The energy associated to the elecromagnetic (EM) field due to thermal agitation in homogeneous ohmic media is evaluated. To this goal I propose an expansion of electromagnetic fields in terms of orthogonal modal functions that are eigen-vector of Maxwell's. These fields are recognized as the degrees of freedom of the field and the energy associated to each of them is represented using the Johnson thermal voltage sources associated to the dissipative part of the impedance of the modes. At low frequency the conduction currents are dominant and in phase with the agitation of the electrons: the thermal energy is thus efficiently transferred to the EM field. At higher frequencies the radiated displacement currents overshadow the conduction currents. However, since the displacement currents are out of phase with the agitation of the electrons, the thermal energy is not efficiently transferred to the EM field. The model, which is entirely analytical, first provides the thermally generated EM energy per unit of volume. Then, relying on a number of simplifications commonly adopted in radiometry, provides the energy radiated outside the warm bodies. This latter radiated energy is finally compared with recent measurements for moderate conductivity silicon presented in a companion paper [11]. The model's accuracy with respect to measurements is within 1 dB, while the application of Planck's law, even mediated by the emissivity, is clearly off target.

The thermal energy radiated by silicon wafers is characterized experimentally in the mm and sub-mm wave ranges. The samples are heated and the energy they radiate thermally is captured by horn antennas and then detected. The energy radiated is in the tens and hundreds of pico-Joule per second ranges. The Zero Bias Schottky Detectors adopted are characterized by Noise Equivalent Powers in the few pico-watts, rendering the measurements possible. For moderate conductivities the measurements show a descending pattern as a function of the frequency which is not in line with expectations from Planck's law. Should these experiments have been available 120 years ago, the community of the time would probably have been concerned about avoiding the THz catastrophe, rather than the Ultraviolet catastrophe! A companion paper provides a possible theoretical classical explanation of the results [1].