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# Electromagnetic Fields Help

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By — McGraw-Hill Professional
Updated on Sep 18, 2011

## Electromagnetic Fields

The optical telescope was invented long before scientists knew that visible light represents only a tiny part of a continuum of energy wavelengths. Isaac Newton believed that visible light was composed of tiny particles or corpuscles . Today we recognize these particles as photons . However, light is more complex than can be represented by the simple corpuscular theory . The same is true of all forms of radiant energy.

The wave nature of visible light, and of other forms of radiant energy, is the result of synergistic interaction of electrical and magnetic forces. Charged particles, such as electrons and protons, are surrounded by electrical (E) fields . Magnetic poles produce magnetic (M) fields . The fields extend into the space surrounding the charged particles or magnetic poles, and when the fields are strong enough, their effects can be noticed at a considerable distance. When the E and M fields vary in intensity, the result is an electromagnetic (EM) field .

Orderly, well-defined EM fields are generated by voltages or currents that vary in a rhythmic way. Conversely, an EM field can give rise to alternating voltages or currents. These effects can occur over vast distances in space.

## Static E And M Fields

If you’ve ever played with permanent magnets, you have noticed the attraction between opposite poles and the repulsion between like poles. Similar effects take place with electrically charged objects. These forces seem to operate only over short distances under laboratory conditions. This is so because static (steady, unchanging) E and M fields weaken rapidly, as the distance between poles increases to less than the smallest intensity we can detect. In theory, the fields extend into space indefinitely.

Physicists have known for a long time that a constant electric current in a wire produces an M field around the wire. The lines of magnetic flux are perpendicular to the direction of the current. It is also known that the existence of a constant voltage difference between two nearby objects produces an E field; the lines of electrical flux are parallel to the direction in which the voltage varies most rapidly with distance. When the intensity of a current or voltage changes with time, things get more interesting.

## Fluctuating Fields

A fluctuating current in a wire or a variable voltage between two nearby objects gives rise to both an M field and an E field. These fields regenerate each other, so they can travel for long distances with less attenuation than either type of field all by itself. The E and M lines of flux in such a situation are perpendicular to each other everywhere in space. The direction of travel of the attendant EM field is perpendicular to both the E and M lines of flux, as shown in Fig. 18-1.

Figure 18-1. An EM wave is comprised of fluctuating, mutually perpendicular electric and magnetic lines of flux. The field travels perpendicular to both sets of flux lines.

In order for an EM field to exist, electrons or other charge carriers not only must be moving, but also must be accelerating. That is, their velocity must be constantly changing. The most common method of creating this sort of situation is the introduction of an alternating current (ac) in an electrical conductor. It also can result from the bending of charged-particle beams by E or M fields.

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