Biomedical Engineering Reference
In-Depth Information
For an EM wave, the energy associated with the electric field is equal to the
energy associated with the magnetic field. That is,
H
2
H
2
ε
E
=
or
E
=
(8.5)
0
μ
εμ
0
00
Hence, the energy density is written in terms of just one or the other. J. C.
Maxwell, who showed that electricity and magnetism could be described by four
basic equations, also showed a connection between
υ
,
μ
0
, and
ε
0
(the permeability
of free space):
1
V
=
light
εμ
00
(8.6)
Substituting the above relation in (2.5),
EV H
=
(8.7)
light
EXAMPLE 8.2
A certain plane EM wave has a maximum electric field strength of 30 V/m. Find the maxi-
mum magnetic field strength and magnetic flux density.
Solution:
From (8.7), 30 [V/m]
=
3.00
×
10
8
[m/s]*
H
Hence
H
=
1.0
×
10
−
7
[V.s/m
2
]
=
1.0
×
10
−
7
A/m
B
=
1.2566
×
10
−
13
T
=
1.2556 nGauss
One way to assess the energy of an EM wave is to evaluate the energy carried
by the wave from one place to another. The radiative power (
P
radiative
) or radiant
flux is a physical quantity expressed in units of watts. If the number of photons per
second is
n
with a wavelength of
λ,
then
nhV
light
(8.8)
P
=
radiative
λ
A common measure of the photometric quantity of light is the intensity of the
wave or luminous flux, which is represented in lumens. Luminous flux correlates
to the visual sensation of light. Intensity (
) is the power that passes perpendicularly
through the unit area seen by an observer. The intensity [units are W/m
2
] is writ-
ten as
ι
