The Poynting vector P is equal to

Concept:

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 

\( \vec P = \vec E × \vec H\)

Or, it can be written as,

\(⇒ S = \frac{Power}{Area}\)

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

• 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.

EXPLANATION:

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

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

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

• 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 gives the instantaneous power density. 
• And average Poynting vector gives the average power crossing a unit area placed perpendicular to the incident wave.