Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] T dS versus dQ

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, 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