internal energy

The internal energy of a thermodynamic system is the energy as a state function, measured as the quantity of energy necessary to bring the system from its standard internal state to its present internal state. It excludes the kinetic energy of motion of the system and the potential energy of position of the system as a whole, with respect to its surroundings and external force fields. It includes the thermal energy, i.e., the constituent particles' kinetic energies of motion relative to the motion of the system as a whole. Without a thermodynamic process, the internal energy of an isolated system cannot change, as expressed in the law of conservation of energy, a foundation of the first law of thermodynamics.