Internal Energy

  • The energy stored in the system by virtue of the configuration and motion of molecules constituting the system is called internal energy (U). The energy due to mass motion of the system as a whole (the kinetic energy), and that due to its external position in gravitational, electrical, or magnetic field (the potential energy) are not included in the internal energy.
  • The molecules constituting the system possess kinetic energy of translation, rotation, and vibration. They also possess potential energy due to the forces of attraction between them. These molecular potential and kinetic energies contribute to the internal energy of the system.
  • A system under a given set of conditions has a definite internal energy.
  • It is impossible to measure the total internal energy of the substance and therefore absolute values of internal energy are not known. This is not a serious limitation because it is the changes in internal energy and not the absolute values that are usually needed in thermodynamic analysis