* For a completely isolated system, S is maximum at thermodynamic equilibrium.
* For a closed system at controlled constant temperature and volume, A is minimum at thermodynamic equilibrium.
* For a closed system at controlled constant temperature and pressure without an applied voltage, G is minimum at thermodynamic equilibrium.
The various types of equilibriums are achieved as follows:
* Two systems are in thermal equilibrium when their temperatures are the same.
* Two systems are in mechanical equilibrium when their pressures are the same.
* Two systems are in diffusive equilibrium when their chemical potentials are the same.
* All forces are balanced and there is no significant external driving force.