<P> Though it is a macroscopic quantity, internal energy can be explained in microscopic terms by two theoretical virtual components . One is the microscopic kinetic energy due to the microscopic motion of the system's particles (translations, rotations, vibrations). The other is the potential energy associated with the microscopic forces, including the chemical bonds, between the particles; this is for ordinary physics and chemistry . If thermonuclear reactions are specified as a topic of concern, then the static rest mass energy of the constituents of matter is also counted . There is no simple universal relation between these quantities of microscopic energy and the quantities of energy gained or lost by the system in work, heat, or matter transfer . </P> <P> The SI unit of energy is the joule (J). Sometimes it is convenient to use a corresponding 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 . </P> <P> 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 . </P> <P> The internal energy, U (S, V, (N)), expresses the thermodynamics of a system in the energy - language, or in the energy representation . Its arguments are exclusively extensive variables of state . Alongside the internal energy, the other cardinal function of state of a thermodynamic system is its entropy, as a function, S (U, V, (N)), of the same list of extensive variables of state, except that the entropy, S, is replaced in the list by the internal energy, U . It expresses the entropy representation . </P>

The sum of the kinetic and potential energies of all particles in the system is called the