The notion of internal energy at a level appropriate for students of the Introductory Physics Course (IPC) is examined. The methodology proposed offers a general frame for analysing practically any physical system or process in a system of particles, from thermal phenomena and phase transitions to chemical and nuclear reactions, including processes involving elementary particles. The role of interactions among particles in a system is emphasized. The notion of statistical equilibrium of a system is discussed in this context. Finally, two structure factors are introduced to show the relation between the kinetic, interaction and particle energies of a system in maintaining stable structures.