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Berlin Environmental Atlas

08.05 Electromagnetic fields

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Overview

Since its discovery, electricity has radically transformed people's lives and has become an indispensable part of our civilisation. Electricity can be converted into any other kind of energy, such as mechanical work, heat or light, which means that its applications are universal. The use of electric energy inevitably entails the occurrence of electric and magnetic fields. These are almost always oscillating fields, as most technical devices are powered by alternating current or generate it themselves. Because the polarity of flux in an alternating current changes, field direction changes constantly, too. The number of cycles per second is known as the frequency, which is measured in Hertz (Hz). Fig. 1 summarises the spectrum of electromagnetic fields.

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Fig. 1: Electromagnetic Spectrum: Applications and Manifestations of Electromagnetic Energy in Relation to Frequency f (or Wavelength lambda) (VEÖ 91)

Strictly speaking, the term "electromagnetic field" only applies to high frequencies, where electric and magnetic fields are inextricably linked and can propagate freely in space as electromagnetic waves. At low frequencies, on the other hand, there are two independent fields, magnetic and electric. Electric field strength is described as E and measured in units of V/m or kV/m. The electric field is represented visually as field lines standing at right angles to the surface of the conducting object. Every geometry creates its own characteristic electric field. By way of example, Fig. 2 shows the field lines around a double-wire cable.

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Fig. 2: Electric Field Lines around a Double-wire Cable

Magnetic field strength H is measured in amperes per metre (A/m), and magnetic flux density B in units of T (tesla). As magnetic flux densities are often very small, we usually refer below to a millionth of a tesla, or µT. Magnetic field lines run in circles around the conductor (cf. Fig. 3).

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Fig. 3: Circular Magnetic Field B around a Conductor Carrying Current I

Electric and magnetic fields always spread out in space from a source. The electric field is a source field which occurs between separate charges (battery, mains socket). The magnetic field is a vortical field which only occurs when charges move, i.e. when a current flows. Any charged conductor has an electric field, whereas the magnetic field is only created when a flux begins, e.g. a lamp is switched on.

Field strengths decrease very rapidly as their distance from source increases.

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