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The Poynting vector (named after its inventor John Henry Poynting) is the cross product of the electric field and the magnetic field. For an electromagnetic wave it points in the direction of energy flow and its magnitude is the power per unit area crossing a surface which is normal to it. (The fact that it points perhaps contributes to the frequency with which its name is misspelt.) It is given the symbol S (in bold because it is a vector) and is given by
- <math>\mathbf{S} = \mathbf{E} \times \mathbf{H} = \frac{1}{\mu} \mathbf{E} \times \mathbf{B}<math>
where E is the electric field, H and B are the magnetic field (see magnetic field for the difference) and <math>\mu<math> is the permeability of the surrounding medium. For an electromagnetic wave propagating in free space <math>\mu<math> becomes <math>\mu_0<math>, the permeability of free space.
Since the electric and magnetic field of an electromagnetic wave oscillate, the magnitude of the Poynting vector changes with time. The average of the magnitude over a long time T (longer than the period of the wave) is called the irradiance, I:
- <math>I = \left \langle S \right \rangle_T<math>.
See also
Poynting flux, Poynting theorem
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