POYNTING VECTOR

a quantity describing the magnitude and direction of the flow of energy in electromagnetic waves. It is named after John Henry Poynting, an English physicist, who introduced it in 1884. The Poynting vector S is defined as S = (1/m)E B, where m is the permeability of the medium through which the radiation passes, E is the electric field intensity, and B is the magnetic induction. Thus, the magnitude S of the Poynting vector equals (1/m)EB sinq, where E and B are, respectively, the magnitudes of the vectors E and B and q is the angle subtended by the two vectors. The direction of the vector product S is perpendicular to the plane determined by the vectors E and B. For a traveling electromagnetic wave, the Poynting vector points in the direction of the propagation of the wave.