# Poynting Theorem and Poynting Vector

Poynting vector represents the energy flux (in W/m2) of an electromagnetic wave. It is named after its inventor, the British physicist John Henry Poynting. It is defined as,

where,  is the electric field and  is the magnetic field.

The Poynting theorem provides a statement about the conservation of energy of an electromagnetic wave. The Poynting theorem is applicable only for time- varying fields. It relates the time derivative of the energy density, u, to the energy flow and the rate of work done (i.e., power) by the electromagnetic field.

Poynting vector for a plane wave