Thermodynamic potentials The four ways to say energy
This chapter introduces the thermodynamic potentials (enthalpy, Helmholtz free energy, and Gibbs free energy), which are the three Legendre transformations of the internal energy function that can be formed from the products of variables. It emphasizes their physical interpretations as energies stored in the system under controlled conditions. J. Willard Gibbs formulated some equivalent criteria for characterizing equilibrium, the entropy maximum principle and the energy minimum principle, that for systems in equilibrium. Legendre transformations provide a way of obtaining equivalent descriptions of the energy (known as thermodynamic potentials) involving variables that may be easier to control. Heat death will occur if thermodynamic equilibrium can be established over cosmological length scales with all inhomogeneities leveled, when matter and energy in all forms are evenly distributed throughout the universe. In the Joule-Kelvin (or throttling) process, a steady flow of gas is forced through a porous plug under conditions of thermal isolation.