Electromagnetic Intensity Previously, it has been shown that when a linear electric field varies in only one direction in space, a perpendicular magnetic field is created according to dE/dx = dB/dt , and that the end result is a pair of transverse sinusoidal fields E = E ₀ cos(kx-ω t) along with B = B cos(kx-t) that compose an electromagnetic wave as depicted. If these two field equations are inserted into the differential equation above, the result is E = Bc where c is the propagation speed for the wave moving perpendicular to the fields according to the Right-Hand Rule. Since the RHR normally accompanies the cross product, it is worth asking what happens when the electric field is crossed into the magnetic field for such a wave. The result is called the Poynting Vector , an amusing coincidence in that it points in the direction of propagation of the wave. The Poynting vector is given by I ѐ = E ѐ × B / µ , where µ has been included because I then has units of intensity in watts per meter squared. For an EM
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