By combining two forces known since ancient times into one working package,
nineteenth-century science laid the basis for a twentieth-century
technological revolution
ELECTRICITY and magnetism are two facets of the forces of nature that
humans have effectively brought under limited control. Nuclear forces, by
comparison, are hardly the sort of thing you want to unleash in your living
room. And we are still far from being able to generate gravity by machines, so
there are few domestic uses for it. But electromagnetism is the servant that
gives us light to see by, radio and TV waves for communications, power for
microwave ovens and computers, and a host of other machines and domestic
appliances. Without electricity and magnetism, technological civilisation
could not exist.
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In fact, electricity and magnetism are two aspects of the same force. Put a
piece of metal near an electric current and you can make it into a magnet.
Move a wire in a magnetic field and you can generate electricity flowing in
the wire.
People have known about electricity and magnetism for a long time. The
ancient Greeks knew that a piece of amber rubbed with a woollen cloth attracts
bits of straw or paper. The ancient Chinese noticed that loose pieces of
magnetic minerals always line up in the same direction. But it was not until
relatively recently that we harnessed the power of electricity and magnetism 鈥�
or even began to understand it.
In the early 19th century, scientists realised that these two fundamental
forces are closely related. The Scottish physicist James Clerk Maxwell
succeeded in defining that relationship mathematically in 1873. He came up
with a theory of electromagnetic fields that brought together aspects of both
forces. Maxwell also showed that electromagnetism includes light, X-rays and
cosmic rays.
Electromagnetic waves carry information to Earth from distant galaxies. Radio
waves carry messages and pictures around the world. X-rays allow us to see
inside some things that are opaque to ordinary light. Electromagnetic
radiation forms a network that enables us to send and to receive messages
throughout our world and beyond.
But what are these fundamental forces?
Electricity and magnetism are so closely related that, as an old song has
it, 鈥測ou can鈥檛 have one without the other鈥�. Moving electrical charges produce
magnetic forces and moving magnets produce electric forces. A changing
electric field cannot exist without producing magnetism.
The close relationship between electricity and magnetism can be found
within atoms, the building blocks of matter.
In simple terms, atoms consist of a cloud of negatively charged electrons
swarming around a positively charged nucleus. The electrons gather around the
nucleus because they are attracted by its positive charge (they do not fall
into the nucleus because quantum effects hold them at bay; see Inside Science,
Number 25). The fact that unlike charges attract and like charges repel each
other is what lies at the heart of interactions between atoms.
The behaviour of the spinning electrons which move around the nuclei of its
atoms is what determines the magnetic properties of a material. Each spinning
charge also acts as a magnetic dipole 鈥� a sort of tiny bar magnet. In magnetic
materials, the combined effect of the electrons makes the atoms behave like
tiny bar magnets too. Each atom has its own north and south pole.
Most atoms do not behave in this way, however. Usually, electrons with
opposing spins pair up and cancel out each other鈥檚 magnetic effects. But
sometimes an atom has an electron in one of its inner orbits that is not
paired up with one that has an opposite spin, and this produces strong
magnetic properties (see Inside Science, Number 26).
Both magnetic and electrical forces affect the area around their source.
They produce fields of force. A field is any physical property that takes on
different values at different points in space. If, for example, we put an
electron in an electric field, it would 鈥渇eel鈥� a force and be pulled in a
particular direction. The strength of the pull would depend on the position of
the electron. Physicists represent such a field by lines of force, which join
positive and negative charges. Where the lines are closest together, the field
is strongest, just as contour lines that are close together on a map indicate
a steep hill.
Although you have to imagine what the lines of force in an electrical field
look like, you can illustrate the magnetic lines of force and the two magnetic
poles of a dipole quite easily. Place a magnet beneath a piece of card and
scatter a thin layer of iron filings on it. Now tap the card 鈥� to reduce the
effects of friction. Many of the filings will gather at the two ends or poles
of the magnet. Some of them will form curved lines between the poles. These
lines are a picture of the magnetic field of the magnet.
When charged particles move they create an electrical field, which always
has a magnetic field associated with it. A current passing through a conductor
(a material which allows electrical charges to pass easily) creates a magnetic
field around the conductor. These ideas seem obvious to us now, but it was not
until 1820 that Hans Christian Oersted, a Danish physicist, showed that
electricity could produce magnetism. This led many scientists to wonder
whether the reverse could be true: Could magnetism produce electric currents?
Eleven years later, the answer emerged.
A bright idea
Making electricity
IN 1831, Michael Faraday, a British scientist, and Joseph Henry, an
American scientist, discovered independently that they could produce an
electric current by placing a coil of wire in a moving magnetic field. Faraday
connected a coil of wire to a meter for measuring current 鈥� a galvanometer.
When he quickly moved one pole of a bar magnet towards the coil, the meter
showed a brief pulse of current. When he pulled the magnet away, the meter
again showed current for a short time, but in the opposite direction. When he
held the magnet still, there was no current. By moving the magnet rapidly to
and fro in this way, you can produce an alternating current 鈥� so called
because it moves first in one direction and then in the other. The speed at
which you move the magnet back and forth (or rotate it) determines the
frequency of the alternations in direction.
This discovery was a great breakthrough in understanding the relationship
between the two forces. It led to many practical applications, not least the
generation of electricity, such as that supplied through the mains in Britain.
Before Faraday鈥檚 discovery, scientists used batteries as their main source of
electricity. Batteries produce a direct current, which flows in only one
direction.
When scientists first studied electric currents, they agreed that they
would consider current to flow from the positive terminal to the negative
terminal of a battery. Later, when they discovered electrons, they realised
that electrons flow from the negative terminal to the positive terminal.
Because like charges repel, this is the same as saying that electrons carry a
negative charge. It would be logical to get rid of the confusion by swapping
the labels on the two terminals, and calling the charge on the electron
positive, but the convention is too familiar.
Faraday showed that we can really consider magnetic forces and electrical
forces as two different aspects of the same thing.
Electromagnetic waves
Making light work
IN the 1860s, James Clerk Maxwell took Faraday鈥檚 work a step further. He
developed a mathematical theory of electromagnetic waves.
What Maxwell did was to combine the laws of electricity and magnetism into
one set of mathematical equations. Although Faraday had shown that a changing
magnetic field causes an electric field, scientists then did not understand
what it was that related the electric and magnetic forces. Nor did they
understand exactly how the two forces influenced each other.
Maxwell found that he was forced to add an extra mathematical term to make
the laws of electricity and magnetism completely compatible. The improvement
meant that scientists soon discovered solutions to the equations that
described waves travelling forever through space. In effect, the varying
electric field produced a varying magnetic field, which in turn produced a
varying electric field, and so on as the wave went along. These solutions to
the equations contained a constant which could only be the speed at which the
waves move. That constant turned out to be the speed of light 鈥� the value that
the Danish astronomer Ole Romer had estimated as long ago as 1676. Maxwell
realised that his new equations were describing how light travels through
space. His new theory united light with electricity and magnetism.
Maxwell鈥檚 equations have remained a foundation of physics for more than 100
years, even though there has been a revolution in physics during this time.
Albert Einstein found that Newton鈥檚 laws of motion did not fit with Maxwell鈥檚
equations. He modified Newton鈥檚 laws but kept Maxwell鈥檚 equations intact when
he developed his special theory of relativity. The fundamental constant that
is so important in Einstein鈥檚 theory (the speed of light, c, aproximately
300脳106 metres per second) emerges naturally from Maxwell鈥檚
equations.
Electromagnetic waves can travel tremendous distances. They appear in many
different guises, as light, as radio and television broadcasts, as radar,
X-rays and cosmic rays.
Outer space
Tuning in to the Universe
SPACE is full of electromagnetic fields. The twinkling light we see from
stars on a clear night is just one example 鈥� a form of electromagnetic
radiation which has a frequency within our visible range.
Stars also give out electromagnetic radiation with shorter wavelengths, but
we cannot see it. At longer wavelengths, scientists on Earth can monitor radio
waves coming from space, even from remote galaxies. They can tune in to the
Universe in the form of the famous cosmic microwave background radiation left
over from the big bang itself (see Inside Science, Number 1). By monitoring
radio waves from outer space, astronomers can pinpoint objects immense
distances away. Other forms of electromagnetic radiation from space, such as
microwaves, X-rays and gamma-rays, also give us valuable clues about the
origin and nature of the Universe.
Back on Earth we can generate and use our own electromagnetic fields. In
1888, the German physicist Heinrich Hertz showed that it was possible to
transmit electromagnetic energy without using wires. A spark jumping across a
gap in one electric circuit produced electromagnetic waves which crossed to a
similar circuit. This induced an electric current that produced a
corresponding spark across the equivalent gap in the receiving circuit.
In the 1890s, this discovery inspired a young man of mixed Irish and
Italian blood, Guglielmo Marconi, to develop his system for 鈥渨ireless鈥�
transmission of radio frequency electromagnetic waves. Marconi鈥檚 work paved
the way for reliable communication throughout the world, and made possible the
age of television and radio broadcasting. He laid the foundation for radar,
radio navigation and eventually for satellite communications.
We can listen to radio broadcasts because an alternating voltage is induced
in the antenna of a radio receiver by the electromagnetic field of radio waves
transmitted from a broadcasting station. Waves representing the sounds to be
broadcast are superimposed on carrier waves. The receiver separates the two
sets waves, and the 鈥渟ound waves鈥� operate a loudspeaker that reproduces the
original sound vibrations.
Television companies use similar principles to broadcast pictures, but they
need waves of a higher frequency. The television camera scans the scene and
produces a series of electrical impulses. These impulses are amplified and
transmitted on a carrier wave. When they reach the receiver they activate a
picture tube in which a narrow beam of electrons moves across and down a
fluorescent screen. It is this beam of electrons that reproduces the image.
Radio or television signals reach the receivers by travelling through the
Earth鈥檚 atmosphere. The outer atmosphere, or ionosphere, is exposed to
electromagnetic radiation from the Sun. As a result, several layers of the
atmosphere become ionised. The various layers have particular levels
of ionisation, and radio waves of different wavelengths respond differently to
them. The ionised layers will reflect certain wavelengths of radio waves. The
amount of ionisation is greatly influenced by events on the surface of the
Sun, particularly the sunspot cycle, which affect the Earth鈥檚 magnetic field
(see Inside Science, Number 29). The position of the ionised layers also
changes at night when that part of the ionosphere is facing away from the Sun.
In general, the higher the frequency of the waves, the more easily they can
pass through the charged layers. Very high and ultra high frequency (VHF and
UHF) waves are able to pass through all the ionised layers and continue out
into space. These are the frequencies that scientists use to send messages to
probes or missions in outer space. Before they can be heard on Earth, these
waves must be reflected back. In fact, signals that are said to be 鈥渂ounced鈥�
off communication satellites are not. Instead, receivers on the satellites
pick up the signals and rebroadcast them back to Earth.
The worldwide system of short-wave radio communication takes advantage of
the ionised layers. Short waves can penetrate the weakly charged layers
nearest the surface of the Earth but they cannot penetrate the more densely
ionised layers higher up. Instead, they are bounced off these charged layers
and back to Earth. They can skip around the world, bouncing between the upper
layers of the ionosphere and the surface of the Earth.
Lower-frequency medium waves find it difficult to penetrate even the
lowest, least-charged layers. However, this weakly ionised layer disappears
when the Sun goes down. At night, medium waves sometimes travel up to the
higher layers and are reflected back to a point on the Earth which is far from
their source. This is why you can sometimes pick up foreign broadcasting
stations on your radio at night.
Units of electricity and magnetism
THE ELECTRICITY that runs our homes comes from electric currents. These
occur when electrons 鈥� which have become detached from atoms 鈥� hop from one
atom to the next. The strength of electrical currents is measured in amperes,
often shortened to amps. Amperes represent the amount of charge passing per
unit time and 1 amp is equal to the flow of 1 coulomb of charge (about
6.3脳1018 electrons) per second.
Current only flows through a conductor if there is a pressure to push the
electrons along it. One way to think of this pressure is to imagine what
happens to a tank of water placed above a sink. If a pipe connects the tank to
the sink, the pull of gravity will force water in the tank to move down the
pipe. If the pipe is wide, there is little resistance, but if the pipe is
narrow, the flow of water will be reduced.
In electrical terms, a conductor takes the place of the pipe and the
鈥減ressure鈥� which pushes the current down the conductor is the voltage or
potential difference (pd) between its ends. Voltage is a measure of the loss
of electrical potential energy when one coulomb of charge flows down the
conductor. One volt is one joule per coulomb.
Electrical resistance to current flow is measured in ohms. A resistance of
1 ohm will allow a current of 1 amp to flow for each volt of pd across it.
The strength of a magnetic field is measured by its flux density. This
describes the density of lines of force passing through a loop in the field.
The SI unit of magnetic flux density is the tesla. One tesla is about a
hundred thousand times stronger than the magnetic field at the surface of the
Earth. The magnetic field near the poles of a toy horseshoe magnet is about
0.5 tesla.
Waves
THE essential difference between the many forms of electromagnetic waves is
the frequency at which they oscillate. Frequency, usually expressed in hertz
(Hz), is the number of cycles of oscillation per second that a wave undergoes.
This represents the number of times the wave oscillates each second.
Wavelength is inversely proportional to frequency, and you will often see
electromagnetic waves described in terms of their wavelength. To convert
between wavelength and frequency you can use the formula f=c/位
where f is the frequency, 位 is the wavelength and c is the speed of
light, approximately 300脳106 metres per second in air or
space.
The faster the oscillation, the higher the frequency and the shorter the
wavelength.
Using electromagnetism
WE use a wide spectrum of electromagnetic waves in our daily lives. For
example, electricity generating companies use low-frequency electromagnetic
fields to generate domestic electricty. Anyone who uses a microwave oven is
familiar with the power of high-frequency electromagnetic waves.
Faraday鈥檚 discovery of electromagnetic induction, the process by which
magnets and coils produce currents, is used in electricity generators or
dynamos. A modern generator consists of a rotating magnet surrounding a coil
connected to an outside circuit. By rotating a magnetic field around the coil
of wire it is possible to produce continuous, but alternating, currents. The
speed at which the magnet is rotated determines the frequency of the
alternation. Older generators had a rotating coil within a fixed magnet: this
design meant that collecting the current from the coil was not very efficient.
In Britain, the frequency of mains electricity is 50 Hz, or 50 cycles per
second.
Alternating currents are convenient becuase it is easy to change the
voltage and current values by means of a transformer. Transformers consist of
two coils of wire wound on an iron core. Any change in the current in the
first coil induces a voltage across the second. If the second coil has a
greater number of turns, the voltage across the second coil will be 鈥渟tepped
up鈥�, or made higher than the voltage across the first coil. The current is
reduced in proportion, and the power (given by voltage脳current) remains
constant. If there are more turns in the first coil, the voltage is 鈥渟tepped
down鈥�. Step up and step down transformers are very important in large systems
for distributing electricity. Power plants step up the electricity they
generate to very high voltages. This reduces the current, which in turn
reduces the amount of power that the electricity loses as it is being
transmitted. When it arrives, the electricity is stepped down to the lower
voltages we use.
Electric motors are simply dynamos reversed in their functions: the same
principles govern their design as that of generators. They are central to a
wide range of traction and domestic devices. In 1821, Faraday made the first
electric system to produce continuous rotation. Engineers, though, did not
develop practical electric motors until the 1870s and 80s, when they had
advanced greatly their knowledge of generators.
At its simplest, the motor consists of an armature, a coil of wire of many
turns wrapped around an iron cylinder, freely suspended in a magnetic field.
The armature turns when a current is passed through its coil.
Virtually all domestic devices with an electric motor, including tape decks
and washing machines, use alternating current and convert, or 鈥渞ectiify鈥�, it
to direct current. Most large motors, such as railways and metros use, depend
on direct current, but engineers are now beginning to adapt them to
alternating current.
On the higher frequency side, microwaves, which have frequencies in the
range 300 to 30 000 megahertz or MHz (1 MHz=1脳106
hertz) and wavelengths between about 1 metre and 1 centimetre, can be used to
heat food. They do this by causing molecules within the food to vibrate.
Microwaves encourage water molecules in food to align themselves in the
direction of the electric field. In a microwave oven, the electric field
changes direction 2450脳106 times per second. Each time the
field changes direction, the water molecules are flipped over. This makes them
jostle one another and so the temperature of the food rises. The process cooks
the food from the inside.
Further reading
Understanding Radio Waves, by Peter Bubb (Lutterworth Press, 1984),
introduces the technical world of broadcasting and provides a clear
introduction to the basics of electromagnetic waves. A more detailed and
lively treatment is The Feynman Lectures on Physics, volume 2 (R.P. Feynman,
R.B. Leighton and M. Sands, Addison-Wesley, 1963).
In search of Schr枚dinger鈥檚 Cat, by John Gribbin (Corgi, 1984) places
the wave theory in the context of modern quantum physics.