Gas flows from cylinder A to cylinder B

1. Sep 10, 2007

mune

Hi all,

Let's say we have 2 closed cylinders A and B, same volume, containing air inside. Two cylinders are connected to each other with a tube, with valve. Initially the valve is closed.

cylinder A: P=100 Pa, T= 350 k.
cylinder B: P=50 Pa, T= 300 k

now if the valve is opened, and closed when their pressure are the same.

assume no heat loss to or gain from surrounding.

here my problem:
will their temperature be the same when their reach the same pressure?

my guess are....

1. yes. By ideal gas law, PV/T = mR, if P1=P2, V1=V2, then T1=T2, so the temp will be the same.

2. no. When the valve is opened, the air flows from A to B, which means the air does some flow work. By conservation of energy, the temp in B will be higher than A.

which one of me should I believe??

thanks.

2. Sep 10, 2007

Staff: Mentor

When A and B become (A+B), and it's adiabatic, what is the equilibrium state.

Certainly the pressure will equilibrate, but what can be said of the temperature?

How? B is starting at a lower temperature than A. How could B end up at a higher temperature?

Those are lower pressures btw. Are those gage or absolute pressures?

Remember that 1 atm = 14.7 psi = 101.325 kPa.

Last edited: Sep 10, 2007
3. Sep 10, 2007

mune

what if they started with the same temperature, other condition remains the same?

assume the values are gage pressure values(but does make any difference?)

4. Sep 10, 2007

Staff: Mentor

Then they remain at the same temperature. At some point in time, the pressures have to equilibrate.

I'm thinking along the lines of PV=nRT, in which the pressure is in absolute terms.

Perhaps the pressures are in absolute, in which case, they are partial vacuums, or at least very low pressures.

5. Sep 11, 2007

mune

what about the flow work? the air molecules which flow from A to B have some KE right? The KE will change into thermal E after the valved is closed?