What a Wonderful World (12 page)
Read What a Wonderful World Online
Authors: Marcus Chown
Faraday noticed that, when he moved a magnet in the vicinity of a coil of conducting wire, an electric current flowed fleetingly
in the coil.
15
A current is a flow of charges, and charges are propelled by an electric field. What Faraday had noticed was that
a changing magnetic field creates an electric field
.
There is a pleasing symmetry between electric and magnetic fields. Not only does a changing electric field create a magnetic field but a changing magnetic field creates an electric field. Spin a coil of wire in the magnetic field of a permanent magnet. Arrange the coil and magnet in the right way – and this also takes some ingenuity – and an electric field will be created in the coil, which will drive a current.
Voilà
. You have created an electric generator.
At a power station, a coil of wire is spun in the force field of a magnet. The thing doing the spinning could be wind or water, or steam from water heated by coal or oil or nuclear power.
16
The key thing is that the spinning coil cuts through the magnetic field. In other words, the magnetic field through the coil
changes
. And the changing magnetic field creates an electric field, which pushes electrons around the coil. It creates a current. The same current that surges out of the power station down a cable and powers your home.
In practice, the current goes in and out of each home
in the same cable
. The current arrives from the power station in the live wire and returns to the power station in the neutral wire, thus completing the circuit.
17
There is a twist, however. Although the statement above is basically true, power stations do not generate current that flows in one direction only. Instead of direct current, or DC, they generate alternating current, or AC, which sloshes back and forth, rapidly changing its direction. The reason for this is to overcome a major problem with the long-distance transmission of electricity.
Think of an electric current flowing from a power station like a stream flowing down a hillside to a valley bottom. If you were to intercept the stream near the top of the hill, you would be able to exploit the long drop of the water to the valley bottom to power a piece of machinery such as a water wheel. However, if you were to intercept the stream close to the valley bottom, there would be very little drop left that could be exploited. And this is also the problem with an electric current flowing from a power station. The further away a home, the less energy can be extracted from the flow of electrons. While those close to the power station might be able to light their homes with a multitude of bright light bulbs, those far away will have to make do with the faintest of glimmers from a single light bulb.
The amount of energy a current can deliver is characterised by its voltage, which is analogous to the height of that stream above the valley bottom. In the UK, domestic appliances work on 240 volts (110 volts in the US). This would seem to imply that a power station would have to generate electricity at near 240 volts. But, of course, if it did, those living far away might have to make do with 100 volts, or 10, or even a measly 1 volt. In New York in the 1880s, the only way Edison could overcome this Achilles heel of electric power transmission was to build a power station about every 2.5 kilometres. Although this is just about doable, it is clearly an unworkable solution for distributing electrical power nationally.
The solution to the transmission problem, pioneered by Nikola Tesla and others, is to generate the electrical power not at 240 volts but at a voltage about
a thousand
times greater. In the UK’s
National Grid, electricity is transmitted over long distances at 110,000 volts or higher.
18
This means that, if the electrons lose energy travelling down the wire from a power station – and they inevitably do, banging into atoms and losing energy as heat – the voltage drop is hardly noticeable compared with 110,000 volts. Consequently, it is possible to transmit electricity over huge distances, and power stations do not have to be near homes. The problem is that 110,000 volts is too enormous for household appliances. Somewhere between a power station and people ’s homes, the voltage must reduced, or stepped down. This is not possible with a direct current. But it is possible with an
alternating current
.
An alternating current switches direction rapidly, commonly between 50 and 60 times a second. Think of the electrons in a wire as sloshing back and forth like waves on a sea shore. And, since an alternating current is ultimately made of countless
moving
electrons exactly like a direct current, it can carry energy as efficiently as its uni-directional cousin.
19
Furthermore, it is possible to design both a generator to
create
alternating current and a motor to
run on
alternating current. There remains only the problem of stepping down an AC current from 110,000 volts to the 240 volts required domestically. But this can be done with a transformer.
In a transformer, a current changing in one coil of wire – that is, an alternating current – causes a changing magnetic field in a second coil. This, in turn, creates a changing electric field, which drives a changing current in the second coil. If the second coil has fewer
turns
than the first, then the voltage
goes down
.
So there you have it: our electrical power system in a nutshell.
But electricity and magnetism have some more tricks up their sleeve. Recall that, if an electric charge is moving relative to you, you see not only an electric field
but a magnetic field
. However, if you travel alongside the charge, so that it is not moving relative to you, you see no magnetic field. And this is not all. If a magnet is moving relative to you, you see not only a magnetic field
but an electric field
. But, if you travel alongside the magnet, you see only a magnetic field.
How is it possible that, from one perspective, there is a magnetic field and, from another, no magnetic field? How is it possible that, from one perspective, there is an electric field and, from another, no electric field? There is only one way it can be possible: if magnetic fields and electric fields are
not fundamental things at all
.
As Einstein realised in 1905, an electric field and a magnetic field, just like space and time, are simply different facets of the same thing – an electromagnetic field. How much of each facet you see depends on your speed relative to the source of the electromagnetic field. This is why what one person sees as an electric field someone else sees as an electric field
and a magnetic field
. This is why what one person sees as a magnetic field someone else sees as a magnetic field
and an electric field
. No wonder there is a pleasing symmetry between the behaviour of electric and magnetic fields. How can there not be? They are essentially the
same
thing.
But there is yet more. In 1863, the Scottish physicist James Clerk Maxwell, in a scientific tour de force, distilled all known electrical and magnetic phenomena into a single neat set of
equations.
20
Studying those equations, he noticed something remarkable. It appeared possible for a ripple to propagate through the electric and magnetic fields just like a wave on a lake. Not only was the ripple self-sustaining but it travelled at a very particular speed:
the speed of light
.
Maxwell had discovered a surprising connection between electricity and magnetism
and light
. Light, it turns out, is a wave of electromagnetism – an electromagnetic wave.
21
And there is more. Maxwell’s equations reveal that it is possible to have electro magnetic waves that oscillate both more rapidly and more sluggishly than visible light. In 1888, their existence was proved by the German physicist Heinrich Hertz. With the aid of a spark, he transmitted an electromagnetic wave. The invisible-to-the-naked-eye radio wave crossed his laboratory and induced a measurable current in a coil of wire. It was an epochal, worldchanging moment. All radio and television communications around the world began with that one triumphant demonstration. Our connected modern world was born that day. ‘From a long view of the history of mankind, seen from, say ten thousand years from now, there can be little doubt that the most significant event of the nineteenth century will be judged as Maxwell’s discovery of the laws of electrodynamics,’ said American Nobel Prizewinner Richard Feynman.
22
Electricity opened up technological possibilities that were unimaginable to earlier generations. Not only was it possible to transmit a signal around the globe so that one person could talk to another without any material connection existing between
them but it was possible to run huge extended power systems. In the evocative words of Feynman, ‘Ten thousand engines in ten thousand places running the machines of industries and homes – all turning because of the knowledge of electromagnetism.’
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But, hand in hand with the harnessing of electricity came a dawning realisation that electricity is of central importance in nature. We live in an electrical world. Nobody had realised this before because, in pretty much all everyday circumstances, the enormous electric forces are perfectly balanced and so nullified. This is not the case, however, in the realm of the atom, the building block of all matter. There charge imbalances are ubiquitous.
As the joke goes …
Two atoms are walking down the street when they collide. One says to the other, ‘Are you all right?’
‘No, I lost an electron.’
‘Are you sure?’
‘Yeah, I’m positive.’
In a piece of matter with only a few atoms, there will usually not be an equal number of positive and negative charges. And, even if there are, there might still be large electrical forces. This is because the negative charge of one piece of matter might be closer to the positive charge of another piece of matter than its negative charge. Since the electrical force weakens with distance, attraction will win out over repulsion. Thus it is possible for two small pieces of matter to attract each other fiercely even if neither has a net charge.
Atoms, it turns out, are totally dominated by the immensely strong electric force. The glue that holds them together, and sticks them to other atoms to make molecules, is the electrical force. All chemistry, which involves the rearrangement of electrons
in atoms, is electrical. The attraction of the electrical force not only holds the atoms in the molecules of your body together but the repulsion between the electrons on the outside of those molecules keeps you rigid, preventing the Earth’s gravity from crushing you flat. And your cells have learned how to tap the energy of the electric force. Electrons from food create electric fields across cell walls that drive the creation of power-pack molecules such as adenosine triphosphate, or ATP. And electrons help to store and carry our very thoughts.
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Biology runs on electricity. We are electrical beings. We are as animated by the electrical force as much as a battery-powered toy is animated by the electrical force. Which explains why electricity is not only miraculous but
dangerous
. ‘My nephew tried to stick a penny into a plug,’ said American comedian Tim Allen. ‘Whoever said a penny doesn’t go far didn’t see him shoot across that floor.’
1
The force, like gravity, weakens according to an inverse-square law – that is, if two bodies are moved twice as far apart, the force is four times weaker; three times farther apart, nine times weaker; and so on.
2
To be precise, the electric force between an electron orbiting a proton in an atom of hydrogen, the lightest element, is about 10
40
– that is,
10,000 billion billion billion billion
– times stronger than the
gravitational force between them. Crudely speaking, this means at least 10,000 billion billion billion billion atoms must clump together before gravity – which is cumulative – can overcome the electrical repulsion between those atoms. This corresponds to a body about 600 kilometres across if made of rock and about 400 kilometres if made of ice, which is less stiff than rock and more easily crushed by gravity. Above this threshold, gravity can overwhelm the
electrical
force, pulling every component of a body as close to the centre as possible and creating a sphere. This explains why all bodies in the Solar System smaller than the threshold are potato-shaped while all bodies bigger are spheres like the Earth.
3
Why, if the protons in an atomic nucleus and the electrons in orbit around the nucleus attract each other with such a tremendous force, do atoms not simply shrink to zero size? The answer is that the fundamental building blocks of matter have a peculiar wave-like nature, and waves are fundamentally
spread-out
things. It is because the electron wave needs a lot of elbow room that the electron violently resists being squeezed too close to a nucleus. Without this quantum effect, atoms would not exist (see Chapter 15, ‘Magic
without
magic: Quantum theory’).
4
As much energy would be needed to remove all the electrons from a mosquito as would be liberated by its explosion. This is because the law of conservation of energy dictates that energy cannot be created or destroyed, merely changed from one form into another.
5
Benjamin Franklin advocated a single fluid model of electricity in which objects with a deficiency of the fluid had a negative charge and objects with a surplus a positive charge. He was right about the single fluid but wrong on the rest. Most commonly, the fluid –
electricity
– is made of negatively charged electrons – and it is a
surplus
of them not a deficiency that makes a body negative. By the time this was realised, however, an electric current had already been
defined
as a
flow of positive charge
and the idea was so well established that no one wanted to change it. Because of a historical accident, therefore, an electric current flows in the opposite direction to the actual flow of electrons. Incidentally, Franklin coined the terms
‘negative’ and ‘positive’. He was also responsible for other electrical terms such as ‘battery’ and ‘conductor’.
6
Fast photography actually reveals not a single discharge in each lightning bolt but many, each lasting only a millisecond or so. The first, known as the leader, is actually from the ground upwards to the cloud. It is followed by alternating discharges from the cloud to the ground. To the human eye all the discharges blend into a single strike, though it may appear to flicker.
7
Typically, a single stroke of lightning contains enough energy to light 250 homes for an hour. Most of the 100 lightning bolts that occur each second across the world are in the tropics. This is because the key to creating charge imbalances is updrafts of air. The
equatorial
region has the most rising hot air since it intercepts the lion’s share of solar heat.
8
Edison famously tested hundreds of possible filament materials to find one that glowed brightly without disintegrating. ‘I have not failed,’ he said, describing his method. ‘I’ve just found 10,000 ways that won’t work.’
9
The mechanism by which electric charge becomes separated in a thunderstorm is very similar to rubbing a balloon against a nylon sweater. Moist air, caught in a violent updraught, rises and cools to form ice crystals. In the turbulent air, the crystals run against each other, friction transferring electrons between them and building up a large charge imbalance between one cloud and another or between a cloud and the ground.
10
At first sight it appears impossible that a scrap of paper that is an equal mix of negative and positive charges can be attracted by a charged balloon. It happens in the following way. Say, for instance, the balloon is negatively charged. Its negative charge repels the negative electrons in the surface of the paper, causing them to move deeper into the paper. This leaves the surface of the paper with a net positive charge. It is this positive charge that is attracted by the negative charge of the balloon. A charged body, like the balloon, is said to separate, or polarise, the charge of an uncharged body, like a scrap of paper.
11
In the modern, or quantum, picture, force fields are the result of the exchange of force-carrying particles. The electric force is a
consequence
of the exchange between charged particles of virtual photons, a type closely related to the photons of light.
12
‘I encountered a wonder … as a child of four or five years when my father showed me a compass,’ said Einstein. ‘That this needle behaved in such a determined way did not fit into the way of
incidents
at all … There must have been something behind things that was deeply hidden.’
13
Just as electric charge comes in two types – positive and negative – magnetic charge comes in two types – north poles and south poles. And, just as like charges repel and unlike charges attract, like poles repel and unlike poles attract. But here the similarity ends. Although it is perfectly possible to have an isolated negative charge or an isolated positive charge, nobody has ever observed either an isolated north magnetic pole or an isolated south magnetic pole. A north pole
always
comes with a south pole.
14
From Walter Elsasser,
Memoirs of a Physicist in the Atomic Age.
15
The wire must be a conductor. This is a material such as copper or silver, whose atoms possess loosely bound electrons that can be detached and driven through the material by an electric field.
16
Nuclear power, as Terry Jones of
Monty Python
remarked, is ‘a very silly way to boil water’ (
New Scientist
, 18 June 1987, p. 63).
17
Think of the electric field generated by the power station as
extending
all the way along the wire as it loops from the power station around a city and back to the power station. The field stays
concentrated
in the wire and does not leak out because the wire is sheathed in an insulator such as PVC. This is a material that has no easily detachable electrons that can be pushed by the electric field. It
therefore
stops the electric field leaking out into the air and dissipating its energy.
18
For comparison, the voltage difference between a typical lightning bolt and the ground is
several hundred million volts.
19
In fact, lightning shows the heating effect of an alternating current since a single strike consists of multiple current flows, back and forth
between the ground and a cloud, or between a cloud and a cloud.
20
Einstein, with his recognition that electric and magnetic fields are aspects of the same thing, was able to distil all electric and magnetic phenomena into a set of equations that was
even more compact
than Maxwell’s.
21
Actually, a connection between electricity and magnetism and light had been suspected earlier by Michael Faraday. ‘I happen to have discovered a direct relation between magnetism and light, also
electricity
and light, and the field it opens is so large and I think rich,’ he wrote in a letter on 13 November 1845 (Georg W. A. Kahlbaum and Francis V. Darbishire, eds,
The Letters of Faraday and
Schoenbein
, 1836–1862,
p. 148). Among other things, Faraday had found that a magnetic field could change the plane of vibration, or
polarisation
, of a light wave, a phenomenon known today as Faraday rotation.
22
The Feynman Lectures on Physics,
vol. ii, pp. 1–11.
23
Ibid., pp. 1–10.
24
See Chapter 2, ‘The rocket-fuelled baby: Respiration’ and Chapter 5, ‘Matter with curiosity: The brain’.