Box 16D Measurement and concepts of physical fields

The electromagnetic spectrum is presented in Table 16.2. Electromagnetic waves propagate at the speed of light with an approximate velocity, c, of 3 x 108 metres/second. The wavelengths of these waves, *, depend upon their frequency, f, according to the relation *= c/f. The wavelength therefore varies inversely with frequency, as shown in Table 16.2.

Physicists use the term 'field' to refer to a region in which objects separated in space exert a force on each other.

The strength of an electric field at any given point is determined by the voltage of the conductor and the distance of the point from the conductor. The electric field strength, E, is thus measured in volts per metre (V/m).

Magnetic fields are those in which magnetic objects (ones in which there are electric currents) exert force on other magnetic objects. The strength of a magnetic field at any given point is determined by the power of the source of electric current and the distance of the point from the source. The magnetic field, H, is thus measured in amperes per metre (A/m). Other special units of measurement are often used instead of A/m, for example: 10 milligauss (mG) = 1 microtesla (µT) = 0.80 (A/m); some of these are used in this chapter.

In high-frequency electromagnetic fields, such as radio-waves or visible light, electric and magnetic fields interact and their effects are not separable. At very low frequencies (less than 1000 Hz), however, electric and magnetic fields have different effects and can be characterised separately (for further information, see WHO, 1993b).

Static fields (not to be confused with static electricity) are created by objects that have a constant charge or current flow (DC or direct current). Objects whose charge or current periodically fluctuates create fields that also fluctuate, as in the case of radar (AC or alternating current).

Table 16.2 Electromagnetic spectrum and some typical examples of applications

Source: Gandhi, 1990 and others

Wavelength
(metres)		 Frequency range
(in cycles/sec and Hertz)		 Typical sources/applications/types
­ 0 DC power lines
106­107 10­100 Power line frequency 50/60 Hz
Telephone
105 1000 Telephone
104 104  
103 105 VLF communication systems
102­103 106 (1MHz) RF induction heaters
Medium-wave radio (535­1605 kHz)
1­102 107 (10MHz) Short-wave radio (3­30 MHz)
Radio frequency sealant, short-wave diathermy,
hyperthermia for cancer therapy
10­1­1 108 (100MHz) VHF television
FM radio (88­108 MHz)
UHF television
10­2 109 (1GHz) Radar for sea and air navigation
10­3 109 (1GHz) Microwaves
Microwave oven (2.45 GHz)
Satellite communications; electrical power lines
10­3 1010 (10 GHz) Microwave links for long-distance telephone and TV
communications
Cellular telephones
10­6­10­3 1012 to 1014 Infrared
10­6 1015 Visible light
10­8­10­7 1015 to 1016 Ultraviolet light
10­11­10­8 1016 X-rays
10­14­10­11 1017 Gamma (*) rays
shorter than 10­14 above 1017 Cosmic rays