Irreversibility

Abstract

An expanding gas does work in pressing against its expanding container. Is there not some way in which it can expand without pressing against an expanding container? Yes. Take, for instance, the somewhat oversimplified diagram of Figure 17.01. Suppose that there is gas in the left compartment, vacuum in the right. If the dividing partition is suddenly broken, the gas will redistribute itself uniformly throughout the total volume. But nowhere, if the breaking is done expertly enough, will the gas molecules bounce off a receding wall. None of the supply of energy of random motion is used up; the average kinetic energy per molecule, or per degree of freedom, remains the same. The temperature, then, remains the same, in spite of the expansion.