Internal energy is a fundamental thermodynamic property representing the total energy contained within a thermodynamic system. This total energy comprises the kinetic energy associated with the random motion of the system's constituent particles—such as atoms and molecules—as well as the potential energy stored in the forces and chemical bonds between those particles. Changes in the internal energy of a closed system are quantified by the First Law of Thermodynamics, which dictates that the net change in internal energy is equal to the heat transferred to the system minus the work done by the system. This concept is crucial for understanding chemical reactions, phase changes, and the energy transformations occurring in physical processes.