5

Introduction

1.2.1 Electromagnetism

Te properties of electromagnetic waves necessary for understanding the various efects of electromag­

netic felds on biological systems will be briefy presented. Tey describe the fundamental aspects of

electric and magnetic felds as well as the electromagnetic wave. To begin, consider the case where a

direct current (DC) power supply is connected to a single wire stretched in the air. Te wire is being

charged by the electric charge from the power source, and electric lines of force are generated from the

wire to the ground. Although the electric lines of force are invisible to the eye, their existence can be

confrmed by the fact that when another charged particle is placed between the wire and the ground, it

can receive either an attractive or repulsive force in the direction of the electric lines of force. Te space

where these electric lines of force exist, that is the feld where the electric force act, is called electric feld.

Its strength is defned by the force that acts when a unit charge is placed there. Now, if an alternating

current (AC) power source is connected to the same cable instead of a DC power supply, the polarity

of the cable charge reverses direction with its frequency. Terefore, the direction of the electric lines of

force is also reversing with its frequency, but the pattern of distribution remains the same. In short, an

electric feld is a region of space over which an electric charge exerts a force on charged objects in its

vicinity. Te unit of the electric feld strength is Newton/Coulomb (N/C), and in practical use, the unit

is expressed in Volt/Meter (V/m).

When a resistor is connected to one end of a wire and an electric current fows through the wire,

the current creates magnetic lines of force around the wire. Magnetic lines of force are also invisible,

but when a magnet is placed near the wire, its existence is confrmed by the experience of attractive or

repulsive forces in the direction of the magnetic lines of force. Te space where these magnetic lines of

force exist or the feld where they act is called magnetic felds. Its strength is proportional to the density

of the magnetic lines of force. Even if an AC power supply is used instead of a DC power supply, the dis­

tribution pattern remains the same as in the case of a DC power supply, except that the direction of the

magnetic lines of force reverses corresponding to the direction of the current. Te units of magnetic feld

strength are Newton/Weber (N/Wb) and Ampere/Meter (A/m). Magnetic fux density, which is defned

as the amount of magnetic feld passing through a unit cross-section area, is used in place of magnetic

feld. Te unit for magnetic fux density is Wb/m or Tesla (T) which is equal to 10⁴ Gauss (G). If H and B

are made to be the magnetic feld and the magnetic fux density, respectively, it becomes

B = µH

where μ is the permeability in unit of Henry/Meter (H/m). In biological materials μ is equal to the free­

space’s value, i.e., μ0 = 4π × 10⁻⁷H/m.

If an AC power supply is connected to two short wires, electric lines of force corresponding to the

applied voltage are generated in the space between the two wires, and magnetic lines of force propor­

tional to the current fowing through the wires (displacement current) are generated around the wires.

It is clear that the electric and magnetic lines of force reverse their directions with the frequency of the

power source. It is easy to imagine that if the tips of the two wires are spread out, the distribution pattern

of electric and magnetic lines of force will spread outward. If the two wires are further spread out until

they are lined up on the same line, there is no longer any space between the wires to confne the electric

and magnetic lines of force, so further they will both spread outward. If the frequency of the power sup­

ply is chosen so that the length of the two wires is equal to a half wavelength, a resonant state occurs, and

the magnitude of the voltage and the current fowing through the wires are maximized. Tis also maxi­

mizes the density of the electric and magnetic lines of force in the surrounding space. At higher frequen­

cies, the electric and magnetic felds spread outward like waves. Using this property, high-frequency

energy can be sent to distant places by a transmitting antenna, and the electrical vibrations radiated

are called electromagnetic waves. In this sense, an electromagnetic wave consists of the oscillation of

electric and magnetic felds in wave motion. In an electromagnetic wave, an electric feld and a magnetic