy, the final temperature, the final entropy, and 
> the final pressure in Carl's scenario will all be higher for *any* 
> process-- oscillatory, jerky, smooth, or otherwise--during which the 
> gas is *ever* once out of equilibrium than they would be for a "slow" 
> (or "quasi-static" or "reversible") adiabatic process, i.e., an 
> isentropic process. This is a relatively simple consequence of the 
> second law.
> The somewhat confounding experimental fact is that, for liter-sized  
> containers of gas, one can closely approximate an isentropic process  
> even with fairly rapid piston motions.  But that is simply a matter  
> of experimental precision.  Indeed, pedagogical lab experiments often  make use of this fact and may leave students (and faculty?) with  confused ideas about "adiabatic" processes.  For instance, Experiment  #3a on pages 11 and 12 in the lab manual for Pasco's Adiabatic Gas  Law Apparatus (see http://tinyurl.com/yb4f7cs), explicitly instructs  students that "the gas should be compressed as rapidly as possible to  make the experiment approximately adiabatic."  The reason for the  instruction is to get around the fact that the gas is NOT thermally  insulated and it works DESPITE the fact that the rapidity itself  technically insures that the process does NOT precisely follow the  adiabatic gas law.
>
> John Mallinckrodt
> Cal Poly Pomona