I.3.2. ELECTROMAGNETIC POLLUTION
I.3.2.1. The Electromagnetic Fields. An Introduction
Electric Current. Magnetic Field
Electric charge in motion is an electric current. A current generates a magnetic field. The time varying electric field together with its co-dependent magnetic field give rise to electromagnetic (EM) radiation. EM radiation is a transport of energy in the form of an EM wave. This wave may be regarded as an oscillatory wave (Figure I.3.6.) propagated through a volume of space.
Figure I.3.6. Representation of an electromagnetic wave
This is the number of complete oscillations or cycles of an EM wave detected by a stationary observer in one second. The unit is the Hertz (Hz).
Example: the oscillatory electric charge motion which comes as an alternating current (a.c.) in all houses with electricity; 60 Hz in North America, 60 Hz elsewhere.
This is the distance occupied in space by one EM cycle. The unit is the meter (m).
Velocity of the Electromagnetic Wave (v)
The frequency and wavelength of an EM wave are related by the simple formula:
v = f (1)
Example: the velocity of EM waves in vacuum (air) is equal to 3x108 m/s, the speed of light.
The Electromagnetic Wave Spectrum
EM waves are thus transversal waves, i.e. the vibrations are perpendicular to the direction of motion. As all waves, EM waves show properties such as absorption, reflection, refraction, diffraction and interference. Moreover, as transversal waves, they also show polarization effects.
The classification of EM waves by their wavelength or frequency is behind the idea of the EM spectrum (Figure I.3.7.). The frequencies corresponding to the energy power supplies (50; 60 Hz) are related to the so-called “extremely low frequency” electric and magnetic fields. As the frequency goes higher, the audio-range is reached, then radio frequencies, television and microwaves. Higher regions define the infrared, visible and ultraviolet light and lastly, the X-rays and gamma rays.
FigureI.3.7. Spectrum of the electromagnetic waves
In 1996 the World Health Organization (WHO) defined the EM fields (EMF) as those with frequencies in the range from 0 to 300 GHz. Generically, this domain is divided into the following regions:
(1) extremely low frequency (ELF): 0-300 Hz;
(2) intermediate frequency (IF): 300 Hz-10 MHz;
(3) radiofrequency (RF): 10 MHz-300 GHz.
EM energy in the natural fields is almost entirely in the ELF spectrum. Moreover, there is no significant level of natural energy at radio/microwave frequencies.
Intensity. Flux Density
An electric field may be described by its magnitude (E), and the electric flux density (D). The units are (V/m) and (coulomb/m2), respectively. The two quantities are related by the permittivity (), which defines the electrical properties of the medium:
D = E (2)
A magnetic field is described by its magnitude (H), and the magnetic flux density (B). The units are (A/m) or Øersted (Ø) and Tesla (T), respectively. In this case, the two quantities are related by the permeability () of the medium:
B = H (3)
The permeability of biological materials is taken to be equal to the permeability of free space.
This is the rate of EM energy flow. The unit is the Watt (W). Often, the flow of EM radiation is described energetically in terms of its power density, the power incident upon an area of one square meter. The unit is W/m2.
Example: the EM power density of the Sun upon the Earth equals a value of 1.4 kW/m2; the power density from a typical 800 kW TV broadcast transmitter at a distance of 55 km is equal to 20 mW/m2.