The Electro-Magnetic Link

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The Electro
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NEW SCIENTIST

June 2, 1990, Inside Science No. 34, pp. 1
-
4


Re
printed with permission. This article first appeared in New

Scientist, the weekly review of sciences and technology,

available from Publications Expediting, Inc., 200 Meacham Avenue,

Elmont, NY 11003.


THE ELECTRO
-
MAGNETIC LINK

by Nina Morgan


By com
bining two forces known since ancient times into one working package, nineteenth
-
century
science laid the basis for a twentieth
-
ce
ntury technological revolution


Electricity and
magnetism

are two facets of the forces of nature that humans have effectiv
ely 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 electro
magnetism 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 coul
d not exist.


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 el
ectricity 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
--
o
r 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 i
ncludes
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. Elect
romagnetic radiation forms a network that enables us to send and to receive messages
th
roughout our world and beyond.



But what

are these fundamental forces?

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Electricity and
magnetism

are so closely related that, as an old song has it, "you can't

have one
without the other". 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

ca
n 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 p
ositive
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
--
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's magnetic e
ffects. 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 I
nside 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 "feel" 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 a
s contour lines that are close together o
n 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
--
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 whic
h 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 ele
ctricity


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 meas
uring current
--
a galvanometer.


When he quickly move
d one pole of a bar magnet towards the coil, the meter showed a brief pulse of
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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
--
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 main
s in Britain. Before Faraday's 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 woul
d 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 t
he 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 t
he convention is too familiar.


Faraday showed
that we can really consider magnetic forces and electrical forces as two different
aspects of the same thing.



Electromag
netic waves: making light work


In the 1860s, James Clerk Maxwell took Faraday's work a step further. He developed a mathemati
cal
th
eory 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
--
the value that the
Danish a
stronomer 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's 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's laws of
motion did not fit with Maxwell's equations. He modified Newton's laws but kept Maxwell's equations
intact

when he developed his special theory of relativity. The fundamental constant that is so important
in Einstein's theory (the speed of light, c, approximately 300 x 10 E6 metres per second) emerges
natur
ally from Maxwell's equations.


Electromagnetic wa
ves can travel tremendous distances. They appear in many different guises, as
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light, as radio and television broadcasts, as radar, X
-
rays and cosmic rays.



Outer sp
ace: Tuning in to the Universe


Space is full of electromagnetic fields. The twinkl
ing light we see from stars on a clear night is just one
example
--
a form of electromagnetic radiation which has a frequ
ency 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 Sc
ience, 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 ori
g
in 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 acro
ss 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 i
nspired a young man of mixed Irish and Italian blood, Guglielmo Marconi,
to develop his system for "wireless" transmission of radio frequency electromagnetic waves. Marconi's
work paved the way for reliable communication throughout the world, and made poss
ible 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
re
ceiver 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 "sound waves" operate a loudspea
ker that reproduces
the original sound
vibrations.


Television companies use similar principles to broadcast pictures, but they need waves of higher
frequency. The television camera scans the scene and produces a series of electrical impulses. These
im
pulses 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 electr
ons that reproduces the image.


R
adio or television signals reach the receivers by travelling through the Earth's 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 variou
s 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's magnetic field (see Inside Science,
Number 29). The position of the ionised layers also changes at night when that part of the ionospher
e is
facing away from the Sun.


In general, th
e higher the frequency of the waves, the more easily they can pass through the charged
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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 tha
t 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 "bounced" off communication satellites are not. Instead,
receivers on the sa
tellites pick up the signals and r
ebroadcast 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 the
y 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. T
hese occur when electrons
--
which
have become detached from atoms
--
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 x

10 E18 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 "pressure" 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 i
s about 0.5 tesla.



Waves


The essential difference between the many forms of electromagnetic waves is the frequency at which
they oscillate. Frequenc
y, usually expressed in hertz (Hz), is the number of cycles of oscillation per
second that a wave undergoes. This represent the number of times the wave oscillates each second.
Wavelength is inversely proportional to frequency, and you will often see elect
romagnetic waves
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described in terms of their wavelength. To convert between wavelength and frequency you can use the
formula f = c/(lambda) where f is the frequency, (lambda) is the wavelength and c is the speed of light,
approximately 300 x 10 E6 metres p
er 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 electricity.
Anyone who uses a microwave oven is familiar with the power of high
-
fr
equency electromagnetic
waves.


Faraday's 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 possibl
e 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 coi
l was not very efficient. In Britain, the
frequency of mains electricity is 5
0 Hz, or 50 cycles per second.


Alternating currents are convenient because 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 "stepped up", or made higher than the

voltage across
the first coil. The current is reduced in proportion, and the power (given by voltage x current) remains
constant. If there are more turns in the first coil, the voltage is "stepped down". Step up and step down
transformers are very importa
nt 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 a
rrives, 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 domes
tic 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 sim
plest, 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 curre
nt 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 "rectify", it to direct current. Most large motors, such as railways and
metros use, depend on direct current, but engineers are now beginning to adap
t them to alter
nating
current.


On the higher frequency side, microwaves, which have frequencies in the range 300 to 30,000
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megahertz or MHz (1 MHz = 1 x 10 E6 hertz) and wavelengths between about 1 metre and 1 centimetre,
can be used to heat food. They do this by ca
using molecul
es 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 x 10 E6 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.



Nina Morgan is a science writer specialising in physics and Earth

sciences.



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 t
reatment is THE FEYNMAN LECTURES ON PHYSICS, volume 2 (R.P. Feynman, R.B.
Leighton and M. Sands, Addison
-
Wesley, 1963).



IN SEARCH OF SCHRODINGER'S CAT, by John Gribbin (Corgi, 1984) places the wave theory in the
context of modern quantum physics.