john l. lewis - electrons and atoms

8/17/2019 John L. Lewis - Electrons and Atoms

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LONGM N PHYSICS TOPICS


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LONGMAN PHYSICS TOPICS

Materials D. W. Harding and L. Griffiths

Crystals Sr M. M. Hurst (Sr St Joan of Arc)

Pressures A. R. Duff

Forces R. D. Harrison

Electric Currents

J.

L. Lewis and P. E. Heafford

Heat A. 1. Parker and P. E. Heafford

Planetary Astronomy E. J. Wenham

Radioactivity J. L. Lewis and E. 1. Wenham

Using Light W. Llowarch and B. E. Woolnough

Ideas and Discoveries in Physics Sir Lawrence Bragg

Electromagnetism J. M. Osborne

Mass in Motion J. Jardine

From Darkness to Light: Renaissance Science D. D. Lindsay

Supersonic Flight-Bernoulli to Concorde F. R. McKim

Waves D. C. F. Chaundy

Electrons and Atoms J. L. Lewis

Waves or Particles H. F. Boulind

Time G. Dorling

Magnetism G. W. Verow

Energy R. Stone and R. Dennien

Rutherford and the Nuclear Atom E. S. Shire

front cover Teltron Maltese Cross tube: the shadows of the cross

in the white light from the filament and in the cathode

rays are both sharp and coincident; the shadows

separate in a magnetic field

back cover Teltron double-beam tube: the stream of electrons is

both focused into a fine beam and made visible by

helium at low pressure. In a uniform magnetic field,

the electrons move in a circular path at right-angles to

it. The light from the luminous gas consists of several

distinct wavelengths, the strongest of which are green,

but because the colour film is less sensitive to these than

to other colours, the circle appears to be blue.


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LONGMAN PHYSICS TOPICS General Editor: John L. Lewis

ELECTRONS

AND ATOMS

John L. Lewis

Senior Science Master, Malvern College

and formerly Associate Organiser,

Nufjield O-level Physics Project

Illustrated by T. H. McArthur

~ ~ ~

••••

•••

LONGMAN

1 1 1 ~ ~ l r l l f l l ~ ~ I I I ~ n l l l l l

N25702


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LONGMAN GROUP LIMITED

London

A ssociated companies, branches and representatives throughout the world

© Longman Group Ltd 1972

A II rights reserved. No part of this publication may be reproduced, stored

in a retrieval system or transmitted in any form or by any means - electronic

mechanical, photocopying, recording or otherwise - without the prior

permission of the copyright owner.

First published 1972

ISBN 0 582 32215 4

Printed in Great Britain by Butler and Tanner Ltd, Frome and London

The author and publisher are grateful to the following for permi

sion to reproduce photographs: Cavendish Laboratory, Cambridge

page 42: Esso, pages 26 (above) and 30: Geological Survey Museum

Crown

©,

page 7 (centre); Philip Harris Ltd, page 31; Kodansh

Ltd, page 9 (left and right); Gunter Lutzow, page 34 (left a

right); Mullard Ltd, pages 25, 57 (left and right) and 58 (above

Mr H. E. C. Powers (retired, Tate and Lyle Ltd), page 7 (lef

Science Museum, London, pages 20 (all photos), 43, 50 (all photo

and 60 (above and below right); Telequipment Oscilloscopes

courtesy of Teltronix U.K. Ltd, page 53 (left and right); Teltro

Ltd, pages 22, 24, 26 (below), 27, 28 (above and below) and cov

photographs. The photograph on page 8 (above) is from Martin

Thirteen Stops to the A tom (H arrap), and on page 60 (above a

below left) from Aston, Mass Spectra and Isotopes (Arnold).

CKNOWLEDGEMENTS


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NOTE

TO THE

TEACHER

This book is intended for use in the final year of the

Nuffield O-level Physics course. It attempts to bring

together topics discussed earlier in the course and to

show how they are relevant to the model of the atom

which we build up at the end of it. It should provide a

useful summary prior to examinations and emphasise

the logical development of our ideas.

An appendix at the end includes material beyond

what is strictly necessary for an O-level course, but it is

included so that the boy or girl who wants to take the

subject a little further can do so. The book, together

with the appendix, will also be suitable for the pupil

in the first year of an A-level course. It is unfortunate

that most A-level books are written in a language and

style suitable mainly for the second year. It is hoped

very much that the style in which this book is written

will be more suitable for those in their first year and

that they will therefore find it useful.

Although it has been written with the needs of

Nuffield courses in mind, this book is also useful as a

background book for the traditional course in which

ideas about the atom and its structure are increasingly

finding a place.


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ICONTENTS I

WHY DO WE BELIEVE IN ATOMS?

The place of models

Early speculations

The evidence from crystals

X-ray diffraction by crystals

Solid to liquid to gas

Evidence from the oil-drop experiment

Evidence from Brownian motion

Support for the model from a consideration of

pressure

Evidence from diffusion

Powerful evidence from chemistry

Conclusion

THE EVIDENCE FOR CHARGED PARTICLES

What we learn from electrostatics

Electrostatic charge and current electricity

1

1

1

1

1

1

1

1

1

IONS

A new model of the atom

Ions in liquids

Ions produced by a flame

Ions produced by radioactive radiations

Ions produced by a hot filament

THE THERMIONIC EFFECT

The diode

Uses of the triode

Other experiments with thermionic vacuum tubes

Conclusion

THE MILLIKAN EXPERIMENT

The experiment

The theory of the experiment

Result of the experiment

Conclusion

3

3

3

3

3

The speed of the electrons

The force on the electrons due to the magnetic field

Calculation of e/ m

The mass of the electron

THE MASS OF THE ELECTRON

The fine-beam tube


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THE MASS OF THE ATOM

Mass of the proton

The Avogadro constant

Mass spectrometers

Isotopes

39

39

39

40

40

42

42

44

45

46

46

48

50

5 1

1.1. THOMSON AND THE ELECTRON

Cathode rays

Measurement of e/ m

The electron as a constituent of all matter

MODELS OF THE ATOM

Nuclear model of the atom

The Bohr model of the atom

The wave-mechanical model

The future

ELECTRONS AT WORK

The cathode-ray oscilloscope

The television tube

The X-ray tube

5 2

5 2

56

57

59

PPENDIX: MASS SPECTROMETERS

5


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WHY DO WE

BELIEVE IN

ATOMS?

6

You have grown up believing in the existence of atoms

We have all heard of the atomic bomb. We all kno

that atoms exist. But why do we believe in them? Th

most probable reason is that we have read about them

in papers and books, that we have heard about them o

the radio or television, that people talk about them s

much that we have come to believe in them ourselves

But no good scientist will take things like this on trust

we should look for evidence.

THE PLACE OF MODELS

The story is not a simple one. There is not one conclu

sive experiment you can do to prove the existence

atoms. But we get a clue about atoms from studying

matter and this clue enables us to suggest a model o

what matter is like. We can start thinking about th

model, and this may lead us to expect certain con

sequences which can then be put to the test. If we fin

these consequences are confirmed by experiment, th

provides further evidence in support of the model.

You cannot ever prove that a model is correct; a

you can do is to go on collecting more and mor

evidence which supports it. You can go on believing

is valid until you find evidence that contradicts it. It

at that point that you have either to abandon the mode

or modify it to fit the new evidence. But models d

not have to be correct in all respects to be useful. Pro

vided one knows how far one can safely go, model

known to be incomplete may be extremely valuable

we shall see later in this book.

A good theory is one which can be put to the tes

Newton's theory of gravitation was a good theory

many deductions could be made from it and these wer

confirmed by experiment. None of these experiments,

however, proved that the theory was true in the sens

that a geometrical theorem can be proved. Newton's

theory was, nevertheless, accepted until an experiment

showed that it had limitations (although it is still usefu

within these). A new theory of gravitation, Einstein's


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WHY DO WE

BELIEVE IN

ATOMS?

anulated sugar

was proposed. The evidence now supports this and it

will doubtless go on being accepted until it too has to be

superseded by another theory which gives us yet deeper

insight into the nature of the physical universe. It is

much the same with models.

Let us now look at the evidence that matter is made

of atoms.

EARLY SPECULATIONS

The first ideas about a particulate nature of matter

came from the Greek philosopher Democritus, as re-

corded by the Roman poet Lucretius in his work De

Rerum Natura. But these ideas were little more than

speculations as they were unsupported by experimental

evidence. The rival speculation that matter was made

up of four basic elements - earth, air, fire and water -

was preferred to any idea of atoms for nearly two

thousand years.

THE EVIDENCE FROM CRYSTALS

Fluorite

A lum crystal

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WHY DO WE

BELIEVE IN

ATOMS?

If you have not already done so, look

at Crystals in this series.

Growth of crystals under a microscope

8

Early in your course you will have studied crystals

You will have noticed the regularity of shape

different kinds of sugar, in salt and other substances

You will have seen what happens when you gro

crystals of alum and copper sulphate. You may perhap

have seen crystals of salt or salol growing under

microscope. The angles always seem to be the same f

the particular substance under consideration.

What could account for this regularity? One possib

explanation is that the substances were made up

basic building blocks; so it was that you came to

model of matter made up of particles.

You found support for the model when you s

crystals of calcite being cleaved. If the regular shape

crystals meant that they were made up of layers

particles, we might expect them to cleave along certa

planes and that is just what happened. So our model

supported.

Cleaving calcite

Cleaving a polystyrene model


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WHY DO WE

BELIEVE IN

ATOMS?

This is considered in greater detail

in Waves or Particles in this series.

Ray of light falling on a diffraction

grating (left)

Laue spots (right)

X-RAY DIFFRACTION BY CRYSTALS

Very powerful evidence that solid matter consists of a

regular array of particles comes from X-ray diffraction

by crystals. If waves in a ripple tank strike a double

slit in a barrier there are certain definite directions in

which there are lines of constructive and destructive

interference.

there is a series of regularly spaced

slits, there are again definite directions in which con-

structive interference occurs. The same thing occurs

when light falls on regularly spaced lines (the diffrac-

tion grating).

If X-rays (like light waves but with a very much

smaller wavelength) are directed in a fine beam at a

crystal, there are definite directions in which construc-

tive interference occurs and a series of dots representing

those directions is obtained on a photographic plate.

directions for

constructive interference

X-rays

lead

photographic

plate

9


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WHY DO WE

BELIEVE IN

ATOMS?

10

In

all these experiments the spacing must be regular fo

constructive interference to occur. The crystal behaves

like a three-dimensional 'grating'. The fact that Lau

spots (named after Professor Laue who first suggested

crystal diffraction of X-rays) can be obtained provides

further strong evidence in support of our theory tha

crystals are made of regularly spaced particles.

SOLID TO LIQUID TO GAS

A very familiar property of matter is that a solid turn

to a liquid when sufficient heat energy is given to i

when further heat energy is supplied, it turns to ga

What does our model of matter made of particles sa

about this?

Perhaps the heat energy supplied breaks some of th

bonds holding the particles together in the solid, s

that they can flow more freely. We know that liquid

can flow and you have probably seen how two di

similar liquids placed one above the other in a cylinde

diffuse into each other.

When further energy is added, perhaps all the bond

are broken and the particles move around quite freely

If this happened we would expect the gas to occupy

a much larger volume than the solid - and that

exactly what we find. Support for our model also come

from the fact that a gas occupies all the space avai

able to it. None of this proves that matter is made

particles, but it does lend support to the idea.

EVIDENCE FROM THE OIL-DROP

EXPERIMENT

When you did this experiment, you took a very sma

drop of oil

G

mm across) and put it on the surface of

tray of water which had been dusted with lycopodium

powder. It spread out into a thin film on the surface.

If oil were made of 'continuous juice' and not o

particles, we might expect the layer of oil to go o


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WHY DO WE

BELIEVE IN

ATOMS?

spreading out more and more as it got thinner. But i

does not. The drop formed a circle (about 20 cm

across). This is similar to taking a quantity of lead shot

and pouring it on to a flat surface so that it forms an

area one lead shot thick.

This oil-drop experiment does not prove the exist-

ence of atoms, but the experiment does give us a

maximum value for the size of oil drop particles if they

do exist.

If the diameter of the drop was ~ mm,

4

( 1 ) 3

the volume

=

: 3 ' [ 4 mm '

If the diameter of the circle was 20 em,

the area

=

n(100)2 mrn

If

x

is the thickness of the oil film in mm, then

10

4 _

4 1

n :» ; xx-

r

43

And this gives

x =

3 X4 ;X 10

4

2 X 10-

6

mm

or 2x 1 O - 7cm

Remember that this is only a crude estimate of size:

it is not easy to make precise measurements and the

size of the oil drop is subject to a lot of error. But a

least this crude experiment gives an order of magnitude

to molecular size.

There is another complication in that the olive-oil molecule

is in fact nothing like a sphere: it is considerably elongated.

But at least the calculation gives an answer of approximately

the right size.

More precise experiments show that the size of an

atom is of the order of

lO r =cm

and this is a convenient

number to remember.

EVIDENCE FROM BROWNIAN MOTION

You will be familiar already with the Brownian motion

experiment: smoke particles are put in a cell containing

air, illuminated from the side and viewed through alow-

powered microscope. They are seen to be moving about

1


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WHY DO WE

BELIEVE IN

ATOMS?

molecule

~ ~ ~ ~ ~ = = = = = ~ ~ = ~ = ~

: This theory is considered in greater

detail in Kinetic Theory in this series.

12

wall

rapidly in a random fashion. This important experi-

ment gives us direct evidence that gases, such as air

consist of randomly moving small particles, too smal

to be seen, though their effect is clearly visible in th

experiment when they buffet around the much larger

smoke particles in a random manner.

SUPPORT FOR THE MODEL FROM A

CONSIDERATION OF PRESSURE

If a ball hits a wall and bounces off it, a force is exerted

on the wall because of the change of momentum.

Likewise if a gas consists of fast-moving particles w

would expect it to exert a force on the walls of any

container and thus there would be a pressure.

One of the triumphs for our model is that if we apply

to these tiny particles in a gas the same laws o

mechanics that we have derived for large-sized objects

we can deduce an expression for the pressure in clos

agreement with what is observed in practice.

We start by considering one gas particle hitting th

wall. The rate of change of momentum gives us th

force on the wall. We consider the effect of all th

particles, which gives the total force on the wall, and

dividing by the area we get the pressure. The theory';

predicts that where p is the pressure, V the volume,

v the root mean square value of the particle velocities

and M the total mass of gas,

p V =

iMv2.

If the assumption is made that the kinetic energy o

the particles is constant at a particular temperature,

it follows from the above that the product of the pres

sure and the volume is a constant for a given mass o

gas. But Boyle discovered experimentally that this pro

duct is a constant if the temperature is kept constant

and provided the pressure does not get too big

Although we must not lose sight of the fact that a

extra assumption was made, it is encouraging tha

our model has thus predicted a result which is con

firmed by experiment.


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The model suggests that the gas particles are in rapid

motion. The above formula enables us to deduce the

average speed of the particles at a particular tempera-

ture. For air at atmospheric pressure and room

temperature the speed is about 500 m/s, You saw in the

diffusion of bromine gas into air how the particles o

bromine gradually mix amongst the air molecules,

explained by the random motion of the gas. At first

sight the rate of diffusion seems slow in view of the high

speed calculated above, but this is because the gas

particles do not travel far in anyone direction without

a collision, so that diffusion takes time.

If by contrast the bromine is released into a vacuum

the result is dramatic: one can well believe in a speed of

500 rn/s.

A detailed study of the diffusion of bromine into air

leads to an estimate of molecular size in agreement

with other estimates, so this also provides further

evidence for our theory of a particulate nature of

matter and tells us that our model of a gas consisting of

For this treatment, see also Kinetic particles moving randomly at high speed is not a bad

Theory.

one.

EVIDENCE FROM DIFFUSION

POWERFUL EVIDENCE FROM

CHEMISTRY

So far all our evidence for atoms comes from physical

considerations. But we must not forget what chemistry

has to say. One of the joys of studying science is when

we find evidence from one branch supports our studies

in another.

The chemists find, for example, that water is made up

of hydrogen and oxygen, that common salt is made up

of sodium and chlorine and that nitrogen can be com-

bined with oxygen in a variety of different ways. They

find for example when combining nitrogen and oxygen

that the masses are in the ratio 28: 16,14: 16 or 28:48.

They find it convenient to postulate the existence of

13


16/68

atoms which can be combined to form molecules and

this suggests that the mass of a nitrogen atom to the

mass of an oxygen atom is in the ratio 14: 16 and that

the atoms combine in the ratio 2: 1 (conveniently

written N

2

0), 1: 1 (NO) or 2: 3 (N

2

0

a

).

The chemists therefore support our theory of a parti-

culate nature of matter and from them we shall adopt

the idea that the particles consist either of atoms or

combinations of atoms which we call molecules.

WHY DO WE

BELIEVE IN

ATOMS?

CONCLUSION

All this evidence - and much more besides - supports

the atomic model of matter. Let us therefore assume

that it is correct and see where it leads us.

n

particular,

let us try to find out what is inside the atom.

14


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THE EVIDENCE

FOR CHARGED

PARTICLES

[ ] [ ]

+++++++

+++++

~[ J

WHAT WE LEARN FROM

ELECTROSTATICS

You have experienced the crackling when a comb

becomes charged as it is pulled through hair or when

a nylon garment is pulled off. You know that a poly-

thene strip becomes charged when rubbed, as also does

a cellulose acetate one. Your work in the laboratory

will have shown that when two conducting spheres are

hung up side by side and each is touched by a rubbed

polythene strip they repel each other. Similar repulsion

is seen when the spheres are touched by a rubbed cellu-

lose acetate strip.

/\ /\

\ /

-7 ~ -7 ~~

spheres both touched spheres both touched one sphere touched

by polythene strip by cellulose acetate strip by polythene, one by

cellulose acetate

But when one of the spheres is touched by a rubbed

polythene strip and the other by a rubbed cellulose

acetate strip the spheres are attracted to each other.

This suggests that charge is of two kinds, which we call

positive

and

negative.

Positive charge repels positive

charge, negative charge repels negative charge, but

unlike charges attract. In fact the polythene strip be-

comes negatively charged when rubbed and the

cellulose acetate strip positive.

The gold-leaf electroscope is a useful tool in

investigating charge. When charge is deposited on the

insulated plate, it spreads over the plate and the gold

leaf, and because like charges repel each other the

leaf rises.

If a positively charged rod is brought near the plate

of a positively charged electroscope, it will repel the

positive charge on the top of the electroscope and the

leaf will rise more. If a negatively charged rod is

brought up it will attract the positive charge and the

leaf will fall.

15


18/68

An electroscope can be charged positively by con-

necting the plate to the positive terminal of a battery

or a power supply, at the same time connecting the

outer case to the negative terminal. (This should give

you a clue as to how you could confirm that a cellulose

acetate strip is positively charged and a polythene rod

negatively charged.) A high voltage is necessary to get

the leaf to rise. When the leaf is fully up there may be

a potential difference of 1000 or more volts, though it

depends on the design of the electroscope.

THE EVIDENCE

FOR CHARGED

PARTICLES

ELECTROSTATIC CHARGE AND

CURRENT ELECTRICITY

Is there some connection between this electrostatic

charge and current electricity? This can be shown by

charging up a van der Graaff generator and then dis-

charging it to Earth through a current-measuring

instrument. Alternatively you can charge the sphere of

the van der Graaff generator continuously and get a

continuous discharge through a delicate meter. You

may also have seen a similar experiment in which a

table-tennis ball, coated to make it conducting, is

suspended between two plates as illustrated above.

When the van der Graaff generator is operating the

ball moves rapidly from one plate to the other, carrying

charge across the gap, and the galvanometer records a

current flowing.

From such experiments we accept that the flow of

electric charge is equivalent to a current.

16


19/68

1

1

:1----------,

anode cathode

-_- _- CuSO. solution

= - - = - - -

A NEW MODEL OF THE ATOM

The charging of the polythene and cellulose acetate

strips, discussed in the previous section, might be due

to the duster used for rubbing them. But if a duster is

not used and the two strips are rubbed together, the

same thing happens: one becomes negative, the other

positive.

We might modify our model of the atom and think of

it as having a positive and a negative part, perhaps

something like:

a negative part

of the atom

a neutral atom a positively charged

atom with a

negative part removed

a neutral atom

with a negative part

added leaving

the whole negative

The neutral atom with a negative part knocked off is

usually referred to as a positive ion. The negative part,

or the neutral atom with a negative part added, is a

negative ion. The process of forming ions is called

ionisation. (We will see later that the negative part

knocked off is an electron.)

I t is not suggested that the atom really does look like

this, but it is a convenient model which fits the facts

discussed so far, and we shall use it until we find the

need for a different model.

IONS IN LIQUIDS

Early in your course you found that pure water

(distilled water) did not conduct electricity, but that it

became conducting when common salt was added to

it. Similarly an electric current could pass through

water which had copper sulphate crystals dissolved in

it. We can explain this passage of current by assuming

that positive and negative ions carry the current

through the liquid.

17


20/68

For details of the chemical reactions,

you should consult a chemistry textbook.

18

The positive ions are attracted to the negativ

electrode (the cathode) and the negative ions to t

positive electrode (the anode). At the electrodes th

either give up their charge or are neutralised by t

current from the battery.

When copper electrodes are used in a solution

copper sulphate, the movement of the ions leads to

deposit of pure copper on the cathode and the chemic

action which takes place at the anode when t

negative ions reach it leads to the copper being take

from it. :;: (This has an important application

purifying copper: the impure metal can be made t

anode and pure copper is deposited on the cathode.)

The presence of ions in a liquid solution is al

illustrated by passing a current through water co

taining a little sulphuric acid. Positive and negative io

are again responsible for carrying the curre

through the liquid. The positive ions are attracted

the cathode, where they are neutralised by the curre

from the battery, producing hydrogen. The negativ

ions travel to the anode, where they give up the

charge and the chemical action produces oxygen. T

two gases, hydrogen and oxygen, bubble up from t

cathode and the anode respectively, and can

collected in apparatus similar to that shown on the le

IONS PRODUCED BY A FLAME

Positive and negative ions are produced in the air ov

a Bunsen flame. For this reason a cool Bunsen flame

a very good way of ensuring that a rod is discharged

If a flame is waved over a polythene rod the ions pr

duced will neutralise any charge on it.

A good demonstration to show that a flame produce

both positive and negative ions can be given using

candle, above and on either side of which are put tw

vertical plates. One plate is connected to the positiv

terminal of a high-voltage supply, the other to t

negative terminal. If a strong light source is set up

that the shadows caused by the currents of air abo


21/68

the flame are visible on a screen it will be noticed tha

they divide as the positive ions are attracted to th

negative plate and the negative to the positive.

IONS PRODUCED BY RADIOACTIVE

RADIATIONS

A match held near a charged electroscope will cause

to be discharged because of the ions it produces.

Similarly a radioactive source, such as a radium one


22/68

Track of an alpha particle

Track of a beta particle

Drops formed in pairs

20

held near an electroscope will discharge it, showing

that it produces ions.

In

a cloud chamber a radioactive source produces

ions on which water drops condense, producing the

well-known tracks. The first photograph above shows

the dense tracks produced by alpha particles; the less

dense tracks produced by beta particles (which produce

less ions for the water to condense on) are shown in the

second photograph. The third photograph is an

interesting beta track: many droplets in this magnified

picture are in pairs. Ionisation produces both positive


23/68

and negative ions on which water can condense and

this explains why the droplets form in pairs. ,;:

IONS PRODUCED BY A HOT FILAMENT

If a piece of resistance wire is coiled up and held near a

charged electroscope, the electroscope is again dis-

charged when the resistance wire is heated by passing

a current through it. This discharge occurs whether

the electroscope is either positively or negatively

charged. Ions are clearly being produced by the hot

filament. This may be due to something given off by the

hot filament or it may be due to air molecules hitting

the hot wire and becoming ionised. This is such an

important phenomenon that we must consider it in

greater detail in the next section.

: For further details see Radioactivity in this series.

21


24/68

The evidence discussed already encourages us t

believe that it is the flow of electric charge which

equivalent to an electric current. It is suggested that

liquids it is the flow of both positive and negative ion

which is responsible for the current. But what happens

in a metal? Is it positive charge or negative charge

or both, that is flowing? At this stage we have n

evidence to help us decide, but later experiments wit

the Hall effect tell us that in most metallic conductors

the current is due to the flow of negative charge.

If charge is flowing in a conducting wire, could w

get the charge out of the wire by some means? We ca

get water vapour out of water by heating it and th

suggests that we might try heating a wire to see if w

can get the charge out. An experiment to do this wa

described at the end of the last section. The hot wir

certainly caused the electroscope to discharge, but

would do so whether the electroscope was positively

or negatively charged and there is some confusion

about how the ions are produced in the air. To find ou

if any charge comes out of the hot wire, we mu

investigate it in a vacuum.

THE

THERMIONIC

EFFECT

THE DIODE

1-------1

r o - ~

5 0 0 V

1

r------

The tube shown above contains a filament which ca

be made white hot when 6.3 volt is put across it. It als

contains an isolated plate. Because it contains tw

electrodes, the filament and the plate, such a tube

referred to as a diode.

22


25/68

Plate voltage

VA/volt

200

150

100

50

o

50

100

150

200

250

300

350

400

450

500

either

Current fA/

milliampere

0 0

0 0

0 0

0 0

0 0

0 2

0.6

1.6

2.2

2.5

2.6

2.7

2 8

2 8

2 8

To investigate what is happening a voltage i

applied between the plate and the filament. These

readings are typical for such a tube.

2

3 2 1

o

1 2 3 4 5

VA/V

I t is seen first that no current flows through the

milliammeter when the plate is negative relative to the

filament. But a current flows as soon as the plate is

made positive. In other words, current will flow only

one way through the tube. Just as valves in bicycle

tyres and in water pumps are constructed so that the

air and water can only flow one way, this tube which

only allows current to pass one way also came to be

known as a valve. (Somewhat illogically, the word diode

is now often used for a semiconductor device which

only allows current to pass through it one way: valve

would be more appropriate but is never used.)

The fact that an electric current only flows when the

plate is positive suggests at first sight that something

negative is being given off by the hot filament and that

this negative charge moves to the positive plate, carry-

ing the current.

But there could be another explanation. It could be

that the light from the hot filament hits the positive

plate, causing it to give off positive charge which

travels through the tube towards the filament (negative

relative to the plate). We must therefore devise some

experiment to decide whether it is negative charge

23


26/68

moving to the left as shown in the first diagram or

positive charge moving to the right.

The crucial test comes when we introduce a grid into

the tube between the filament and the plate.

Suppose the current is due to negative charge. If the

grid IS made positive, it would encourage negative

charge to move to the plate and we would expect the

current to increase. If the grid is made negative, it would

discourage the negative charge from flowing and the

current would fall.

On the other hand, suppose the current is due to

positive charge. If the grid is made positive, it would

discourage the flow of positive charge and the current

would fall, the opposite to the above. If the grid is made

negative, it would encourage positive charge and the

current would increase.

THE

THERMIONIC

EFFECT

=O - 25V

}-----j ~- - - 5 0 0 V -II------..J

When the experiment is done, with a fixed voltage o

400 volt applied to the plate, the current readings are

as follows:

Grid Voltage/V

Curr en t lm .A

4

0.6

o

0.4

-4 -8

0.2 0.04

+8

0.8

This confirms that the current must be due to negative

charge flowing from the filament to the plate.

That negative charge or negative ions are released

from the hot filament when it is heated led to this being

called the thermionic effect. In fact, as we shall see later,

it is electrons that are emitted and they are often

referred to as thermionic electrons from the nature o

24


27/68

THE

THERMIONIC

EFFECT

their orrgm, though of course they are no different

from any other electrons.

USES OF THE TRIODE

The fact that a small change in the voltage applied to

the grid can produce a large change in the current

through the tube led to the extensive use of the triode,

for example as an amplifier. The effect is particularly

pronounced when the grid is placed near to the fila-

ment. The large tubes described here, which you wil

have seen in your laboratory, have been specially

designed for teaching purposes; valves used in

electronic equipment are generally much smaller

and more compact. The photograph on the left shows

a triode valve.

Valves, however, have now become almost obsolete

for most applications; transistors have taken their

place. Not only is the transistor smaller and more

robust, but it does not require any voltage supply to

heat a filament and it is consequently more efficient.

OTHER EXPERIMENTS WITH

THERMIONIC VACUUM TUBES

You have probably seen the Maltese-cross tube. The

charge given off by the hot filament is accelerated

towards the anode, and passes through the hole in it

A metal Maltese cross is fixed in the beam.

When the filament is heated to white heat, but before

the accelerating voltage is applied, an optical shadow

of the cross will be seen on the end of the tube due to

the light from the filament. When the voltage is applied

the end of the tube glows with a distinctive green glow

produced when the electrons hit the glass which is

coated with a fluorescent material such as barium

platinocyanide. A sharp image of the Maltese cross will

be apparent on the screen: the cross has obstructed the

electron flow.

25


28/68

THE

THERMIONIC

EFFECT

It is interesting to bring a magnet near the beam

The optical shadow is unaffected as a magnet has

effect on light, but the shadow due to the electron beam

is easily deflected. The photograph below shows th

two shadows.

26


29/68

THE

THERMIONIC

EFFECT

The tube illustrated below is based, on the origina

experiment of the French scientist Perrin. The electrons

from the hot filament again pass through a perforated

anode and produce a fluorescent spot on the end of th

tube. At the side of the tube there is a small collecting

cylinder which can be connected through the outpu

terminal to an electroscope. The beam is deflected b

a magnetic field so that the electrons are collected

the cylinder and it is found that the electroscope

becomes negatively charged.

Another interesting tube is illustrated below,

which there is a vertical screen set inside the tube at

slight angle. The anode has a horizontal slit in

This results in a horizontal line along the centre of th

screen when the electron beam strikes it. (Can you se

2


30/68

deflecting

voltage

from the drawings opposite why the screen is inclined?)

Above and below the screen are two horizontal plates.

If a potential difference is applied between the plates

so that the lower plate is positive relative to the top,

there will be a downwards electric force on the moving

negative charge, which will move in a curved path.

As the force is a constant downward force (as long a

the electrons are in the field) and as the electrons

initially move in a horizontal direction, the path wil

be a parabola, exactly as it is a parabolic path for

ball moving horizontally under the influence of

constant downward gravitational force.

o o m l l l m 9 1 1

parabolic path

circular path

You know already that if a current flows in a wire a

right angles to a magnetic field there will be a force on

it at right angles both to the current and to the magnetic

field. A similar force will act on a stream of electrons

which moves at right angles to a magnetic field, even

though the charge is flowing in space and not along

wire. This can be demonstrated with the above tube.

It is convenient to produce a uniform magnetic field

28


31/68

rawing to show why the screen is

by putting a coil on either side of the tube and passing

a current through them. The magnetic field is then

across the path of the electron stream, the force is at

right angles to the path and the beam will move either

upwards or downwards depending on the direction of

the field. This time the force is always at right angles to

the beam, so the path will be circular.

no field applied

field applied

source

CONCLUSION

This sequence of experiments tells us:

(1) that negative charge is given off by the hot filament,

(2) that it travels in straight lines (the Maltese-cross

experiment),

(3) that the charge can be deflected in both electric

and magnetic fields, whereas light cannot be deflected

in this way.

On the other hand, it has not told us that the charge

given off consists of particles. It could, for example,

have been a continuous 'juice'. The crucial experiment

which confirms that charge is particulate (in other

words, that electrons exist) is discussed in the

important section which follows.

29


32/68

THE MILLIKAN

EXPERIMENT

3 0

You may have seen in your laboratory an experimen

in which a light metallised sphere is suspended from

light glass spiral spring between two large horizonta

plates. The sphere is charged and it hangs centrall

between the plates under the balancing forces of gravit

and the spring. If the sphere is negatively charged and

potential difference is applied between the plates

that the top plate is positive relative to the bottom

the sphere will rise under the influence of this extr

force. The support for the spring must be lowere

slightly to bring the sphere back to the middle again

The greater the voltage applied, the greater is th

electric force on the charged sphere, and the more th

support must be lowered. Similarly if the potentia

difference is reversed, there will be a force downwards

on the sphere and the greater the voltage the greate

the force. This principle is used in the Millikan exper

ment in which a small charged drop is used instead

the metallised sphere.


33/68

THE MILLIK N

EXPERIMENT

upward force

due to electric field

charged drop

downward force

due to gravity

A version of the Millikan apparatus

This version of the Millikan apparatus

can be seen in use in the Nuffield film

Are There Electrons? obtainable from

the Rank Film Library.

THE EXPERIMENT

Millikan did his first experiment in 1913 with two me

plates 16 mm apart. He sprayed oil drops through

small hole in the top plate and in the process som

would become charged by friction. They w

illuminated from the side and viewed through

microscope as they fell under gravity. The fall of

charged drops could be controlled by voltages appli

between the plates. His experiment achieved t

things: it showed that electric charge always appeare

as a definite multiple of a single basic charge and

enabled that basic charge to be measured.

The apparatus above is a school version of

Millikan apparatus. :;: In this a potential difference

applied between the plates is adjusted so that

upward electrical force on a charged drop is exac

equal and opposite to the downward force due

gravity, and the drop neither rises nor falls.

Concentrating on the one drop, it is briefly irradiate

by a radioactive source (Millikan used X-rays for t


34/68

purpose). This may have the effect of changing the

charge on the drop (in fact, by knocking off or adding

one or more electrons). A different voltage is then

required to balance it. The experiment can be repeated

many times and a whole series of voltage readings

obtained depending on the different amount of charge

on the drop.

A

typical set of voltage readings when

studying one .drop is: 226,452,361,301,604,449,905,

303, 450, 1805, 904 volt.

What is significant is that certain definite voltages

seem to occur rather than any arbitrary values. They

group together: 226; 301, 303; 361; 449,450,452;

604; 904, 905; 1 805. Let us look at this in more detail.

HE MILLIK N

EXPERIMENT

THE THEORY OF THE EXPERIMENT

If the potential difference between the plates is V, we

know that the energy transferred in taking charge q

from the lower plate to the top plate is Vq (from the

definition of a volt as 1 joule per coulomb).

We know there is an electric force

F

on the charge

drop. So we can also calculate the energy transferred

by saying that it is

Fx d,

where

d

is the distance apart

of the plates. (Normally we measure in SI units, so that

d

will be in metres and

F

in newtons. The energy trans-

ferred, Fx d, will thus be in newtons

X

metres, i.e. in

joules, since a joule is

1

newton

X 1

metre.) Thus

Fd

must equal Vq. This gives us an expression for the force

on the charge drop, namely

F= Vq

d

Note that this agrees with what we found in the

experiment with the metallised sphere described at the

beginning of this section, namely that the greater

V

the greater is the force F.

In the Millikan experiment with the oil drop, the

upward electric force is balanced by the downward

force due to gravity, namely mg.

mg= Vq

d

hus:


35/68

THE MILLIKAN

EXPERIMENT

From which we find that

Vq

=mgd.

For a single drop in a particular apparatus mgd is a

constant, so that we would expect

Vx q

always to have

the same value. Let us now look again at the voltages

listed above.

RESULT OF THE EXPERIMENT

We have set out the voltages in the first column. When

we multiply them by the whole numbers in the second

column we get the number in the third column.

Voltage V

1805

9 4

9 5

6 4

452

450

449

3 61

3 03

3 01

226

Charge q

1

2

2

3

4

4

4

5

6

6

8

Vxq

1

805

1

808

1

810

1

812

1 808

1800

1 7 9 6

1 805

1

818

1

8 6

1 8 0 8

Within the limits of experimental error the values in

the last column are always the same. We notice the

values of the voltage are such that the charge is always

an integral multiple of one basic charge.

q

never turns

out to be a fraction of this charge, but is always a

whole-number multiple of it.

In other words, charge always seems to come in

definite lumps. It suggests that charge is particulate

and we call this basic charge the charge on the electron.

This confirms that charge cannot be a continuous

'juice' .

CONCLUSION

From this type of experiment it is possible to calculate

the actual charge on the electron, and this is found to

be l.60 X 10-

19

coulomb. But the most significant thing

is that it confirms the existence of this basic unit of

charge and that all charges are direct multiples of it.

33


36/68

THE MASS OF

TH E ELECTRON

l fine beam

:

I

~ ,:\ conical

300V:: ~ : anode

I I

---L- I

~ r t ] filament

The mass of the electron is so small that it would b

quite impossible to measure it directly. But fortunately

measurements of the deflection of electrons in magnetic

and electric fields enable us to measure the value

e/ m, the charge on the electron divided by its mas

This may not seem very interesting at first sight, b

when this is combined with the value of e which w

have obtained from the Millikan experiment, we ca

calculate the value of m.

THE FINE-BEAM TUBE

The fine-beam tube, which we use in measuring e/ m

contains a hot filament from which electrons are give

off. Above the hot filament is a conical anode, such tha

when a positive voltage is applied to it the electron

are accelerated rapidly towards it and a fine beam

passes through a hole in the top so that they can trave

onwards into the tube with a constant velocity.

A particular feature of the fine-beam tube is that

contains hydrogen at very low pressure. When th

electrons collide with the hydrogen they ionise

When the ions recombine afterwards, they give o

a faint light so that the path of the electrons is clearl

visible in the tube. We do not see the electrons them

selves, merely where they have been, in much the sam

way that in a cloud chamber you are not seeing alph

particles but their tracks.

34


37/68

THE MASS OF

THE ELECTRON

Round the tube are placed two large coils

illustrated. When a current is passed through these,

uniform magnetic field is produced at right angles

the electron stream. Consequently the electrons mov

in a circular path. The greater the current in the coil

the greater the magnetic field, the greater the force o

the electrons and the greater the curvature of the path

There will come a stage when the electrons can mov

round in a complete circle inside the tube. Th

diameter ofthis circle can be measured.

THE SPEED OF ELECTRONS

Suppose the voltage applied between the filament an

the anode is

V.

This means that for a charge of

coulomb the energy transferred is V joule. For

electron with charge e coulomb the energy gained w

be Ve joule. This will therefore be the kinetic energ

Gmv2)

of the electron if it leaves the wire with negligibl

kinetic energy.

Thus ~mv2 =Ve

and this gives the velocity of the electron as

v=V2~e


38/68

THE MASS OF

THE ELECTRON

6

THE FORCE ON THE ELECTRONS DUE

TO THE MAGNETIC FIELD

Suppose the force on the electrons due to the magnetic

field is F. The electrons move in a circle whose radius

is

Y.

From our knowledge of circular motion this means

that

This force depends in some way on the strength of the

magnetic field, and to find out more about it we have

to look first at the force on a current-carrying con-

ductor in such a magnetic field.

You will remember the experiment on the left. The

force on the wire was at right angles to the magnetic

field and to the direction of the current. It is found that

the force depends on the current and the length of the

conductor in the field. In other words,

F=BIL

where

F

is the force (in newtons),

I

is the current (in

amperes), L is the length (in metres) and B is a force

constant dependent on the strength of the field. In later

work B is used as a measure of the strength of the field.

How can we measure this force constant B? The

apparatus used is shown below. There are the same

- battery to

- provide current

-=- through coil


39/68

THE MASS OF

TH E ELECTRON

~

iF = IL

~

i.,

7\

mg = F = IL

= mg

IL

two coils producing the magnetic field which deflected

the electrons, but the fine-beam tube has been removed

and a current balance put in its place. (The washer

suspended by cotton from the end of the balance hang

in water with a little detergent in it, and helps to damp

the balance, so that it does not sway too easily.) Th

wire frame is balanced without the magnetic field

When the field is switched on, the force acts and th

balance is destroyed. A small piece of copper wir

(a rider) is added to the wire frame and the current

adjusted so that the balance is restored. As we know th

mass of the rider, the force can be calculated. As th

current I can be found with an ammeter and L can b

measured, we have a value for the force constant B

However, there is one thing more we need ver

badly. We have considered the force on a current i

a wire. We now need to find the force on a single electron

in the magnetic field, and this important step needs th

following argument.

I observer

-l - watching here

n electrons

A B

at time 0

L

A

B

after time t

L

. /

Consider a section AB of the wire of length L

Suppose there are n electrons in this length each with

charge e, and that when there is a current

I

in the wire

it is due to the electrons moving along the wire wit

speed

v .

Imagine an observer at the point shown who counts

the electrons as they pass him. An electron which is a

A when the observer starts counting will pass him afte

some time t having travelled the distance L with speed

v . In that time t all the n electrons in AB will have

3


40/68

passed the observer. Thus in time t he counts a tota

charge of ne passing and he says that the current is th

charge flowing per unit time, nett, But L = vt. So tha

IL = nte)

X

vt =nev

We have already discussed the force on the con

ductor as being BIL, and this therefore equals Bnev

But there are n electrons in this length. Therefore th

force on a single moving charge is Bev.

CALCULATION OF m

Now we have exactly what we needed. When th

electrons move in a circle, the force on them, which

mv?

equals (-, will be Bev. We have already shown on pag

r

35 that v= V 2~e.

Substituting this into our new equation,

mv

Bev=-

r

e: 2V

m - B2

r

2

we find that

V is the accelerating voltage, which can be measured

with a voltmeter.

B

is found with a current balance a

described above. r is the radius of the electron path an

can be measured. This enables us to calculate elm

The generally accepted value of elm is

1.76x 10

coulomb per kilogramme.

THE MASS OF THE ELECTRON

The above gives us elm. The Millikan experiment tol

us that e was

1.60 X 10-

19

coulomb. From these two w

can calculate a value of m.

Thus m

=1.60 X 10-

19

-

9

X 10-:)1

kg

1.76x10

11

-

and you will probably agree that it is remarkable to b

able to measure something as small as this.

3 8


41/68

THE M SS OF

THE TOM

: Cjkg

=

coulombs per kilogramme.

MASS OF THE PROTON

The experiment on electrolysis already mentioned o

page 18 enables a value to be calculated of e/ M fo

hydrogen ions. We find how much hydrogen is release

by a certain current in a definite time. The charge tha

passes can be calculated as it equals the current X time

It is found in such an experiment that 1.008 kg

hydrogen is released by 96.5 million coulomb.

This gives a value of 95.7x lO G C/kg for hydrogen

ions. ,;:We now believe that a hydrogen ion consists

a hydrogen atom which has lost an electron; it

usually called a proton. The positive charge of th

hydrogen is thus numerically equal to the charge

the electron: e is the same for both. We can therefore

compare the mass of the electron with the mass of th

proton, since e/ m for the electron is 1.76 X lO ' C/k

and

e

M

for a proton is 95.7

X lO G

C/kg. This shows tha

the mass of the electron is only about 1/2 000 (

more accurately 1/1 840) of the mass of the proton.

We can summarise our knowledge as follows:

Charge of electron

Mass of electron

Mass of proton

Mass of electron

Mass of proton

1.60

X

10-

19

C

9 X 10-

31

kg

1.67 X 10-

27

kg

1

1840

THE AVOGADRO CONSTANT

In your chemistry you will have heard of 1 mole of

substance. The mass of 1 mole equals the relativ

molecular mass of the substance in grammes. Thus

mole of hydrogen has a mass of 2 g, 1mole of nitrogen

28 g, and 1 mole of oxygen 32 g.

As there are two hydrogen atoms in a hydrogen

molecule, the mass of one hydrogen molecule

2 X 1.67 X 10-

27

kg. We can now deduce the number

molecules in 1 mole of hydrogen. This is

2 g 6

X

lO>' molecules per mole

2 X 1.67 X 10-

2

-1 g


42/68


43/68

%

%

% %

i

H

99·98

I ~ G O

99·76

1 1 2 S

n

1·1

~ ~ O } P b

1·5

,0

iH

0·02

1 ~ 7 0

0·04

1 1 4 S

n

0·8

~ , ? : ? P b

23·6

)O

1 . ~ 8 0

0·20

1 1 5 S

n

OA

~ , ? ; P b

22·6

;0

~Li

7·9

I } G S

n

15·5

~ ~ 0 2 8 P b

52·3

,0

~Li

92·1

TgNe

90·00

1 _ 1 7 S n

9·1

,0

nNe

0·27

~ ; J 2 4 U

0·006

lO B

20·0

1 1 8 S

n

22·5

s

[)O

7 6

Ne

9·73

~ ~ } U

0·720

\ I B

80·0

1 _ 1 9 S

n

9·8

,0

~ ~ } ' U

99·274

n S

95·0

1 _ 2 0 S n

28·5

, 0

lG

2 C

98·9

l i S

0·74

1 _ 2 2 S n

5·5

,0

16

3 C

1· 1

n S

4·2

\ 2

0

4 S

n

6·8

1 7

4N

99·62

f ~ S

0·016

?N

0·38

r ~ C I

75·5

n C I

24·5

4


44/68

J. J. THOMSON

AND THE

ELECTRON

1.

J.

Thomson (left) with Ernest

Rutherford

cathode

anode

to pump

Discharge tube

4 2

This book would not be complete without reference t

J. J. Thomson, to whom is ascribed the discovery o

the electron. He was born in 1856, went to Owen

College, Manchester, and then to Trinity College

Cambridge. At the age of twenty-eight he succeeded

Lord Rayleigh as Cavendish Professor of Physics. I

was an unexpected appointment and remarks wer

made about 'mere boys being made professors'. Bu

during J. J. Thomson's professorship, the Cavendish

Laboratory in Cambridge became perhaps the mos

famous laboratory in the world.

CATHODE RAYS

It was through the study of cathode rays that J. J

Thomson was led to suggest the existence of electrons.

In

the year before he was born the German physicist

Heinrich Geissler produced a vacuum pump capable o

producing pressures much lower than had ever been

achieved before. Such a pump enabled Geissler's friend

Plucker to discover that electricity could flow through

an evacuated tube into which electrodes had been fixed


45/68

cathode

Crookes Maltese-cross tube

Thomson s original apparatus

when a high voltage was connected across it. In 187

Sir William Crookes developed tubes which established

that the glow visible on the end of the tube was pro

duced by something coming out of the cathode an

travelling down the tube to hit the glass wall. Th

radiation came to be known as cathode rays. For twent

years there was controversy as to whether these ray

were electromagnetic waves (like light) or particles.

It was 1. 1. Thomson who finally resolved the issue

His paper on cathode rays in the Philosophica

Magazine of October 1897 begins as follows:

The experiments discussed in this paper were undertaken

in the hope of gaining some information as to the nature

the cathode rays. The most diverse opinions are held as

these rays; according to the almost unanimous opinion

German physicists they are due to some process in th

aether to which - inasmuch as in a uniform magnetic fie

their course is circular and not rectilinear - no phenomenon

hitherto observed is analogous; another view of these rays

that, so far from being wholly aetherial, they are in fa

wholly material, and that they mark the paths of particles

of matter charged with negative electricity. It would seem

first sight that it ought not to be difficult to discriminate

between views so different, yet experience shows that this

not the case, as amongst physicists who have most deeply

studied the subject can be found supporters of eithe

theory .

In those days it was considered necessary to have an aether as a medium

which electromagnetic waves could travel.

4


46/68

J J THOMSON

ND THE

ELECTRON

undeflected

in electric field alone

magnetic field alone

The crucial point in Thomson s work

was getting a better vacuum. It was this

that enabled the electrostatic deflection

to be observed. Previous scientists had

tried this, but the pressure of residual

ionised gas neutralised the electro-

static field and no deflection was

observed. It was this that led to the

divergence of views mentioned in the

quotation above.

44

MEASUREMENT OF

m

His paper continues by describing his experiments in

which a beam of rays from a cathode c were deflected

by the electric field between plates

d

and e. The deflec-

tion of the spot on the screen was measured. ,;:

A

magnetic field, produced by coils placed around the

tube, was then superimposed so that the spot was no

longer deflected. This experiment enabled Thomson to

calculate the velocity of the particles and hence to

deduce a value for

elm,

where

e

is the charge and

m

the

mass of each cathode ray particle. From his experi-

ments, he found that the value of

elm

was independent

of the nature of the gas in the discharge tube and that

its value was much larger than that of the hydrogen

ion found from electrolysis. This could be due either to

a larger size of

e

or a smaller

m.

His paper continues:

The explanation which seems to me to account in the most

simple and straightforward manner for the facts is founded on

a view ... that the atoms of the different chemical elements

are different aggregations of atoms of the same kind. If, in

the very intense electric field in the neighbourhood of the

cathode, the molecules of the gas are dissociated and are

split up, not into the ordinary chemical atoms, but into

these primordial atoms which we shall for brevity call

corpuscles; and if these corpuscles are charged with

electricity and projected from the cathode by the electric

field, they would behave exactly like cathode rays.

Thus on this view we have in cathode rays matter in a new

state in which the subdivision of matter is carried very much

further than in the ordinary gaseous state; a state in which

all matter - that is, matter derived from different sources

such as hydrogen, oxygen, etc. - is of one and the same kind;

this matter being the substance from which all the chemical

elements are built up.

J. J. Thomson also made measurements of the charge

e.


47/68

J J

THOMSON

ND THE

ELECTRON

The method he used was not very accurate (accuracy

had to wait until Millikan began his series of experi-

ments in 1909), but it enabled him to show that the

mass m of the cathode rays was much less than that of

hydrogen ions. These small negatively charged

'primordial corpuscles' came to be known as electrons.

THE ELECTRON AS A CONSTITUENT

OF ALL MATTER

Thomson found that the value of elm was always

the same whatever the residual gas in the discharge

tube and whatever the material of which the cathode

was made. His quantitative experiments suggested that

the electron was a common constituent of all kinds of

matter. Support for Thomson's suggestion, based on

his work with gases, came from others who studied

electrons from metals (photo-electrons and thermionic

electrons). In all cases the value of elm was the same.

In 1906

J. J.

Thomson received the Nobel Prize and

in 1908 he was knighted. He was President of the

Royal Society from 1915 to 1920 and Master of

Trinity College, Cambridge, from 1918 until his death

in 1940. But he was probably never happier than when

he was working in the Cavendish Laboratory and it was

his discovery of the electron which marks the begin-

ning of a new epoch in physical science.

45


48/68

Our first model of the atom pictured it as consisting

of a positive and a negative part. This was sufficient

explain the facts of ionisation. After the discovery

the electron J. J. Thomson came to picture the atom

MODELS OF

THE ATOM

an impenetrable sphere, positively charged, wit

negative electrons embedded in it: the 'plum pudding

model, which was accepted until Rutherford replaced

it by another.

NUCLEAR MODEL OF THE ATOM

source of

o :

particles

The history of the nuclear model began in 1909 whe

Geiger and Marsden were investigating what happened

when fast-moving alpha particles were directed at gol

foil. They found that a small proportion of the particle

was deflected through angles greater than 90

0

•

A

Rutherford said long afterwards, this was 'almost a

incredible as if you had fired a IS-inch shell at a piec

of tissue-paper and it came back and hit you'. Th

number scattered was small (only 1 in 8000 through a

angle greater than 90

0

, but it was characteristic o

Rutherford to appreciate the significance of these few

microscope

He showed that such large-angle scattering could b

explained only if the alpha particles were able to mov

4 6

,

\

,

I

f \

\

f

,

\

\ I

I

\ I

- . ,//

~

,,

•• • _

--t----- e

deviation of alpha particles

in Thomson atom

deviation of alpha particles

in Rutherford atom


49/68

MODELS OF

THE TOM

within a very close distance of a positive charg

contained within a very small volume at the centre

the atom, in fact a distance very much closer than t

size of the atom itself. For this to be possible Ruther

ford proposed the nuclear model of the atom consis

ing of a very small positively charged nucleus at

centre with electrons round it like planets round th

sun.

size of atom In this model a hydrogen atom of diameter abou

(' V10-

IO

m) 3 X 10-

10

m has a nucleus about 3 X 10-1:, m in diameter

The diameter of the nucleus is approximately 1/100000

of the diameter of the atom, the volume is 1/10

15

th

volume of the atom and the rest is empty space. If th

nucleus were represented by a ball 5 em in diameter

the atom as a whole would be a virtually empty spher

5 km in diameter, assuming that the size of the ato

is determined by the rotating electron.

The chemical properties of an atom are determined

by the number of electrons around the nucleu

Hydrogen has 1 electron, helium 2, lithium 3, ..

carbon 6, nitrogen 7, oxygen 8, ... and so on up

uranium with 92 electrons (and now transuranic

elements have been produced, each having successively

one more electron). As the atom as a whole is electric

ally neutral, the nucleus must have a positive charg

numerically equal to that of the electrons. Thus th

positive charge on the hydrogen nucleus is 1 uni

helium 2, lithium 3, ... carbon 6, nitrogen 7, oxyge

8, ... uranium 92.

After the discovery of the neutron by Chadwick

1932 it was accepted that the nucleus was made

protons and neutrons. Both these particles hav

approximately equal masses; the neutron has n

charge, whereas the proton has a positive charge equ

and opposite to the charge on the electron. The oxyge

atom, which is eighth in the periodic table, has eig

electrons around the nucleus. The nucleus, therefore

has a charge of

+

8 units and this is provided by 8 pro

tons. But the mass of oxygen is 16 units and this mean

that there must be 8 neutrons in the nucleus to bring th

mass to 16. The heavier isotope of oxygen - oxygen 17


50/68

MODELS OF

THE TOM

Nuclei

will have the same number of electrons and the same

number of protons, but one extra neutron to bring the

mass to 17. Similarly uranium has 92 electrons in the

outer part of the atom, 92 protons in the nucleus to

make the charge right and 146 neutrons to bring the

mass to a total of 238 units.

1

1 proton

H

•

o

neutron

1

1

2

1 proton

H

1 neutron

1

2

16

8 protons

0

8 neutrons

8

'-6

1 electron outside nucleus

1 electron outside nucleus

8 electrons outside nucleus

17

o

8

8 protons

9 neutrons

17


92 protons

146 neutrons

238


THE BOHR MODEL OF THE ATOM

The idea of electrons moving in orbits round the

central nucleus was attractive: it was similar to the

solar system except that each planet was kept in orbit

by a gravitational force whereas in the atom it was the

electrical force between the orbiting negative electron

and the positive nucleus.

There was, however, a serious snag. If electrons

move up and down in an aerial, they radiate energy.

Imagine an electron moving in a circle round a nucleus:

viewed from one direction, it would be moving up and

down. If it were radiating its energy away, it would


51/68

MODELS OF

THE TOM

soon collapse into the nucleus and the atom would

cease to exist - and yet our experience is that such an

atom lasts indefinitely Why, therefore, does the

orbiting electron not radiate its energy?

i

--------)

radiation

is emitted

electrons moving

up and down

in aerial

//

• . . . .

- --- - ,

// e

I \

I \

I I

I I

\ I

\ I

\ I

,

/

-, -7-- /

electron in orbit why is no radiation emitted?

iew of

electron orbit

sideways on

The Danish physicist Neils Bohr developed a new

model of the atom to overcome this difficulty. The

essence of his model is that the electron going round

the nucleus moves in an ellipse, but it cannot have just

any orbit; there are certain definite ones each of which

corresponds to a definite energy. Bohr's theory

assumed that when an electron is in a particular orbit

with a particular energy it does not send out radiation:

energy is only given out when the electron changes

from one orbit to another with a different energy.

Typical Bohr models of the atom might be as shown

opposite.

Many a book that wants to appear modern, many a

toy that calls itself an 'atomic ray gun', many an adver-

tisement that wishes to appear up-to-date puts on the

container or cover a drawing like that on the left which

is meant to be a Bohr model of the atom. It is perhaps

unfortunate that those responsible for the advertise-

ments do not realise that the Bohr model is in fact only

a model originally suggested in 1913, and that since

1926 it has been replaced by yet another model dis-

cussed below.

49


52/68

Models of Bohr orbits in the Science

Museum, London

THE WAVE-MECHANICAL MODEL

(This section is not easy to understand and is merel

included here for the sake of completeness. It woul

be a pity to leave the impression that the Bohr mode

was the end of the story, because the wave-mechanical

model has now replaced it. Do not worry about an

detail of this new model.)

In 1927 it was discovered that electrons could b

diffracted. In other words it was shown that electrons

had wave properties even though it was 'proved' b

Millikan's experiment that they consisted of particles

This wave-particle duality is one of the strange thing

about modern physics: electrons are shown by on

experiment to consist of particles, by another

consist of waves. ':' This is a fact of life which has to b

accepted even though at first it sounds sel

contradictory.

If electrons can be thought of as waves, how will th

modify our model of the atom? Bohr's model of th

atom was a good model because it was useful an

helped towards a further understanding of the atom

But there were the strange hypotheses made by Boh

about electrons having certain definite orbits wi

certain definite energy states. It was the new model

the atom developed as a result of Schrodinger's wor

in 1927 that gave the clue to why these energy state

existed.

It would not be appropriate to describe in detail th

wave-mechanical model of the atom, but suffice it

say that this model, based on the idea of electron

having wave as well as particle properties, has no

replaced the Bohr model as it has usefully explained

: For further details read Waves or

Particles in this series.

50


53/68

those things which appear inexplicable on the Bohr

model. The electron is no longer a particle in orbit;

instead we have a probability distribution that it is at a

distance r from the nucleus, and although we cannot

know precisely where it is, we do know exactly what its

energy is. The interesting thing is that the maximum of

the probability distribution curve for hydrogen shown

below occurs at a distance which turns out to be the

distance of the first Bohr orbit.

MODELS OF

THE TOM

c:

o

.~

.0

. • . . .

.

~

-0

.?

i5

c o

.0

o

0.

distance

THE FUTURE

The important thing to realise is that no one would

claim that the wave-mechanical model of the atom is

the final correct one, but it provides a convenient model

which fits the known facts as at present observed.

Physicists are now much too modest to think they know

the ultimate truth.

51


54/68

ELECTRONS

AT WORK

52

This book has until now been concerned with th

evidence which led towards a deeper understanding

of the atom. The tiny electron may have seemed a

unreal part of an academic exercise. On the contrary

the electron is continually being put to our use. Th

moving electron is responsible for all the electrica

devices we take so much for granted in our daily lif

It makes possible the sending-out and reception

radio waves. Without it a car engine would not wor

and a computer would be impossible. Its motio

provides our homes with heat and light at the touch

a switch.

It would be an impossible task in this short chapte

to list all the useful jobs the electron does, but we sha

refer briefly to certain devices which make use

electron streams. Pride of place will be given to tha

invaluable tool of the physicist, the cathode-ray

oscilloscope (c.r.o. for short), which will be found

every physics laboratory in the world.

THE CATHODE-RAY OSCILLOSCOPE

In an oscilloscope a hot cathode gives off electron

which are accelerated towards an anode with a centra

aperture. They pass through and travel to the coate

screen, which fluoresces, or glows, when and wher

heated focussi ng deflecti ng

filament anode plates

:-1 I O J -

L7

control

grid

accelerati ng

anode

fluorescent scree

the electrons hit it. The brightness of the spot w

depend on the number of electrons striking the screen

This can be controlled by a grid near the cathode:


55/68

the grid is made negative fewer electrons get through

and the spot is dimmer. The

BRIGHTNESS

knob on the

oscilloscope controls this.

Before reaching the anode, the beam of electrons

usually passes through a focusing cylinder, to which

a positive voltage is applied. This tends to concentrate

the beam. There is an optimum voltage for it: too little

and the beam is not focused, too much and the spot

will again be blurred. This voltage is controlled by the

FOCUS

knob on the front of the oscilloscope.

Inside the cathode-ray tube are two pairs of deflect-

ing plates; one pair will deflect the beam horizontally,

the other vertically. The deflection is proportional to

the voltage applied and this enables the oscilloscope

to be used as a measuring instrument. For example,

.0,5

if it is known that

20

volt deflects the spot

1

em,

40

volt

2

cm and so on, an unknown voltage can be applied

·0,2 and from the deflection its magnitude can be found.

In

order that small voltages may be measured it is usual

·0.1

for an amplifier to be included in the oscilloscope.

A voltage selector switch allows different sensitivities.

53

not focussed

focussed

not focussed

volt/em

2

10.

20·


56/68

If d.c. voltages are applied to the deflection plate

the spot will move up or down depending on whic

way the voltage is applied. If a.c. voltages are applie

the spot will move rapidly up and down and a line w

appear on the screen.

ELECTRONS

AT

WORK

no voltage

applied

d.c. voltage

applied to plates

a.c. voltage

applied to plates

Incorporated in an oscilloscope is a time-base circui

This applies a steadily increasing voltage to t

X-plates (those that deflect the spot horizontally)

that the spot sweeps across the screen at a steady ra

until, at a certain point, the voltage flies back to

original value and the spot consequently returns rapid

to its starting point. The flyback time should be as sho

as possible; it is also usual for an internal arrangement

to reduce or 'suppress' the flyback trace so that it is n

visible. (You can sometimes see the flyback trace if t

brightness is turned up to its maximum value.) T

sweep speed should be variable so that the frequenc

can be changed over a wide range. The voltage vari

tion might therefore be as shown above: it is obviou

why it is called a saw-tooth voltage.

If an a.c. voltage at 50 Hz (50 cycles per second)

applied to the Y-plates and the time-base is n

switched on, the trace will be as (i) opposite. If t

time-base is switched on at a frequency of 25 Hz, t

trace will be as in (ii); if the time-base frequency

50 Hz, it will be as in (iii); if the time-base frequenc

is 100 Hz, it will be as in (iv).

54


57/68

ELECTRONS

AT WORK

(i) (ii) (iii) (iv)

The action of a rectifier can be shown easily on an

oscilloscope.

In

the circuit below, a rectifier is put in

series with an a.c. voltage supply and a resistor.

Connected as shown, the oscilloscope will show the

voltage across the resistor. As the resistor has a fixed

resistance, the voltage will be proportional to the

current, so that the trace will show how the current

passing through the rectifier varies with time. (Note

that this is the usual technique for getting an oscillo-

scope to show how current varies with time.) If the

rectifier is reversed, the trace will appear the other way

up.

o

o

o

o

88

a.c.

d.c.

School oscilloscopes usually also have an

AC/DC

switch on the front panel.

In

the

DC

position the spot

will be deflected by both a d.c. voltage and an a.c. one.

When in the AC position a d.c. voltage has no effect

and the spot is only deflected by an a.c. signal.

In

some ways it is inaccurately named an

AC/DC

switch:

it would be more correct to call it an AConly/ric and

AC switch' To illustrate its use, consider the circuit

below in which an a.c. and d.c. voltage in series are

connected to the input.

In

the DC switch position both

01

: In practice it will be found that a very

slow a.c. signal (2 Hz or less) probably

will not show on the screen in the A C

switch position. A t these very low

frequencies it may be necessary to use

the DC switch position.

rll~.g

CJ

L

~~O 0 0

55


58/68

the a.c. and d.c. voltages will deflect the spot, so that

the trace will be as shown on the left. In the

AC

switch

position only the a.c. voltage acts, so the trace is as

on the right.

There is usually one more facility on school oscillo-

scopes - a Z-input on the back of the oscilloscope.

An input voltage here is superimposed on the grid

inside the tube (see page 24) and this changes the

number of electrons streaming through the anode.

This affects the brightness of the spot on the screen.

If, for example, an alternating voltage at 20 Hz is

applied to this input, the brightness of the spot will

vary 20 times a second. This facility is of considerable

importance in the next device using electron streams,

the television tube.

ELECTRONS

AT WORK

THE TELEVISION TUBE

A television tube is basically a cathode-ray tube with

two time-bases. The first moves the spot horizontally

across the screen and then causes it to fly back very

quickly: this is called the line time-base and is just like

the time-base in an oscilloscope. The other is called

the frame time-base and this operates at the same time,

moving the spot more slowly down the screen until i

too flies back to the top again. In this way the spot

covers the whole of the screen as shown below. In

~,~=

john l. lewis - electrons and atoms

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