Explanation:
Internal energy is the sum of all the energies inside or within a system because molecules of the system will have kinetic, potential, rotational, vibrational etc energies.
Internal energy arises because of random or disordered movement of particles within a system.
Therefore, internal energy of a system is the sum of kinetic and potential energy of all the particles present inside the system.
The variable for internal energy is "U". Also, change in internal energy is represented by .