# Internal energy

From SklogWiki

The **internal energy** of a system is given by the sum of the kinetic energy and the
potential energy. This excludes the external energy of the system, for example motion of the center of mass of the system, or the presence of an external field.

The internal energy in classical thermodynamics is given by

and in statistical mechanics by

where is the Helmholtz energy function and is the partition function.