Biomedical Engineering Reference
In-Depth Information
Infrared light, microwaves, and radio/television waves are lower energy waves
(lower frequency, longer wavelength) relative to visible light. Ultraviolet light, X-
rays, and gamma rays possess higher energy than visible light (higher frequency,
shorter wavelength). Most gamma rays are somewhat higher in energy than X-
rays. It is convenient to describe X-rays in terms of the energy they carry, in units
of thousands of electron volts (keV). X-rays have energies ranging from less than
1 keV to greater than 100 keV. Although the distinction between hard and soft X-
rays is not well defined, hard X-rays have the highest energy (greater than 10 keV)
while the lower energy X-rays are referred to as soft X-rays. The energy and spin
of an atom or a nucleus can be changed by the absorption or emission of a photon.
An X-ray photon with sufficient energy can interact with and remove electrons
bound to an atom (the process of ionization). This is why X-rays and g-rays are
also referred to as ionizing radiation.
The EM waves are made up of two parts: an electric field and a magnetic field.
The two fields are at right angles to each other. EM waves propagate perpendicular
to both electric and magnetic fields. If the EM wave propagates in the
x
-direction,
then the electrical field is in the
z
-direction and the magnetic field will be in the
y
-direction. Since EM waves have both electric and magnetic fields, the energy in
the wave is linked to those fields. The magnetic field is analogous to the electric
field (discussed in Chapter 3), and the same field model is used to describe mag-
netic field. Further, the SI unit for magnetic field strength is Amperes per meter.
A constant current produces a constant magnetic field, while a changing current
produces a changing magnetic field. Conversely, a magnetic field produces current,
as long as the magnetic field is changing, which is termed as induced electromotive
force. A steadily changing magnetic field induces a constant voltage, while an oscil-
lating magnetic field can induce an oscillating voltage.
Similar to the energy density (energy per unit volume) in an electric field given
by (3.43), the energy density in a magnetic field of strength
H
is written as
1
2
w
=
H
(8.3)
m
2
μ
0
10
−6
Weber/Ampere.meter or N/
A
2
). The number of magnetic lines of force cutting through a plane of a given area at
a right angle is known as the magnetic flux density (
B
) or magnetic induction. The
flux density is a measure of the force applied to a particle by the magnetic field. The
unit of magnetic flux density is tesla (equivalent to N/A.m or N.s/C.m). Since tesla is
a very large unit, the smaller magnetic flux density unit is the gauss (1 tesla
where
μ
0
is the permeability of free space (1.2566
×
10,000
gauss), or dyne/A.cm. The Earth's magnetic field is typically a fraction of a gauss.
The relation between magnetic flux density and magnetic field strength in vacuum
as well as in the air or other nonmagnetic environments is constant (
=
μ
0
H
). Using
(3.43), the total energy density associated with an EM wave is written as
β
=
1
1
wE
=
ε
2
+
H
2
(8.4)
0
2
2
μ
0
