Internal Energy (U):

It is the energy possessed by the system on account of its configurations, and motion of atoms and molecules. Not like potential energy and kinetic energy of a system, which is visible and can be felt, internal energy is the invisible form of energy and can only be sensed. In thermodynamics, the major interest of study lies in knowing the change in internal energy than to know its absolute value.

The internal energy of a system is the sum of energies contributed by several configurations and inherent molecular motions. These contributing energies are given below

(1) Spin energy: because of clockwise or anticlockwise spin of the electrons about their own axes.

(2) Potential energy: because of intermolecular forces (Coulomb and gravitational forces), which keep molecules together.

(3) Transitional energy: because of movement of molecules in all the directions with all possible velocities within the system, this results in kinetic energy acquired by translatory motion.

(4) Rotational energy: because of rotation of molecules about the centre of mass of the system, resulting in kinetic energy acquired by the rotational motion. Such form of energy exists invariably in diatomic and polyatomic gases.

(5) Vibrational energy: because of vibration of molecules at the high temperatures.

(6) Binding energy: because of force of attraction between various sub atomic particles and nucleus.

(7) Other forms of energies such as Electric dipole and magnetic dipole energy when system is subjected to electric and magnetic fields. High velocity energy when rest mass of system mo changes to variable mass m according to Eisenstein's theory of relativity). The internal energy of system can increase or decrease during the thermodynamic operations. The internal energy will increase if the energy is absorbed and will decrease when energy is evolved out.