Physical Interpretation of Internal Energy

Internal energy can be defined as the sum of all the microscopic forms of energy of a system. It is related to the molecular structure and the degree of activities at molecular level and can be viewed as the sum of the kinetic and potential energies of the molecules. Let us consider a system for analysis of internal energy at molecular level. Due to different type of movements of molecules, such as translational, rotational, and vibrational, kinetic energy in the system is developed. Vibrational motion of the molecules becomes more significant at higher temperature. If we analyse the system at atomic level, the fundamental particles rotate in their orbits around the nucleus and also spin about their own axis. Thus, rotational kinetic energy and spin energy are associates with the system. The part of internal energy associated with the kinetic energy is known as sensible energy and proportional to the temperature of the system. At higher temperature, degree of activity at molecular level will be larger and system will have higher internal energy.

Internal energy may be presented in the form of binding force at atomic level. If external energy is supplied to break the bond and to change the phase from solid to liquid or liquid to solid, a certain amount of energy is stored as latent energy. This latent energy represents internal energy of the system. Similarly, it may be associated with nuclear and some other forms of energy in the system.