I'm reading D. F. Lawden, Principles of Thermodynamics and Statistical Mechanics, and am trying to understand the first worked problem they have. Below is the problem and start of their solution.

My problem in understanding is this: In the problem below, I am taking a "container" to be a fixed-volume container, and I don't know what it means if it isn't. I am understanding the container being a fixed-volume entity, and that the gas escapes into a variable-volume chamber with a piston. But that doesn't make sense with the explanation given below. Can someone explain a bit more what it means to have a container change volume while gas escapes into the piston chamber? Is the container variable-volume, but always at equilibrium with pressure in the surrounding environment (that is, the environment pressure changes to match the container pressure as the container pressure changes, and so the container can then be a variable volume entity? There's no picture with the problem, and I may fundamentally misunderstand what a "container" is here and that it is a fixed-volume object (obviously not from the problem, but I want to make sure I have the right picture of what that means).

D. F. Lawden, Principles of Thermodynamics and Statistical Mechanics, pg. 17, Problem 1:

$V_0$ moles of an ideal gas are contained in an insulated chamber at pressure $p_0$ and temperature $T_0$. The gas slowly escapes through a valve into an insulated cylinder provided with a frictionless piston to which an external pressure $p_1$ ($< p_0$) is applied. Initially the volume enclosed iby the piston is zero. When the piston comes to rest, calculate the number of moles of gas left in the chamber and its temperature. Find, also, the temperature of the gas in the cylinder.


The expansion of the gas is adiabatic, but not quasi-static, since thee will be a pressure gradient across the valve until the chamber has been reduced to $p_1 $at the end of the process. However, the expansion of that portion of the gas which never leaves the chamber will be quasi-static, and must be governed by [$pV^\gamma$ = constant, governing adiabatic expansion of an ideal gas by a quasi-static process]. Thus, if $V_0, V_1$ are its volumes at the beginning and end of the process

$p_0 V_0^\gamma = p_1 V_1^\gamma$



1 Answer 1


Visualize an imaginary membrane surrounding the portion of the gas in the chamber that eventually remains in the chamber once the system equilibrates. Initially, this portion of the gas occupies only a fraction of the chamber, and it expands adiabatically and reversibly to push the other part of the gas (that which leaves the container) out through the valve. So it is doing work at the membrane interface on the portion eventually being expelled.

This all can be analyzed in a different way by applying the open system (control volume) version of the 1st law of thermodynamics to the gas in the chamber, including the enthalpy of the gas leaving. The analysis leads to the same result.

  • $\begingroup$ Thank you. This helps clarify that I should imagine it more like a balloon (membrane) rather than a rigid metal container, like a propane tank for my grill. Thank you for the clarification. $\endgroup$
    – mpettis
    Mar 18, 2021 at 18:54
  • $\begingroup$ No, check that. As you stated, a balloon in a rigid container that contains the gas that will remain in the rigid container. $\endgroup$
    – mpettis
    Mar 18, 2021 at 19:01
  • 1
    $\begingroup$ Yes, except that the balloon has stretching stiffness, and our imaginary membrane does not. $\endgroup$ Mar 18, 2021 at 19:09

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