5
Coupling of Electromagnetic Fields into Biological Systems
led to the postulate that electromagnetic waves can transport energy and the hypothesis
that light is an electromagnetic wave.
Equation 1.3 is Gauss’ electric law, which states that the net outward flow of electric
flux through a closed surface is equal to the charge contained in the volume enclosed
by the surface. Likewise, Gauss’ law for magnetic fields (Equation 1.4) states that the net
outward flow of magnetic flux through a closed surface is zero. Therefore, magnetic flux
lines are always continuous and they form closed loops.
The auxiliary equations that bridge the fields and flux densities produced by a given
current or charge distribution are
D = εE
(1.5)
B = μH
(1.6)
J = σE
(1.7)
Free space or vacuum is a medium in which the permittivity, ε, is given by
ε 0 = 8.854 × 10 −12 = 1/(36π) × 10 −9 F/m
(1.8)
Free space permeability, μ, is given by
μ 0 = 4π × 10 7 H/m
(1.9)
Finally, the electrical conductivity for free space is σ = 0. For all other linear, isotropic,
and homogeneous media, it is the convention to introduce the following dimensionless
ratios:
ε r = ε/ε 0
(1.10)
μ r = μ/μ 0
(1.11)
Equations 1.10 and 1.11 give the relative dielectric constant and relative permeability,
respectively. Living matters generally have relative permeability equal to that of free
space, with the exception of cells, molecules, or organisms endowed with ferromagnetic
particles. However, the relative dielectric constants show characteristic dependence on
frequency and material medium.
1.3 Electromagnetic Properties of Tissue
Typically, dielectric constants decrease and conductivities increase with increasing frequency (Figure 1.1). Biological materials exhibit very high dielectric constants, especially
at low frequencies, compared to other homogeneous solids and liquids. This is because
biological tissues are composed of macromolecules, cells, and other membrane-bound
substances. Mobile counterions are associated with charges on cell membranes, and
Coupling of Electromagnetic Fields into Biological Systems
led to the postulate that electromagnetic waves can transport energy and the hypothesis
that light is an electromagnetic wave.
Equation 1.3 is Gauss’ electric law, which states that the net outward flow of electric
flux through a closed surface is equal to the charge contained in the volume enclosed
by the surface. Likewise, Gauss’ law for magnetic fields (Equation 1.4) states that the net
outward flow of magnetic flux through a closed surface is zero. Therefore, magnetic flux
lines are always continuous and they form closed loops.
The auxiliary equations that bridge the fields and flux densities produced by a given
current or charge distribution are
D = εE
(1.5)
B = μH
(1.6)
J = σE
(1.7)
Free space or vacuum is a medium in which the permittivity, ε, is given by
ε 0 = 8.854 × 10 −12 = 1/(36π) × 10 −9 F/m
(1.8)
Free space permeability, μ, is given by
μ 0 = 4π × 10 7 H/m
(1.9)
Finally, the electrical conductivity for free space is σ = 0. For all other linear, isotropic,
and homogeneous media, it is the convention to introduce the following dimensionless
ratios:
ε r = ε/ε 0
(1.10)
μ r = μ/μ 0
(1.11)
Equations 1.10 and 1.11 give the relative dielectric constant and relative permeability,
respectively. Living matters generally have relative permeability equal to that of free
space, with the exception of cells, molecules, or organisms endowed with ferromagnetic
particles. However, the relative dielectric constants show characteristic dependence on
frequency and material medium.
1.3 Electromagnetic Properties of Tissue
Typically, dielectric constants decrease and conductivities increase with increasing frequency (Figure 1.1). Biological materials exhibit very high dielectric constants, especially
at low frequencies, compared to other homogeneous solids and liquids. This is because
biological tissues are composed of macromolecules, cells, and other membrane-bound
substances. Mobile counterions are associated with charges on cell membranes, and