Answers (1)
In Poynting's original paper and in many textbooks, the Poynting vector is defined as[3][4]
\mathbf{S} = \mathbf{E} \times \mathbf{H},
where bold letters represent vectors and
E is the electric field vector;
H is the magnetic field vector.
This expression is often called the Abraham form.[5][6] The Poynting vector is usually denoted by S or N.
In the "microscopic" version of Maxwell's equations, this definition must be replaced by a definition in terms of the electric field E and the magnetic flux density B (it is described later in the article).
It is also possible to combine the electric displacement field D with the magnetic flux density B to get the Minkowski form of the Poynting vector, or use D and H to construct yet another version. The choice has been controversial: Pfeifer et al.[7] summarize and to a certain extent resolve the century-long dispute between proponents of the Abraham and Minkowski forms.
The Poynting vector represents the particular case of an energy flux vector for electromagnetic energy. However, any type of energy has its direction of movement in space, as well as its density, so energy flux vectors can be defined for other types of energy as well, e.g., for mechanical energy. The Umov–Poynting vector[8] discovered by Nikolay Umov in 1874 describes energy flux in liquid and elastic media in a completely generalized view.