Clas Blomberg - Physics of life-Elsevier Science (2007)

42 Part II. The physics basis
The last equation is similar to this one as it relates the time derivative of an electric field,
the D-field to the H-field. It provides the generation of a magnetic field by a current—the
Ampere’s law (eq. 5.15)—but in this the time derivative of the displacement field D
appears in the same way as the current. The relation is expressed in terms of vectors and the
current density j:
D
rot H t
j
(5.21)
or in component form:
H
H
y
z
D
t
z y x
H
H
D
H
x z y
j
H
x; jy;
z
x
t
x
y
D
j z
t
y x z
We do not go further here, and the equations are not needed in the rest. (They will be mentioned
at some places, but not further developed). The combination of the two last equations
gives rise to a wave equation, the equation for electromagnetic radiation.
5E
Radiation
We shall conclude this section by some discussion about electromagnetic radiation and its
effects. The basis is already mentioned: electric and magnetic fields influence each other,
and this becomes more and more relevant the higher the frequencies are. The electric and
magnetic fields together provide what we see as radiation, waves that propagate in free space,
generated by, for instance, high-frequency currents. As a wave, there is a propagation and
a kind of oscillation in some directions(s). The oscillations are provided by the electric and
magnetic fields that are directed perpendicular to the propagation direction. (This is what is
referred to as transverse oscillations.) There are thus two directions for oscillations, referred
to as polarisation directions. As well known, electromagnetic radiation covers a very large
spectrum of different frequencies with quite different effects. Much can be said about this, but
I take up here what is most relevant for the themes of this book.
Relatively low frequencies as the situation described for low-frequency magnetic fields
can induce currents. And, as also said, any induced current has a tendency to counteract the
fields. Biological matter is not a good conductor, although there are significant currents along
in particular nerve cells, but it is difficult to see how weak low-frequency radiation would
influence biological organisms, although there are (controversial) suggestions about effects
of low-frequency magnetic fields as discussed above.
When going upwards in frequency range, one gets to the higher range called radio frequencies,
the conventional frequencies for radio transmission. These certainly penetrate our
bodies, but there are no suggested effects. What yields a controversial subject is when we get
to what is called microwaves, frequencies around gigahertz (10 9 periods/sec). This provides