Internal energy

Energy contained within a system

The internal energy, in thermodynamics, is the property that defines the energy of a thermodynamic system usually in the absence of effects from electric, magnetic or any other external fields which can be source of potential energy. It also excludes the kinetic energy of motion. The notion of internal energy is present in the expression of the first law of thermodynamics as a state variable, as originally formulated by the german scientist Rudolf Clausius, one of the founders of thermodynamics who is also the author of the notion of entropy as a state variable.

From Wikipedia, under CC BY-SA. More on occurri.