Internal Energy

“Internal” energy is so named since it is closely related to the internal structure of the substances, in words of the internal microscopic energy modes. This involves the rotational, translational, and vibrational energies of the molecules. It is suitable to divide the macroscopically measurable mechanical energy (symbolized by potential and kinetic energy of the system) from the energy in such “hidden microscopic modes”, this is the internal energy. It cannot be measured directly, though should be determined by inference. The evaluation of internal energy as a function of the state of the system is one of the vital troubles in thermodynamics.

Internal energy symbolizes the specific mode of energy that can be directly influenced by heat communication. For steady specific heat, temperature increase is a measure of the rise in internal energy, and temperature increase can be affected via heat transfer.