What is meant by internal energy?

Explanation

(i)Internal energy (U) of a system is equal to the energy possessed by all its constituents namely atoms, ions and molecules.
(ii) The energy of a molecule is equal to the sum of its translational energy (`U_t`), vibrational energy (`U_V`), rotational energy (`U_r`), bond energy (`U_b`), electrical energy (`U_e`) and energy due to molecular interactions `(U_i)`.
(iii) `U=U_t+U_v+U_r + U_b + U_e + U_i`
(iv)The total energy of all the molecules of the system is called internal energy.