Re: joule-thompson expansion

[Robert Cohen <Robert.Cohen@PO-BOX.ESU.EDU>]

The site
http://www.chem.arizona.edu/~salzmanr/480a/480ants/jadjte/jadjte.html
implies that air at room temperature (or atmospheric temperatures) is below
the J-T inversion temperature.  Does anyone know what kind of temperature
changes we are talking about for, say, air?

Also, suppose the gas was really ideal.  Is there an equation that gives the
cooling that would occur given the initial pressures of the two containers?
If we assume adiabatic expansion, I would think that the gas that expands
should cool and the other gas should warm.  Is this correct?  I'm having
trouble getting an expression that predicts the equilibrium temperature and
whether the final temperature change is zero as the initial pressure in one
of the containers goes to zero.

> -----Original Message-----
> From: John S. Denker [mailto:jsd@MONMOUTH.COM]
> Sent: Thursday, August 22, 2002 11:50 AM
>
> 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.
> [snip]
____________________________________________
Robert Cohen; rcohen@po-box.esu.edu; 570-422-3428; http://www.esu.edu/~bbq
Physics, East Stroudsburg Univ., E. Stroudsburg, PA 18301