Download 3.1 Electric Charge

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Revolutions in Physics Notes
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REVOLUTIONS IN PHYICS
ELECTROMAGNETISM AND SPACE-TIME
1. Introduction
One of the most exciting things in Physics is to discover relationships between observed effects (or
phenomena) that were previously thought to be quite distinct. What happened in electromagnetism in
the nineteenth century is a wonderful example. In the year 1800 there were only the vaguest
indications that magnetism had anything to do with moving electric charges, and no evidence at all
that light had anything to do with electricity or magnetism. By 1900 magnetism and electricity had
been firmly linked, and light had been shown to be an electromagnetic wave. How this came about,
sometimes in small steps, and sometimes by seemingly bizarre lines of reasoning, is the subject of this
option.
In one respect the new theory that linked electricity, magnetism and light seemed not to agree with the
facts, as found in experiments. In 1905 Einstein’s Special Theory of Relativity came to the rescue. At
the same time, it actually simplified the theory of electromagnetism (and light). Included in this option
there is a small taste of Relativity theory.
2. Questions and answers about this option
Q What is the point of studying this option?
A • It reinforces some of the non-optional A-Level material, coming at it from a different angle,
giving it a wider context, and adding ‘human interest’.
• It brings the student into contact with great minds and great ideas.
• Sheer self-indulgence – it’s a wonderful story.
Q
A
Q
A
Q
A
Q
A
How can the material presented in this option, derived from what others have written, give the
promised ‘contact with great minds’?
A few extracts from some of the key figures (Young, Faraday, Maxwell and Einstein) are
provided. The extracts are not very long, but are to be studied closely. Guidance is given.
Does the student have to learn dates?
No, but having the right half-decade is good. In fact people often ‘absorb’ dates easily when
there’s a chain of events – and when there’s no stress to learn dates!
What has to be left out in order to fit the story into an A-level option?
This is a real problem. Looking back on past events and ideas, it’s easy to see, or to think we see,
which of them led nowhere or were of secondary importance, and to leave them out. But at the
time they may have been considered very important. They may have influenced the way physicists
thought, in ways we cannot now know. By omitting them we distort history. Please be aware that
this option cannot tell the whole story.
Can anything be done to give a more balanced picture?
Websites references are sprinkled throughout this WJEC material. Two thinnish and very
readable books which provide good support are…
Michael Faraday and the Royal Institution by John Meurig Thomas (ISBN 0-7503-0145-7).
Relativity and its Roots by Banesh Hoffmann (ISBN 0-486-40676-8).
Chapter 4 tells pretty much the same story as this course, but, as the book’s title makes clear,
Hoffmann has a special agenda, and his emphases are different.
All the material to be tested in the PH5 examination is contained in this WJEC printed
material, but students are urged to visit the websites, as they help to bring the basic
material of the option alive and make it easier to learn. They often contain pictures and
diagrams.
3. Electricity, Magnetism and Light: What was known in 1800
3.1 Electric Charge
• It had been known from ancient times that objects, in particular lumps of amber, could be
‘charged’ by rubbing, and could sometimes attract attract or repel other objects. [Our word
electricity comes from the greek word for amber.]
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• Around 1730, Stephen Gray (www.sparkmuseum.com/BOOK_GRAY.HTM) had found that damp
thread, and metals, would conduct charge from one object to another, whereas many materials
were insulators (when dry). [Charge was often referred to as ‘electricity’ and charging, as
‘electrifying’.]
• Soon after, it emerged that there were two sorts of electric charge, and that these could neutralise
each other. Some years later, the american statesman and scientist, Benjamin Franklin, called them
positive and negative. Amber gains a negative charge when rubbed with fur; glass, a positive,
when rubbed with silk.
• Franklin showed, by extremely dangerous experiments, that thunder clouds contain electric charge,
and
that
lightning
is
an
electrical
phenomenon.
(www.inventors.about.com/cs/inventorsalphabet/a/Ben_Franklin_4.htm )
• In about 1745 Dutch investigators discovered that opposite charges could be stored on conducting
surfaces coating the inside and the outside of a glass bottle, and so separated by the insulator, glass.
The device quickly came to be called a Leyden jar, after Leyden, now Leiden, in the Netherlands. It
was used in demonstrations all over Europe to produce sparks and electric shocks - and much
excitement.
• In the late 1780s, Coulomb (www.en.wikipedia.org/wiki/Charles_Augustin_de_Coulomb ) made the
first quantitative investigation of the forces between charged spheres. These were of small enough
diameter, in relation to their separation, to be considered ‘point charges’. Using a torsion balance of his
own devising, he showed that there was an inverse square law, that is, when the separation of the
centres of the spheres was doubled, the force between the spheres quartered, and so on.
(http://library.thinkquest.org/C001429/electricity/electricity11.htm )
[The reclusive Henry Cavendish had made the same discovery some years earlier, but did not publish
his findings.]
Coulomb and his contemporaries were struck by the similarity between this inverse square law for
charges and Newton’s inverse square law of gravitation for masses.
3. Electricity, Magnetism and Light: What was known in 1800
3.2 Magnetism
In the year 1800, most of the knowledge about magnetism dated from 1600, when William Gilbert
had published his great work De Magnete (‘About the Magnet’). He described his experiments to
magnetise iron bars using a lodestone (naturally occurring magnetised iron ore), reported on the
‘magnets’ having poles at either end (the word ‘poles’ is his), and found that even if you cut a magnet
in half, each of the two halves still had both a North and a South pole. He investigated the effect of the
Earth on a pivoted magnet, and came to the conclusion that the Earth itself was a magnet. He
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demolished many superstitions about magnetism, but we would regard his own view as to the cause of
magnetic effects as very odd. (http://galileo.rice.edu/sci/gilbert.html )
Although the attraction and repulsion behaviour of magnetic poles resembles that of electric charges,
Gilbert was very careful to explain that magnetic and electric effects were quite distinct.
3.3 The Battery
This was hot news in the year 1800. Back in the 1780s, Luigi Galvani had observed the twitching of a
leg cut from a dead frog, when a nerve was touched by a piece of metal which was also in contact
with the foot. The effect, he found, was much greater if two different metals were joined together.
There are various versions of how the discovery was made; see for example
www.bioanalytical.com/info/calendar/97/galvani.htm . Galvani attributed the twitching to ‘animal
electricity’, perhaps in the frog’s nerves.
Alessandro Volta took up the investigation and became convinced that it was the different metals
which played the key role. He devised a cell consisting of a strip of zinc and a strip of copper dipping
into a cup of brine or dilute acid, but not touching each other, and then started putting cells in series
(as we would now say). Two forms of ‘battery’ emerged, the ‘crown of cups’
(www.scienceandsociety.co.uk/results.asp?image=10207373 ) and the famous ‘voltaic pile’
(www.en.wikipedia.org/wiki/Voltaic_pile ). [In French the name still survives: une pile or une pile
electrique is a battery.]
News of Volta’s invention spread quickly, and batteries, sometimes very large ones, were built all
over Europe and in America. They were found to melt wires, connected across their terminals, and to
enable the splitting up of water up into oxygen and hydrogen. Some investigators were nearly killed
by
electric
shocks
from
batteries
of
many
cells.
Humphry
Davy
(www.rigb.org/rimain/heritage/ripeople/davy.jsp ), at the recently founded ‘Royal Institution’ in
London, used batteries to perform electrolyses which isolated sodium, potassium and various other
elements for the first time. He also fascinated audiences with demonstrations of what a battery could
do.
Davy’s audiences weren’t made up entirely, or even mainly, of people we would now call ‘scientists’.
Any intelligent person – with the leisure – could contribute to a scientific debate. Davy himself was
quite a gifted poet and was a friend of Wordsworth and Coleridge. There wasn’t really an ‘artsscience divide’. ‘Galvanism’, the term used then for the study of the battery and what it could do, was
much talked about, and we might guess that it was one of the influences on the young Mary Shelley,
when she was writing Frankenstein (published in 1818).
Volta himself had established a connection between batteries and electric charge. He discovered that
the terminals of his batteries were charged positively and negatively. Charge collected from the
terminals could be used to make bodies attract and repel, in specially designed instruments. The
battery provided for the first time the means of producing a continuous flow of charge, or electric
current. [Charge in this context was often referred to as an ‘electric fluid’, and there was controversy
over whether there were really two fluids or just one. We shan’t follow this particular sub-plot.]
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3. Electricity, Magnetism and Light: What was known in 1800
3.4 Light
In the 1660s Newton had performed a brilliant series of experiments showing that ‘white light’ was a
mixture of colours. He made other major contributions to optics. Naturally he wondered what light
was.
Newton’s rival, Robert Hooke (of Hooke’s Law fame) believed it to be a wave-like disturbance
travelling through, and by means of, a universal medium (often called the aether or ether). Christiaan
Huygens, a strong supporter of a wave theory of light, showed how to predict where a wavefront will
be, and what its shape will be, if we know its position and shape now. He gave convincing wave
theory accounts of reflection and refraction.
(http://encarta.msn.com/encyclopedia_761567208/Christian_Huygens.html )
For Newton and others, the problem with the wave theory was that light doesn’t seem to bend round
corners, for example when opaque objects are put in its path. Water waves, though, do bend and
spread into the ‘shadow’ behind obstacles, sound travels round corners – and so do Huygens’
wavefronts. For this reason, mainly, Newton could not accept that light was a wave, or, more
accurately, just a wave. He held that it consisted of a stream of corpuscles or particles, coming from
its source. But he knew there were problems with this: if light fell on a sheet of glass, some goes
through and some is reflected. Why should some corpuscles do one thing and others another?
Newton wrote of light as having ‘fits’ of easy reflection and fits of easy refraction, and hinted that
possibly some sort of wave-like disturbance might accompany the corpuscles and determine what
they did.
Such was the awe in which Newton was held for showing how an inverse square law of gravitation
accounted for the motion of the planets, the moon and the tides, that his corpuscular theory of light
was given enormous respect. If you challenged it, even long after Newton’s death, you would have to
defend yourself very convincingly.
3.5 Questions on section 3
(1) It was discovered in the 1700s that metals could be charged up by rubbing with a dry cloth. In
what special way would the metal have to be held?
(2) A leyden jar would now be classed as a sort of …………………….. ?
(3) How, mathematically, do we now write Coulomb’s inverse square law for electric charges?
(4) What, according to William Gilbert, was the ‘soul of the Earth’?
(5) In what you have read, have you come across any pre-1800 evidence for a connection between
electricity and magnetism?
(6) What was ‘galvanism’, and why was it so called?
(7) Is it true that none of the effects of an electric current could have been observed before the work
of Galvani and Volta?
(8) How does the wave theory of light account for refraction?
(9) What political upheaval was shaking Europe in the 1790s?
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4. Re-birth of the Wave Theory of Light
4.1 Thomas Young
Thomas Young (born in 1773) was a child prodigy. When four years old, he is said to have read the
bible in its entirety…twice. By the age of fourteen he had mastered several languages, ancient and
modern.
He lived up to his early promise. As a medical
student he discovered the mechanism by which
the eye focuses (or accommodates), and, at the
age of 21 was elected a Fellow of the Royal
Society. This is Britain’s most prestigious
scientific society, dating from the time of
Newton.
In 1801, when Young had set up as a doctor in
London, he was chosen as Professor of Natural
Philosophy (roughly speaking, Physics) at The
Royal Institution. [He turned out not to be as
charismatic a lecturer as Humphry Davy.]
At about this time Young started his researches
on light – see below.
Later in life he made some headway in
deciphering the ancient Egyptian heiroglyphics
on the Rosetta Stone.
www.whonamedit.com/doctor.cfm/1715.html
Writing about light, Young stated two
‘hypotheses’ ;
“A luminiferous [light-carrying] ether pervades the universe.”
“Undulations [waves!] are excited in this ether whenever a body becomes
luminous.”
He explained that:
“an undulation is supposed to consist in a vibratory motion; transmitted
successively through different parts of a medium without any tendency in each
particle to continue its motion except in consequence of the transmission of
successive undulations from a distinct vibrating body.”
Young’s new idea, apparently not grasped by Huygens, was that light had to be a regular sequence of
undulations. This implied that light from the same source, travelling to the same point by different
routes would interfere either constructively or destructively, according to phase difference. Using the
idea of interference, Young was able to explain ‘Newton’s Rings’ a phenomenon which had puzzled
Newton himself. Visit the website below for pictures – strictly ‘for interest only’!
www.physics.montana.edu/demonstrations/video/6_optics/demos/newtonsrings.html
Note that it did not occur to Young at the time that light could be anything other than a longitudinal
wave, like sound.
Young seems [historians argue about it] first to have shown a version of his famous two slits
experiment in a lecture given to The Royal Society in 1803. Here is the account he gives of such an
experiment…
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4. Re-birth of the Wave Theory of Light
4.1 Thomas Young (Continued)
“It has been shown that two equal series of waves, proceeding from centres near
each other, may be seen to destroy each other’s effects at certain points, and at other
points to redouble them; and the beating of two sounds has been explained from a
similar interference. We are now to apply the same principles to the alternate union
and extinction of colours.
“In order that the effects of two portions of light may thus be combined, it is
necessary that they be derived from the same origin, and that they arrive at the
same point by different paths in directions not much deviating from each other.
This deviation may be produced in one or both the portions by diffraction, by
reflection, by refraction, or by any of these effects combined: but the simplest case
appears to be, when a beam of homogeneous light falls on a screen in which there
are two very small holes or slits, which may be considered as centres of divergence,
from whence the light is diffracted in every direction.
“In this case, when the two newly formed beams are received on a surface placed so
as to intercept them, their light is divided by dark stripes into portions nearly equal,
but becoming wider as the surface is more remote from the apertures, so as to
subtend very nearly equal angles from the apertures at all distances, and wider also
in the same proportion as the apertures are closer to each other. The middle of the
two portions is always light, and the brighter stripes on each side are at such
distances, that the light coming to them from one of the apertures, must have
passed through a longer space than that which comes from the other, by an interval
which is equal to the breadth of one, two, three or more of the supposed
undulations, while the intervening dark spaces correspond to a difference of half a
supposed undulation, of one and a half, of two and a half, or more.
“From a comparison of various experiments, it appears that the breadth of the
undulations constituting the extreme red light must be supposed to be, in air, about
one 36 thousandth of an inch, and those of the extreme violet, about one 60
thousandth; the mean of the whole spectrum, being about one 45 thousandth. From
these dimensions it follows, calculating upon the known velocity of light, that
almost 500 millions of millions of the slowest of such undulations must enter the
eye in a single second.”
Young continues with a description of the ‘beautiful diversity of tints’ in the fringes which are seen
when white light is used. The above extract is as Young wrote it, apart from one comma being
removed and one new paragraph created. There were no diagrams (apart from the one below);
readers
were
supposed
to
…
read.
And
visualise!
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4. Re-birth of the Wave Theory of Light
4.1 Thomas Young (Continued)
Here are some must-do ‘comprehension’ questions on this first-ever description of a now famous
experiment.
(1) What did Young mean by a ‘luminiferous ether’? What purpose did it serve?
(2) Draw the set-up described by Young in the second paragraph and the beginning of the third
paragraph in the long extract. It should be familiar!
(3) What – in a word – does Young mean by ‘the breadth of an undulation’ (near the bottom of the
third paragraph)?
ay
(4) WJEC gives the ‘Young’s fringes formula’ as

.
D
(5)
(6)
(7)
(8)
(9)
(a) Re-arrange it to make the fringe separation the subject.
(b) Pick out the phrase from Young’s third paragraph in which he states the effect on the fringe
separation of altering D.
(c) Pick out the phrase from Young’s third paragraph in which he states the effect on the fringe
separation of altering a.
The bright stripe next the central bright stripe is at such a distance, to use Young’s terminology,
that the light coming to it from one of the apertures must have passed through a longer space
than that which comes from the other, by an interval which is equal to the breadth of one of the
supposed undulations. Put this in modern ‘path difference’ language.
1 inch = 2.54 cm. Hence express in metres Young’s results (fourth paragraph) for the
wavelengths of the extremes of the visible spectrum. Do they agree with what textbooks give?
What is conspicuously missing from this account of a quantitative experiment?
When, at the end of the passage, Young refers to ‘the slowest of such undulations, he means
those of the lowest frequency. What does he give as their approximate frequency?
Young refers near the end to ‘the known velocity of light’. [It had been inferred a long time
previously by two different methods based on two quite different sorts of astronomical
measurements.] Work backwards from Young’s figures for longest wavelength and lowest
frequency to deduce what figure he must have been using for the velocity of light.
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4. Re-birth of the Wave Theory of Light (Continued)
4.2 Reactions to Young
Young’s experiment is the classic demonstration that light has wave-like properties. But that is not
how it was seen at the time. Maybe Young’s contemporaries would have been more convinced if
he’d given his actual readings, and explained properly how he’d arrived at his results for
wavelengths. Then there was the long-dead Newton to contend with. How dare this upstart, Young,
challenge the great Newton’s view that light was a stream of particles?
Henry Brougham, a barrister who later rose to become Lord Chancellor, wrote an infamous review
of one of Young’s Royal Society papers. He accused Young of putting forward an (unjustified)
theory, and having to make changes to it …
“A mere theory is in truth destitute of all pretentions to merit of every kind, except
that of a warm and misguided imagination. It demonstrates neither patience of
investigation, nor rich resources of skill, nor vigorous habits of attention, nor
powers of abstracting and comparing, nor extensive acquaintance with nature. It is
the unmanly and unfruitful pleasure of a boyish and prurient imagination, or the
gratification of a corrupted and depraved appetite.
“If, however, we condescend to amuse ourselves in this manner, we have the right
to demand, that the entertainment shall be of the right sort – that the hypothesis
shall be so consistent with itself, and so applicable to the facts, so as not to require
perpetual mending and patching – that the child which we stoop to play with shall
be tolerably healthy, and not of the puny, sickly nature of Dr Young’s productions
[...]”
Not impressed, then? In another paragraph (which no writer today could expect to get away
with) Brougham accused Young of bringing the Royal Society into disrepute…
“Has the Royal Society degraded its publications into bulletins of news and
fashionable theories for the ladies who attend the Royal Institution? Proh Pudor!
[For shame!] Let the professor continue to amuse his audience with an endless
variety of such harmless trifles; but, in the name of Science, let them not find
admittance into that venerable repository which contains the works of Newton, and
Boyle, and Cavendish and Maskelyne and Herschell (sic, the correct spelling is
Herschel).” (http://homepages.wmich.edu/~mcgrew/brougham.htm for interest only!)
Brougham’s reaction was extreme, but, even putting it aside, Young’s work on interference and the
wave
theory
didn’t
attract
much
enthusiasm
at
the
time.
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4. Re-birth of the Wave Theory of Light (Continued)
4.3 Transverse waves
Real trouble soon arrived for the wave theory.
In about 1808 Etienne Malus discovered an
astonishing fact about the light reflected from
a transparent sur-face. The effect is observed
to perfection for the light reflected off a glass
plate, A, when the angle of incidence is 57°.
The reflected ray is found to be reflected from
another glass plate, B, when this is as shown
in the left hand diagram, but not when B is
turned about the ray as axis, so that it is as
shown on the right. The light must be
asymmetrical about its direction of travel! [A
related effect involving certain crystals, called
‘double refraction’, had puzzled natural philosophers for well over a century. Polaroid had
not been invented.]
To an A-level student the solution should be
obvious: light is a transverse wave, and A must be polarising it. But it hadn’t occurred to Young that
light could be anything else but a longitudinal wave, like sound. Eventually, though, (c1818) the
penny dropped.
By this time another powerful wave theorist, Augustin Fresnel, was at work in France.
(http://micro.magnet.fsu.edu/optics/timeline/people/fresnel.html ). He came upon the significance of
interference independently of Young, and developed the wave theory mathematically. He showed
convincingly that the reason we don’t normally see light bending round corners is because of its
short wavelength. He accounted for polarisation by reflection, double refraction and the diffraction
patterns caused by various obstacles. For a spherical obstacle his equations made an unlikely
prediction … (www.physics.brown.edu/physics/demopages/Demo/optics/demo/6c2010.htm )
4.4 Problems with the Ether
Fresnel effectively killed off the corpuscular theory. Most natural philosophers were persuaded that
light was a transverse wave. The only sort of wave anyone could imagine was a mechanical wave, in
which a pattern of displacements transmits itself through a medium, the ‘ether’. Try and follow this
crude and sketchy explanation…
In the diagram a transverse wave is travelling to the right. The medium is stiff, so the shaded slice
experiences an upward tangential or ‘shearing’ force from
the upwardly displaced slice to its left. The shaded slice will
accelerate upwards, and the peak displacement, P, will
move to the right – and so on.
There were severe problems with this ‘mechanical’ theory…
• It is difficult to see why the ether shouldn’t transmit
longitudinal waves as well as transverse waves. Yet no
longitudinal waves were observed.
• Transverse waves need a stiff medium, a solid, rather than a liquid or gas. But we receive sunlight
and starlight, so all space must be full of this medium. How, then can the planets move without
obstruction? Indeed, how can anything move freely?
For the next few decades, elaborate attempts were made to devise ether structures which would not
have these problems. We shall return to the ether…
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