The internal energy of the system is sum of kinetic and potential energy of the particles. For an ideal gas, we assume that the particles do not intract with each other. Hence, there is no potential energy associated with the particles and the internal energy of the system will only be due to the kinetic energy of the system. Note For a real gas, the internal energy of the system is the sum of kinetic and potential energy as the gas molecules are interacting with each other.