Re: [Phys-L] irreversible quasistatic

[Carl Mungan <mungan@usna.edu>]

> > I will draw two identical cycles (in the sense of having the same
> > vertices and connecting curves). Cycle A will consist purely of
> > reversible processes. Cycle B will consist of at least one
> > irreversible quasistatic process;
>
> Those three sentences are incompatible.  Pick any two.
>
> That is to say, cycle A and cycle B should look different on any
> reasonable "indicator diagram".  If not, some key variable is being
> left out, and the diagram is not a meaningful representation of the
> system.

I would be delighted if you can find the incompatibility. That’s what I want, indeed.

Let’s simplify to just one step in the cycle. I would be happy to see an incompatibility for *any* kind of process you like, but since I can’t see what I’m missing, I’m going to choose a process at random, namely an isothermal one.

So here goes. I’ll put numbers into the problem for definiteness.

Draw two PV curves for the isothermal expansion of 1 mole of a monatomic ideal gas. Starting point i is Vi = 1 m^3 and Pi = 100 kPa. Ending point f is Vf = 2 m^3 and Pf = 50 kPa. Curve A is for a reversible expansion, accomplished in the usual way by letting the gas slowly push out a frictionless piston while the gas remains in constant with a temperature-regulated water bath. Curve B is for a quasistatic irreversible expansion, accomplished using a Joule-Thompson plug.

Sorry to say, both curves look *identical* on my graph: two overlapping hyperbolas.

I’m assuming you meant curve B is supposed to deviate somehow. What is it missing?

-----
Carl E. Mungan, Professor of Physics  410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363 
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