# Thermodynamics: 2 Pistons connected by a rod

1. Oct 7, 2012

### flyers

1. The problem statement, all variables and given/known data

2. Relevant equations

First Law, U = Q-W
We also need to apply a force equillibrium on the pistons

3. The attempt at a solution
Since the problem says the walls and pistons are insulated, we know there is no heat transfer and the process is adiabatic but not reversible because of friction. So U = -W. I dont know the path and so I don't know how I could determine the work done but if I consider the system which contains both gases, the pistons and the rod (and the case where atm pressure = 0) then the work done is just 0 so U = 0. Therefore naCv(T2-T1)+nbCv(T2-T1) = 0. I also know that for final pressures, the forces acting on the rod must balance so pressure A*piston area of A = pressure B*piston area of B.

Beyond this I'm not so sure what to do since I don't know much on the final state variables. What confuses me most is how it would be possible to calculate work since the pressure is not constant and the path is unknown.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 7, 2012

### voko

The net work is indeed zero (neglecting the forces of friction). However, the work of gas in one cylinder must be equal to the work on gas in the other cylinder. You could also note that the changes of volumes in both cylinders are related quite simply.