Internal energy (E or U) Internal energy is an important concept in thermodynamics. All the energy present in a system, including the kinetic energy of molecules due to translational, vibrational and rotational motion and the potential energy due to intermolecular forces held in every chemical link between molecules, is referred to as internal energy. Every time a system is changed, there are energy transfers and conversions due to the interplay of heat, work, and internal energy. The total energy stored in a substance by virtue of its chemical nature is called internal energy. E = E translational + E vibrational + E rotational + E kinetic + E potential + … Change in internal energy is given by; If then internal energy is positive. If then internal energy is negative. Characteristics of Internal Energy Internal energy is a state function, meaning it is based only on the system's current state and ...