Re: [Phys-l] T dS versus dQ

[John Mallinckrodt <ajm@csupomona.edu>]

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