The aim of the paper is the analysis of relations that express the energy of the electromagnetic field of a domain bounded by a closed surface placed inside a body. There is considered that the surface which bounds the domain has on its two faces, outwards and inwards, surface densities of the electric polarization charge and of the magnetization current sheet, equal in absolute value but of opposite sign. If the terms of the balance equations are verified, the sources of the internal surface have a significant importance in the computing relationships.

Keywords: Energy, Electromagnetic field.