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Re: [Phys-l] T dS versus dQ

Bob LaMontagne:

If you would take the time to read what I said - I assumed none of the items you mention. I am simply assuming that insulated implies no energy of any kind leaves the inside of the cylinder.

With all due respect, I read what you wrote very carefully and responded respectfully. You didn't say "insulated," a somewhat ambiguous term that might at least conceivably be construed as "isolated." Rather, you (more properly) said "thermally insulated" as did Carl. Thermal insulation rules out thermal energy transfers, but not mechanical energy transfers, i.e. work. Indeed, Carl explicitly indicated that a mechanical process would take place, one that would necessarily do positive work on the system leaving it with more energy than it had originally.

Adiabatic and ideal gas gives specific relationships between P, V, and T that you can look up in any general physics text.

This is only true if by "adiabatic" you mean "isentropic." I think it is pretty common, however, to understand "adiabatic" to mean that the process involves no thermal energy transfer (i.e. "heat"). An "isentropic" process is less ambiguously referred to as a "reversible adiabatic" process. But Carl explicitly specified that his process was irreversible.

My conclusion was simply that the final temperature after the compression would be the same regardless of how fast it was done and that in the end the sound and shock waves didn't matter.. How do you get isothermal out of that?

It seemed to me to be a not completely unreasonable conclusion from the fact that you explicitly (and incorrectly) stated that the final temperature would be the same as the initial temperature. You also said (incorrectly) that a reversible adiabat would take you from the initial state to the final state. I took those two explicit statements to suggest that you might think that a reversible adiabatic process is the same as an isothermal process.

John Mallinckrodt
Cal Poly Pomona