In an electromagnetic wave, Poynting vector representing the flow of electromagnetic energy per unit area per unit time across the boundary is represented by

Poynting vector:

It states that the cross product of electric field vector (E) and magnetic field vector (H) at any point is a measure of the rate of flow of electromagnetic energy per unit area at that point that is 

\( \vec S = \vec E \times \vec H\)

Where P = Poynting vector, E = Electric field and H = Magnetic field

Or, it can be written as,

\(S = \frac{Power}{Area}\)

The Poynting vector describes the magnitude and direction of the flow of energy in electromagnetic waves.

The unit of the Poynting vector is watt/m2.

Additional Information

1) The Poynting vector (i.e. energy flow per unit area per unit time) for a plane electromagnetic wave is given by

\( \vec S = \frac{1}{\mu_o}(\vec E \times \vec H)\)

\( S = \frac{1}{\mu_o}(E H\, sin\theta )\)

2) From the above, it is clear that E and H are mutually perpendicular and also they are perpendicular to the direction of propagation of the wave.

Thus, the direction of the Poynting vector is along the direction of the propagation of the electromagnetic wave.

The Poynting vector describes the magnitude and direction of the flow of energy in electromagnetic waves per unit volume.

∴ The dimensions of both the Poynting vector and the electromagnetic power density are the same, i.e. M1 L-1 T-2

where M = Mass, L = Length, T = Time