where S is the entropy [J K -1 ] and U is the internal energy . In relation to the entropy rate, its entropy production (or generation) rate is eveluated by the (4) . This relation refers to the electromagnetic wave related to the photon inflowing to the atom, so, we consider a black body, defined by Kirchhoff as physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence . For a such body, Planck introduced the concept of resonator for which he evaluated the irreversibility as introduced by Maxwell-Hertz and taking into account the Boltzmann hypothesis of molecular disorder, expressed by the Planck natural radiation concept . The entropy of the black body results: