Work (thermodynamics) of the energy went out as heat, and how much as work. This can be summarized by saying that heat and work are not state functions of the system. This is in contrast to classical mechanics, where net work exerted by a particle is a state function. Pressure–volume work (or "PV" work) occurs when the volume of a system changes. "PV" work is often measured in units of litre-atmospheres where = . However, the litre-atmosphere is not a recognised unit in the SI system of units, which measures P in Pascal (Pa), V in m, and PV in Joule (J),
Internal energy density called "specific internal energy" which is internal energy per unit of mass (kilogram) of the system in question. The SI unit of specific internal energy is J/kg. If the specific internal energy is expressed relative to units of amount of substance (mol), then it is referred to as "molar internal energy" and the unit is J/mol. From the standpoint of statistical mechanics, the internal energy is equal to the ensemble average of the sum of the microscopic kinetic and potential energies of the system. The internal energy, , expresses the thermodynamics of a system in the "energy-language", or in