Re: joule-thompson expansion

["John S. Denker" <jsd@MONMOUTH.COM>]

Robert Cohen wrote:
> Suppose we had two fixed chambers of volume V separated by a stopcock.  In
> one we place a gas at pressure P and temperature T.  The other is evacuated
> (P=0).  If we turn the stopcock, the gas expands to fill both chambers.
> What is the pressure and temperature in the chambers?
>
> Does the answer depend on making certain assumptions?
> This problem always seems to get different answers every time I ask it.

Indeed!  The temperature change is positive, negative, or zero,
depending on assumptions.

The temperature change is
 -- negative for a real gas that is initially below the
    J-T inversion temperature,
 -- positive above the J-T inversion temperature,
 -- zero right at the J-T inversion temperature, and
 -- zero always for an _ideal_ gas.

<nit>
Let's assume that the stopcock is a fair approximation of a
Joule-Thompson porous plug, so that the expansion is quasi-static.
(I don't think this is an important assumption, but without
it the expansion is far from equilibrium and one must ask
all sorts of questions about how equilibrium is finally
obtained:  does the gas slosh back and forth forever?  Does
it radiate sound????)
</nit>

I get 440 hits from
  http://www.google.com/search?q=joule-thompson+expansion

Hit #1 is pretty good:
  http://www.chem.arizona.edu/~salzmanr/480a/480ants/jadjte/jadjte.html