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How it works : Electromagnetism |
ELECTROMAGNETISM
| Below: Joseph Henry (1797-1878) designed this lifting
mechanism for heavy loads. The lifting force is provided by an electromagnet. Henry did much work on electromagnetism. |
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If a permanent magnet attracts a piece of iron or steel, that is a purely magnetic action. If a battery sends electric current through a wire so as to heat it, that is an electric effect. But wherever an action takes place involving both magnetism and electricity, such action is said to be electromagnetic. There are therefore many manifestations of this phenomenon which was first discovered by the Danish Scientist OERSTED and greatly enlarged by the subsequent work of FARADAY in the first part of the nineteenth century.
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| Top: when a magnet is moved through a coil of wire, a
voltage is induced which can be measured on a voltmeter. Above: a transformer consists of two coils wrapped around a common 'magnetic circuit'-a ring of iron. When a changing voltage is applied to the terminals of the primary coil a current is set up which creates a magnetic field. This induces a voltage in the secondary coil. Below: an attempt to construct a physical picture of an electromagnetic wave. The electric part E oscillates in one plane and the magnetic field H at right angles to this. It travels at the speed of light. |
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One common manifestation of electromagnetism is that a current flowing in a wire produces a magnetic field-this is the operating principle of an electromagnet, and can be harnessed to produce motion in ELECTRIC MOTORS through the attractive and repulsive forces of magnetic fields. When a magnet (either a permanent magnet or electromagnet) is moved near an electrical conductor, turbulent EDDY CURRENTS are induced in the conductor and it experiences a 'dragging' force. This dragging force can be used to produce motion, and conversely, the eddy currents can be harnessed to produce a useful electric current (such as in ALTERNATORS and DYNAMOS). This is an example of a moving magnetic field producing an electric current.
A more complex example of electromagnetism is found in devices such as TRANSFORMERS where a changing magnetic field produces a current. Here, two coils of wire are placed close together. When a changing current (changing in amplitude and-or direction) flows through one coil a changing magnetic field is produced, which induces a voltage in the second coil. If this second coil is included in any kind of electric CIRCUIT a current flows.
Understanding by analogy These phenomena are not fully understood by man. But in order to exploit them, we devise mental models called 'analogues' to help us to obtain at least an appreciation and a hope that through this means we may learn to design better machines by using a phenomenon which is no more understood than is GRAVITATION.
For electric circuits we imagine that electrons flow in wires in much the same way that water flows in a pipe. We know that pressure is needed to make water flow so we invent an electrical pressure and call it electromotive force (emf) or voltage. The convenience of this analogue is that it allows us to use the equivalent of the frictional resistance in the water pipe which increases in proportion to the length of the pipe but decreases in proportion to its cross sectional area. Then, by another analogy, we can invent a magnetic circuit, in which the driving pressure is called magnetomotive force (mmf) and the 'substance' which it drives around the circuit is even less 'real' than the flow of electrons in an electric current. We call it magnetic flux. Many authors and teachers declare that, despite its name, flux does not flow. The fact is that it does not exist, except as a human concept, and the only 'right or wrong' about its flow is to be judged on whether the concept is useful to a particular individual. For some, it is more profitable to think of flux as merely being 'set up' because it represents only stored energy, and not a continuous loss of power as is the case when electric current flows in a wire. For others, the analogue is more profitable if flux is considered to be a more precise analogue of electric current so that a magnetic circuit can be given the properties appropriate to INDUCTANCE and CAPACITANCE in an electric circuit.
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| Above: the interlinking of electric and magnetic circuits is fundamental to our way of thinking about electromagnetism. This diagram illustrates electric and magnetic circuits in a transformer. | ||
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Linking electric and magnetic circuits
When discussing electric motors, generators and transformers, it is essential
to note that each machine includes at least one electric and one magnetic
circuit. Since there is no simple equivalent in magnetic circuits to the
insulating materials of electric circuits, it is usual to design a machine
with only one magnetic circuit but two or more electric circuits. Moreover,
for the same reason, electric circuits in machines are usually multi-turn
coils of relatively thin, insulated wire. Magnetic circuits tend to be
single-turn, short and fat.
The subject of electromagnetism can therefore be expressed as the linking of electric and magnetic circuits. In such a linking the driving pressure from one circuit is seen to be derived from the flow in the other, and vice-versa. For example, in a transformer an alternating voltage (emf) across the primary windings produces an alternating current in the windings. This produces an alternating mmf in the magnetic circuit, which creates an alternating flux. The alternating flux induces a voltage in the secondary windings, which, if connected in an electrical circuit, produces current.
Vector quantities
The commodity we seek to produce in an electric motor is force which arises
as the result of multiplication of flux by current, but it is no ordinary
multiplication, for the Only quantities of flux and current which are effective
are those which cross each other at right angles. Quantities which have both
magnitude and direction are called vectors, and when determining the
interactions of vectors with each other the direction as well as the magnitude
must be taken into account. In the above example, the force vector is the
result of the vector multiplication of the flux and current vectors.
Where the flux and current vectors are not at right angles to each other
they must be resolved into parallel and right angular components, but it
is always the right angular components which produce the force vector.
Furthermore, the force vector is always at right angles to both the
flux and current vectors.
Vector multiplication and, more generally, vector mathematics is only a form
of 'shorthand' for handling quantities which have been shown experimentally
to interact in this unusual way. This is another example of an analogue.
Electromagnetic radiation
The principles of electromagnetism are not limited to electric motor
and generator design. ELECTROMAGNETIC RADIATION is the name given
to a variety of phenomena to which we give different names depend ing on
the context in which we study them. Thus gamma-rays, X-rays, ultra-violet
radiation, visible light, infra-red (beat radiation) and wireless (radio)
waves are all of the same nature and can all be expressed in terms of a
continuous interchange of magnetic and electric energy, each of which pulsates
in a plane at right angles to the direction of travel of the radiant waves.
All travel at the same speed, approximately 3
´ 108 (300,000,000) metre/sec (about 186,000
miles). The only thing which distinguishes one kind of radiation from another
is its wavelength (or frequency). The whole spectrum of radiation extends
from very low frequencies with wavelengths of many miles, to incredibly high
frequencies (of the order of over 1022Hz (1 Hz = 1 cycle/second)
and wavelengths less than a millionth of a millionth of an inch.
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Left: Faraday's disc dynamo. The disc is made of copper and
positioned within the arms of a horseshoe electromagnet. Contacts are made
with the disc at its centre and periphery be means of 'brushes' and connected
externally to an electrical circuit. When the disc is rotated, a voltage
is induced in the disc between the brushes. |
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The study of electromagnetism is therefore basic to the whole of physics, if not to the whole of science. The Earth receives most of its energy from the sun by electromagnetic radiation. The average private family house in Britain contains between 30 and 100 electromagnetic devices (although the higher numbers generally occur where there are several children, each of whom has battery-powered toys). Electromagnetism is basic to the operation of radio and television sets, car ignition systems, radar, electric systems, microscopes, electric motors and generators, telephones and many other inventions.
ELECTROMETER (see electrostatics)
ELECTROMOTIVE FORCE (EMF) see circuit, electrical
Reproduced from HOW IT WORKS p870
