Abstract
Nonequilibrium thermodynamics is a phenomenological theory of matter, which describes the behavior of a system due to differences within the system or between the system and its surroundings. These differences give rise to thermodynamic forces which can cause flows of quantities such as charge and energy. One example of such a force is the temperature gradient due to a temperature difference across the ends of a solid. The flow caused by this force is energy flow or heat current.|When two or more thermodynamic forces are applied on a system, they can interfere with each other, and give rise to new effects. Although many thermodynamic physicists, such as Rauss, Peltier, and Seebeck, discovered some important phenomena about these new effects, Onsager was the one who gave a complete phenomenological description about these effects.|Onsager’s equations provide a macroscopic picture in nonequilibrium thermodynamics. Starting from the Boltzmann equation, physicists have been able to successfully derive the Onsager equations microscopically. In section II, we will derive Onsager’s equations by using the variational principle.