Internal energy

The internal energy of a system (abbreviated E or U) is the total kinetic energy due to the motion of molecules (translational, rotational, vibrational) and the total potential energy associated with the vibrational and electric energy of atoms within molecules or crystals. Internal energy is a quantifiable state function of a system. The SI unit of energy is the joule.

Related Topics:
Kinetic energy - Molecule - Translational - Rotational - Vibrational - Potential energy - Electric - Atom - State function - SI - Joule

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For systems consisting of molecules, the internal energy is partitioned among all of these types of motion. In systems consisting of monatomic particles, such as helium gas and other noble gases, the internal energy consists only of the translational kinetic energy of the individual atoms. Monatomic particles, of course, do not rotate or vibrate, and are not excited to higher electrical energies, except at very high temperatures.

Related Topics:
Helium - Noble gas

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From the standpoint of statistical mechanics, the internal energy is shown to be equivalent to the ensemble average of the total energy of the system.

Related Topics:
Statistical mechanics - Ensemble average

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