Thermodynamics of stationary states of the ideal gas in a heat flow

TL;DR

This paper develops a thermodynamic framework for the stationary states of an ideal gas under heat flow, showing that steady state thermodynamics mirrors equilibrium thermodynamics and identifying conditions for stability.

Contribution

It formulates a consistent thermodynamic theory for nonequilibrium steady states of an ideal gas, extending classical thermodynamics to systems with heat flow.

Findings

Steady state thermodynamics has the same formal structure as equilibrium thermodynamics.

The internal energy satisfies a specific differential relation independent of system shape or boundary conditions.

The theory predicts stable nonequilibrium states and reduces to equilibrium thermodynamics when heat flux vanishes.

Abstract

There is a long-standing question as to whether and to what extent it is possible to describe nonequilibrium systems in stationary states in terms of global thermodynamic functions. The positive answers have been obtained only for isothermal systems or systems with small temperature differences. We formulate thermodynamics of the stationary states of the ideal gas subjected to heat flow in the form of the zeroth, first, and second law. Surprisingly, the formal structure of steady state thermodynamics is the same as in equilibrium thermodynamics. We rigorously show that $U$ satisfies the following equation $dU=T^{*}dS^{*}-pdV$ for a constant number of particles, irrespective of the shape of the container, boundary conditions, size of the system, or mode of heat transfer into the system. We calculate $S^{*}$ and $T^{*}$ explicitly. The theory selects stable nonequilibrium steady states in a multistable system of ideal gas subjected to volumetric heating. It reduces to equilibrium thermodynamics when heat flux goes to zero.