Since only the microscopic fields E and B occur in the derivation of and the energy density, assumptions about any material present are avoided. The Poynting vector and theorem and expression for energy density are universally valid in vacuum and all materials.
Since only the microscopic fields E and B occur in the derivation of and the energy density, assumptions about any material present are avoided The Poynting vector and theorem and expression for energy density are universally v.alid in vacuum and all materials.