Poynting vector

The Poynting vector is the cross product of the electric field and the magnetic field, named after its inventor John Henry Poynting. Oliver Heaviside independently co-discovered the Poynting vector, also.

Related Topics:
Cross product - Electric field - Magnetic field - John Henry Poynting - Oliver Heaviside

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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 often misspelled.) It is derived by considering the conservation of energy and taking into account that the magnetic field can do no work. It is given the symbol S (in bold because it is a vector) and is given by

Related Topics:
Electromagnetic wave - Magnitude - Vector

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:mathbf{S} = mathbf{E} imes mathbf{H} = rac{1}{mu} mathbf{E} imes mathbf{B}

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where E is the electric field, H and B are the magnetic field and magnetic flux density respectively, and mu is the permeability of the surrounding medium. For an electromagnetic wave propagating in free space mu becomes mu_0, the permeability of free space.

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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:

Related Topics:
Period - Irradiance

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:I = left langle S ight angle_T.

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