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 RightHand 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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 Spring '11
 Tibbets
 Energy, Photon, Light

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