What is Internal energy?
In [[thermodynamics]], the '''internal energy''' of a [[thermodynamic system]], or a [[physical body|body]] with well-defined [[dimension|boundaries]], denoted by&nbsp;''U'', or sometimes&nbsp;''E'', is the total of the [[kinetic energy]] due to the motion of [[molecule]]s ([[translation (physics)|translational]], [[rotation]]al, [[oscillation|vibrational]]) and the [[potential energy]] associated with the vibrational and [[electricity|electric]] energy of [[atom]]s within molecules or [[crystal]]s. It includes the [[energy]] in all the [[chemical bond]]s, and the energy of the free, [[electrical conduction|conduction]] [[electron]]s in [[metal]]s. The internal energy is a [[thermodynamic potential]] and for a [[closed system|closed]] thermodynamic [[thermodynamic system|system]] held at constant [[entropy]], it will be [[Principle of minimum energy|minimized]]. One can also calculate the internal energy of [[electromagnetic radiation|electromagnetic]] or [[black body|blackbody]] radiation. It is a [[state function]] of a [[system]], an [[extensive quantity]]. The [[SI]] unit of [[energy]] is the [[joule]] although other historical, conventional units are still in use, such as the (small and large) [[calorie]] for [[heat]]. (Calories that are on classic food labels are actually kilo-calories.) == Overview == ''Internal'' energy does not include the translational or rotational kinetic energy of a body ''as a whole''. It also does not include the [[special relativity|relativistic]] [[mass]]-energy equivalent ''E''&nbsp;=&nbsp;''mc''<sup>2</sup>. It excludes any potential energy a body may have because of its [[dimension|location]] in external [[gravitation]]al or [[electrostatics|electrostatic]] [[field (physics)|field]], although the potential energy it has in a field due to an [[electrostatic induction|induced]] electric or [[magnetism|magnetic]] [[dipole]] [[moment (physics)|moment]] does count, as does the energy of [[deformation]] of solids ([[stress (physics)|stress]]-[[strain (materials science)|strain]]