High School Physics

# Internal Energy of a thermodynamic system

In this post, we have defined the Internal Energy of a Thermodynamic System. We have also identified the components of Internal Energy.

## Definition of Internal Energy in Thermodynamics

In thermodynamics, the internal energy of a thermodynamic system, or a body with well-defined boundaries, denoted by U, or sometimes E, is the total of the kinetic energy due to the motion of particles (translational, rotational, vibrational) and the potential energy associated with the vibrational and electric energy of atoms within molecules or crystals. It includes the energy in all of the chemical bonds and the energy of the free, conduction electrons in metals.

It is a state function of a system.

The SI unit of energy is the Joule.

Internal energy does not include the translational or rotational kinetic energy of a body as a whole. It also does not include the relativistic mass-energy equivalent E = mc2. It excludes any potential energy a body may have because of its location in an external gravitational or electrostatic field, although the potential energy it has in a field due to an induced electric or magnetic dipole moment does count, as does the energy of deformation of solids (stress-strain).

We can also say that Internal energy is the sum of all microscopic forms of energy of a system. It is related to the molecular structure and the degree of molecular activity and may be viewed as the sum of kinetic and potential energies of the molecules.

## Internal energy components

Internal energy comprises the following types of energy:

1 ) Sensible energy – the portion of the internal energy of a system associated with kinetic energies (molecular translation, rotation, and vibration; electron translation and spin; and nuclear spin) of the molecules.