統計力学
Statistical mechanics considers any system to be statistically distributed across an ensemble of formula_8 microstates. In a system that is in thermodynamic contact equilibrium with a heat reservoir, each microstate has an energy formula_4 and is associated with a probability formula_10. The internal energy is the mean value of the system's total energy, i.e., the sum of all microstate energies, each weighted by its probability of occurrence: