Download maitland/5230/41270 Ideas Part 1

Physics
HSC Course
Stage 6
From ideas to implementation
PHYHSC41270
XP005615
OTEN
Acknowledgments
This publication is copyright Learning Materials Production, Open Training and Education Network –
Distance Education, NSW Department of Education and Training, however it may contain material from
other sources which is not owned by Learning Materials Production. Learning Materials Production
would like to acknowledge the following people and organisations whose material has been used.
•
Physics Stage 6 Syllabus. Board of Studies NSW, 1999.
All reasonable efforts have been made to obtain copyright permissions. All claims will be settled in
good faith.
Writer:
Richard Morante
Editor:
Julie Haeusler
Consultants: Mike McPhee (Open Learning Program, OTEN-DE)
Colin McKay (Dubbo School of Distance Education)
Illustrator:
Thomas Brown
Copyright in this material is reserved to the Crown in the right of the State of New South Wales.
Reproduction or transmittal in whole, or in part, other than in accordance with provisions of the
Copyright Act, is prohibited without the written authority of Learning Materials Production.
© Learning Materials Production, Open Training and Education Network – Distance Education,
NSW Department of Education and Training, 2001. 51 Wentworth Rd. Strathfield NSW 2135.
Contents
Module overview
Outcomes ........................................................................................... iv
Indicative time ......................................................................................v
Resources ............................................................................................v
Icons .................................................................................................. vii
Glossary ............................................................................................ viii
Part 1: Discharge tubes ......................................................1–44
Part 2: The amazing cathode ray tube ...............................1–38
Part 3: Extraordinary science .............................................1–49
Part 4: Harnessing the electron ..........................................1–46
Part 5: Crystals ...................................................................1–36
Part 6: Superconductors for tomorrow today ......................1–40
Student evaluation of the module
Introduction
i
ii
From ideas to implementation
Module overview
‘Modern physics has taught us that the nature of any system cannot be
discovered by dividing it into its component parts and studying each
part by itself, since such a method often implies the loss of important
properties of the system. We must keep out attention fixed on the
whole and on the inter-connection between the parts. The same is true
of our intellectual life. It is impossible to make a clear cut between
science, religion, and art. The whole is never equal simply to the sum
of its various parts.’
Max Planck (1858-1947)
This module looks at the benefits, realised and potential that can come
from intellectual curiosity of scientists doing basic research. Without
these discoveries the world would be a vastly different place.
The interaction of science and society is complete. One cannot develop
as rapidly or as successfully without the other. Past science has laid the
foundations for the social dividend all of us enjoy today and will most
probably benefit from in the future. Enjoy this module and take the time
to think of the benefits of scientific research you accept as a part of your
daily life.
Introduction
iii
Outcomes
The outcomes to which this module contributes are:
A student:
H1
evaluates how major advances in scientific understanding and
technology have changed the direction or nature of scientific
thinking
H2
analyses the ways in which models, theories and laws in physics
have been tested and validated
H3
assesses the impact of particular advances in physics on the
development of technologies
H4
assesses the impact of applications of physics on society and the
environment
H5
identifies possible future directions of research in physics
H8
analyses wave interactions and explains the effects of those
interactions
H9
explains the effects of electric, magnetic and gravitational fields
H10
describes the nature of electromagnetic radiation and matter in
terms of the particles
H11
justifies the appropriateness of a particular investigation plan
H12
evaluates ways in which accuracy and reliability could be
improved in investigations
H13
uses reporting styles and terminology appropriately and
successfully to communicate information and understanding
H14
assesses the validity of conclusions drawn from gathered data
and information
H15
explains why an investigation is best undertaken individually or
by a team
H16
justifies positive values about and attitudes towards both the
living and non-living components of the environment, ethical
behaviour and a desire for critical evaluation of the
consequences of the applications of science.
Extract from Physics Stage 6 Syllabus © Board of Studies NSW, originally
issued 1999. The most up-to-date version can be found on the Board's website
at http://www.boardofstudies.nsw.edu.au/syllabus99/syllabus2000_list.html
iv
From ideas to implementation
Indicative time
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Introduction
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v
For Part 2 you will need:
•
access to a video and a prerecorded video tape
•
access to a television (nondigital)
•
access to an electrical goods store with a display of televisions
•
a bar magnet
•
access to a photocopier.
For Part 3 you will need:
•
an AM/FM radio with headphones
•
access to a tunnel or underground car park or building basement
•
a car
•
a friend with a licence to assist you
•
two electroscopes or two 10 cm nails, two identical glass soft drink
bottles, two strips of light thin metal foil, and cellotape
•
two sheets of zinc metal or two large galvanised washers
•
a weak acid
•
large glass jar or sheet of glass
•
a woollen jumper
•
stop watch or watch with a second hand
•
a polystyrene piece
•
an inflated balloon.
For Part 4 you will need:
•
a video camera (optional)
•
a ball
•
at least six chairs and six people to assist you.
For Part 5 you will need:
vi
•
access to an unpainted sheet of galvanised iron
•
a cheap diode laser pointer preferably with pattern heads
•
a CD
•
access to equipment in order to study the heating effect of a current
on conductors. This experiment is to be planned and performed by
you for submission with the exercises for Part 6.
From ideas to implementation
For Part 6 you will need:
•
a high temperature superconductor
•
liquid nitrogen
•
access to the Internet
Icons
The following icons are used within this module. The meaning of each
icon is written beside it.
The hand icon means there is an activity for you to do.
It may be an experiment or you may make something.
You need to use a computer for this activity.
Discuss ideas with someone else. You could speak with
family or friends or anyone else who is available. Perhaps
you could telephone someone?
There is a safety issue that you need to consider.
There are suggested answers for the following questions at
the end of the part.
There is an exercise at the end of the part for you to
complete.
Introduction
vii
Glossary
The following words, listed here with their meanings, are found in the
learning material in this module. They appear bolded the first time they
occur in the learning material.
viii
1-2-3 compound
One of the first superconducting ceramics with
the approximate composition YBa2Cu3O7, so
called for its relative atomic proportions of
yttrium, barium, and copper.
absolute zero
The theoretical limit to how cold any given
system can be. The point that all atomic and
molecular motion ceases. 0K = –273.15°C.
analogous
Comparable in function.
anode
Positive electrode.
atom
The smallest unit of any pure substance. All
matter is made up of different kinds of atoms.
An atom itself is made up of smaller particles,
such as protons, neutrons and electrons.
basic research
Research without a specific application or
purpose designed by the researcher to find out
about a phenomenon.
BCS theory
A theory that explains superconductivity in
terms of bound electron pairs that are formed
by the interaction of the electrons with a metal
lattice.
bias voltage
The voltage applied to the emitter of a
transistor.
capacitor
A device for storing electrical charge usually
consisting of metal plates separated by a gap.
The charge jumps that gap when it discharges.
cathode
Negative electrode.
ceramic
An earthenware or glass product made by firing
a mixture in a furnace to produce a
recrystallisation product. A ceramic is usually
a poor conductor of electricity although some
ceramics have superconducting properties at
low temperatures.
coherent
In phase.
composite video signal
All the signals necessary to produce movement
of the electron beam in a television cathode ray
picture tube to produce the picture.
From ideas to implementation
Introduction
Cooper pairs
A pair of electrons that experience an attractive
force due to phonons in the crystal lattice and
are coupled together. They are central to the
BCS theory and are thought to be carriers of
electric current in superconductors.
covalent bonding
Bonds between atoms that involve the sharing
of electrons from each atom to form electron
pairs.
critical current density
The maximum current a superconductor can
carry before its superconductivity breaks down
when it is below its critical temperature.
critical temperature
The temperature below which certain elements
or compounds become superconductors.
diamagnetic
Having a low susceptibility to a magnetic field.
drift velocity
Speed at which electrons actually travel along a
conductor.
electrical resistance
The loss of electrical energy caused by the
collision of electrons with lattice atoms during
the flow of electricity through a conductor.
electron gun
Assembly of positively charged plates (or
cylinders) and the filament that supplies
electrons in a CRO or CRT that makes and
accelerates the beam of electrons.
excites
Causes an emission of electromagnetic
radiation (light).
inductor
A device for producing a large voltage from a
lower voltage.
inert
Unreactive.
ion
An atom with either an excess or deficiency of
electrons. The atom therefore has an electric
charge.
Josephson junction
A superfast switch. It consists of a thin layer of
insulating material sandwiched between layers
of superconducting material.
Kelvin scale
A temperature scale using the same degrees as
the Celsius scale but with zero defined as
absolute zero.
light reflection
microscope
A microscope that produces an image from
light reflected from the surface of the object
being examined.
maglev (magnetic
levitation)
Repulsion of a magnetic field induces a lift of a
device from a surface.
ix
x
magnetic resonance
imaging (MRI)
An important tool to diagnose medical
disorders that uses radio waves and magnetic
fields to image the make up of tissues inside
the body without the need for operating.
median position
The middle point.
Meissner effect
The expulsion of all magnetic fields from the
interior of a superconductor resulting in the
repulsion of a magnet and magnetic levitation.
negative electrode
The electrode connected to the negative
terminal of a DC source. Cathode.
n-type semiconductor
Semiconductors that have been doped to give
an excess of negative charges.
oscillating
Vibrating back and forwards about a middle or
median point.
paramagnetic
A substance with significant magnetic
susceptibility.
pixel
One of the small dots used to make up a picture
in a picture tube.
phosphors
Small packets of phosphorescent material that
make up the pixels when they glow in a
cathode ray picture tube.
photocathode
A metal plate that emits electrons when
exposed to light energy.
photoconductivity
The ability of some substances to have
increased electrical conductivity on exposure to
light.
photon
Small packet of light energy.
photoresist
A chemical that hardens on exposure to UV
light forming a layer of silicon dioxide on a
semiconductor microchip.
photovoltaic cells
Semiconductor cells that produce an electrical
current when exposed to light.
positive electrode
The electrode connected to the positive
terminal of a DC source.
positive ion
Atom that has lost (an) electron(s).
p-type semiconductors
Semiconductors that have been doped to give
an excess of holes because of a deficiency of
electrons surrounding some atoms in the crystal
lattice.
quantised
Made into a discrete sized packet.
From ideas to implementation
Introduction
raster scan
The pattern of movement of the electron beam
in a television screen that produces the picture.
saturation current
The current maximum reached for the emission
of photoelectrons from a metal surface.
standing wave
A wave formed by the superposition of two
waves travelling in opposite directions that
does not appear to move.
stopping potential
The voltage needed to stop the emission of all
electrons from a photocathode.
superconducting
magnetic storage
(SMES)
A large underground loop of superconducting
coil used to store large amounts of electrical
energy potentially indefinitely as a DC current
continually rotating around the loop.
superconducting
quantum interference
device (SQUID)
A super sensitive device used to detect
extremely small magnetic fields in devices such
as a MRI.
superconductivity
The loss of all electrical resistance at very low
temperatures.
temperature
A measure of the average energy of a system of
atoms in a body.
threshold frequency
The frequency of radiation below which no
photoelectrons will be emitted from the surface
of a metal.
UV catastrophe
The term used to describe the failure of a
blackbody radiator to emit radiation that
destroyed the Universe when heated.
valence electrons
Electrons from the outer energy levels of an
atom involved in bonding.
work function
The energy required to release a photoelectron
from a particular metal or material surface.
xi
Physics
HSC Course
Stage 6
From ideas to implementation
Part 1: Discharge tubes
Contents
Introduction ............................................................................... 2
Discharge tubes ........................................................................ 4
Observing the phenomena ..................................................................4
Electric and magnetic fields..................................................... 17
Forces produced by fields.................................................................17
The q/m ratio of the electron .................................................. 25
Thomson’s experiment.......................................................................25
Summary................................................................................. 31
Suggested answers................................................................. 33
Exercises – Part 1 ................................................................... 39
Part 1: Discharge tubes
1
Introduction
In the first part of this module you will learn about the history of the
development of the cathode ray tube. This simple device that had its
beginnings as a scientific curiosity and fundamental research tool has led
to the development of the communication device with arguably the
greatest impact on modern society in the 20th tubes are used is huge.
Its impact on our daily lives has been nothing short of amazing.
During the course of your learning in this part you will have
opportunities to learn to:
2
•
explain that cathode ray tubes allowed the manipulation of a stream
of charged particles
•
identify that charged plates produce an electric field
•
explain why the apparent inconsistent behaviour of cathode rays
caused debate as to whether they were charged particles or
electromagnetic waves
•
describe quantitatively
the force acting on a charge moving through
r
a magnetic field, F = qvB sinq
•
discuss qualitatively the electric field strength due to a point charge,
positive and negative charges and oppositely charged parallel plates
•
describe quantitatively the electric field due to oppositely charged
parallel plates
•
outline Thomson’s experiment to measure the charge/mass ratio of
an electron.
From ideas to implementation
At the end of Part 1, you will have had an opportunity to:
•
perform an investigation and gather first-hand information to
observe the occurrence of different striation patterns for different
pressures in discharge tubes
•
perform an investigation and gather first-hand information to
demonstrate and identify properties using discharge tubes:
–
containing a Maltese cross
–
containing electric plates
– with a fluorescent display screen
–
•
containing a glass wheel and analyse the information gathered to
determine the charge on the cathode rays
solve problem and analyse information using:
r
r
r V
F = qvB sinq and E =
d
Extracts from Physics Stage 6 Syllabus © Board of Studies NSW, originally
issued 1999. The most up-to-date version can be found on the Board's website
at http://www.boardofstudies.nsw.edu.au/syllabus99/syllabus2000_list.html
Part 1: Discharge tubes
3
Discharge tubes
The discharge tube is a sealed glass tube that contains air at a greatly
reduced pressure to that of normal air pressure at sea level. The tube has
a positive electrode (anode) at one end and a negative electrode
(cathode) at the other.
When a large DC voltage is set up between the anode and cathode
strange glowing phenomena occur within the tube if the pressure within
the tube is low enough. The phenomena observed within a tube depend
largely on the air pressure within the tube.
Observing the phenomena
To perform this activity you will need to have some specialised equipment.
In general, that will only be available at your practical session with your
teacher.
Procedure and observations
1
A glass discharge glass tube with a single opening is connected to a
vacuum pump as shown in the photograph following.
2
Reduce the air pressure in the tube while having a large DC voltage
difference between anode and cathode at either end of the tube. The
voltage difference between the anode and cathode is usually set to at
least 2000 V. As the pressure is reduced in the tube a sequence of
glowing patterns can be observed.
3
The observations you could expect to see when performing this
experiment are listed in the table following. The pressures in the
tube when these phenomena occur are also listed.
Normal atmospheric pressure at sea level can support mercury (Hg)
in a sealed inverted tube to a height of around 760 mm. As the
pressure is reduced the height of the column of mercury that can be
supported is smaller.
4
From ideas to implementation
In an absolute vacuum the height of a column mercury that can be
supported in a glass tube is 0 mm. (Air pressure was measured with
a mercury manometer at the time of the early work on discharge
tubes.)
Pressure in the tube
(mm Hg)
Observations
50.0
blue and violet streamers of electrical discharge
appear in the tube
5.0
a faint glow of light appears around the cathode. a
salmon pink stream of light called a positive column
extends from the anode
0.1
the positive column breaks into bands called
striations
A photograph of the equipment set up with the pressure in the tube set at
0.1 mm Hg is shown below.
A Geissler discharge tube with the pressure reduced to 0.1 mm Hg. The
anode or positive terminal is on the left. The cathode or negative terminal is on
the right. (Photo: Ric Morante)
The zones and light patches that appear in the discharge tube at around
0.1 mm pressure have been named as follows, in order from the cathode
to the anode:
•
the cathode glow
•
Crookes’ dark space
•
the negative glow
•
Faraday's dark space
•
the striated positive column.
Part 1: Discharge tubes
5
Look at the photograph of the discharge tube at a pressure of 0.1 mm Hg and
identify and label each area named above on the diagram below and the
photograph above.
to vacuum pump
00
V
20
A Geissler discharge tube with the pressure reduced to 0.1 mm Hg.
Check your answer.
You may think this activity strange but in all likelihood you have seen a
variation of a discharge tube in the past few hours. Examples include
fluorescent lights, the television picture tube and the video monitor for a
desktop computer.
The sequence of photographs below shows how the appearance of the
discharge in the Geissler tube changes as the pressure drops from
2 mm Hg to 0.1 mm Hg.
6
From ideas to implementation
A Geissler discharge tube with the pressure reduced to 2 mm Hg.
A Geissler discharge tube with the pressure reduced to 0.5 mm Hg.
A Geissler discharge tube with the pressure reduced to 0.15 mm Hg.
A Geissler discharge tube with the pressure reduced to 0.1 mm Hg.
(Photos: Ric Morante)
Part 1: Discharge tubes
7
If the pressure in the vacuum tube can be lowered to around 10-5 that of
normal air pressure or 0.01 mm Hg then the radiation from the cathode
fills the whole discharge tube and the sides of the tube take on a
luminous green glow.
Do Exercise 1.1. now.
Discharge tubes in experiments
The low pressure in the discharge tubes be maintained because once the
low pressure has been achieved in the glass tubes it is simple to seal the
glass tube with heat from a flame while the tube was maintained under
vacuum from the pump. The sealed glass tubes were then able to be used
in experiments.
In the second half of the 1800s, experiments with just about any new
technology were popular and the ‘new’ low air pressure tubes provided
an unexplored experimental frontier. There was no apparent use for the
tubes so the experiments were what would be called today ‘basic
research’. That is, research for the sake of it. It is undertaken to find out
what will happen in certain situations.
Once cathode rays could be produced in a discharge tube under vacuum,
and the tube was sealed at that pressure by simply pinching off the glass
tube using a hot flame, scientists were free to begin exploring the
properties and behaviour of cathode rays in sealed low pressure discharge
tubes. This was far more convenient than waiting to achieve a vacuum
each time they wanted to experiment.
You probably gathered from the statement above that the main obstacle
to early research was the development of the vacuum pump. Before it
was possible to make good discharge tubes it was essential that a pump
be invented to enable most of the air to be pumped from glass tubes.
At this point it is important to recognise what people did not know at this
time.
•
They did not know the nature of the radiation in the discharge tube.
•
They had no knowledge of the structure of the atom.
•
They did not know of the existence of electrons.
A mercury pump was invented and built by the German scientist
Heinrich Geissler in 1855 that could get the air pressure in glass tubes to
a low enough level to produce the discharge effects shown in the
photograph below.
8
From ideas to implementation
After the invention of that pump, exploration of the behaviour of
electrical discharges in air at low pressures began in earnest.
A chronology of achievements that eventually led to modern cathode ray
tubes is outlined below.
Discharge tube experiments before 1897
In 1858 Julius PlŸcker, a friend of Geissler showed for the first time that
cathode rays would bend under the influence of a field from a magnet.
In 1865 H Sprengel improved the Geissler vacuum pump so that glass
tubes could be made with even lower air pressures inside them. Almost
immediately PlŸcker used Geissler tubes to show that at lower pressure,
the Faraday dark space grows larger. He also found that there was an
extended glow emanating from the cathode on the walls of the tube and
that the glow now known as cathode rays was affected by an external
magnetic field.
In 1869 J W Hittorf found that a solid body put in front of the cathode
was able to cut off the glow from the walls of the tube. This established
another important property of cathode rays. That property was that rays
from the cathode travel in straight lines.
In 1871 C F Varley published a suggestion that cathode rays were
composed of particles. In the same year the famous experimenter Sir
William Crookes proposed that the cathode rays were molecules that had
picked up a negative charge from the cathode and were repelled by it in
the same sort of way that electrostatic charges that are alike are repelled.
Sir William Crookes in 1875 became the first to systematically study
cathode rays. He designed many different shaped glass tubes with bends
and inclusions in the tubes such as a Maltese cross and a glass paddle
wheel. The results of these experiments led to an understanding of some
of the properties of cathode rays.
¥
If an object is placed in the path of the cathode ray such as a Maltese
cross, a shadow of the object is cast on the glowing tube at its end.
This simple experiment showed that the cathode rays traveled in
straight lines.
This particular experiment shed no light on the debate as to the
nature of cathode rays as particle or wave like in behaviour as both
particles and light could be expected to travel in straight lines.
¥
The cathode rays could push a free turning small paddle wheel up a
slight incline made with a pair of glass rods inside a cathode ray
tube. Because this was up hill it was, against the weight force due to
gravity.
Part 1: Discharge tubes
9
This experiment showed that cathode rays carried energy and could
do work. This also didn't prove that cathode rays were either
particles or waves as it was recognised at the time that light waves
carried energy.
•
The discovery in 1875 that cathode rays were deflected from a
straight line path by a magnetic field, suggested that the cathode rays
and magnetism were related in some way.
The discovery of this effect predated by ten years the unification of
the concepts of electricity and magnetism by James Clerk Maxwell.
To see websites showing a movie of cathode rays being affected by the
influence of a bar magnet brought next to a discharge tube see sites on the
physics websites page at:
http://www.lmpc.edu.au/science
In 1876 Eugene Goldstein was able to show that the radiation in
discharge tubes produced when an electric current was forced through the
tube started at the cathode. Goldstein was the scientist who coined the
term cathode ray to describe the radiation in the tube.
In 1883 Heinrich Hertz was able to show that cathode rays were not
deflected by static electrically charged metal plates. This suggested that
cathode rays couldn't be charged particles. He used this observation to
support the argument that cathode rays are in fact a type of wave motion
phenomena. This later turned out to be incorrect. Hertz was a great
experimental scientist as you will learn in Part 3 of this module. He
simply drew the wrong conclusion from a set of experimental results.
In 1885 Jean Perrin presented a scientific paper to the Paris Academy of
Science in which he identified that there were two opposing thoughts as
to the nature of cathode rays. Those hypotheses are listed below.
•
Cathode rays were waves like light as proposed by Goldstein, Hertz
and Lenard
•
Cathode rays are formed by negatively charged particles moving
with a large velocity as proposed by Crookes and J. J. Thomson.
In 1886 Eugen Goldstein observed that a cathode ray tube with a cathode
fitted as a metal plate with holes drilled into it produced radiation that
travelled in the opposite direction to cathode rays as well as normal
cathode rays.
These rays travelled away from the anode toward the cathode and
overshot the cathode when there were holes drilled in it and into the tube
length behind the cathode.
10
From ideas to implementation
These rays were called canal rays because they were more easily visible
after passing through holes or canals bored in the cathode plate. Later
these were found to be ions.
In 1890 Arthur Schuster was able to calculate the ratio of charge to mass
of the particles making up cathode rays (today known as electrons) by
measuring the magnetic deflection of cathode rays. His answer was not
correct but was able to show that the charge to mass ratio was three
orders of magnitude too small for the rays to be made of even the
smallest mass atom, hydrogen. In the same year Joseph John Thomson
first became interested in the discharge of electricity as cathode rays
through a gas at low pressure.
In 1892 Heinrich Hertz, a German scientist who was advocating
(incorrectly) that cathode rays must be some form of wave, was able to
show that the rays could penetrate thin foils of metal. He interpreted this
as supporting the wave hypothesis for cathode rays as a type of
electromagnetic wave. He did so because it was unclear how any solid
particles could penetrate the seemingly solid metal foil.
In the same year the scientist Philipp von Lenard developed a cathode ray
tube with a thin aluminum window that permitted the cathode rays to
escape from the discharge tube. This allowed the rays to be studied in
the open air at normal air pressure for the first time. Lenard, a highly
respected German scientist, predicted that cathode rays would be shown
to travel at the speed of light just as all electromagnetic radiation did.
In 1894 J J Thomson, an Englishman, announced that he had found the
velocity of cathode rays was much lower than that of light. He had
calculated a measured value of 1.9 x 107 cms-1. This is much lower that
the value 3.0 x 1010 cms-1 for light. This was a much lower value than
the prediction by Lenard of the velocity of the rays and cast doubt about
the hypothesis that cathode rays were an electromagnetic wave
phenomena.
To put the apparently extremely divergent opinion as to the nature of
cathode rays at this time in perspective it must be pointed out that there
was great rivalry between German and British cathode ray researchers.
The Germans favoured the explanation that cathode rays were a wave
phenomena like light. The British cathode ray researchers believed that
cathode rays were a particle phenomena. Research to discover the true
nature of cathode rays were a focus of active research around this time.
In 1895 Jean-Baptiste Perrin was able to show that cathode rays
deposited a negative electric charge where they impacted. This was a
major step in refuting Hertz's concept of cathode rays as a wave
phenomenon. This was because it was difficult to see how a form of
electromagnetic radiation could generate a negative charge. He
suggested that the cathode rays were particles.
Part 1: Discharge tubes
11
In 1896 Pieter P Zeeman discovered that spectral lines of gases placed in
a magnetic field are split, a phenomenon called the Zeeman effect. You
learned about the formation of spectral emission lines in the preliminary
module The cosmic engine. You may wish to review that work now if
you cannot recall it.
In the same year Hendrik Antoon Lorentz explained the Zeeman effect
by assuming that motion of charged particles in the atom produced the
light. Lorentz used Zeeman's observations of the behavior of light in
magnetic fields to calculate the charge to mass ratio of the electron in an
atom. This was a remarkable piece of work that occurred a year before
electrons were discovered and 15 years before electrons were known to
be constituents of atoms.
1
Review the chronology of progress made in the study of cathode rays
presented prior to 1897. In the space below list the properties and
features of cathode rays that were known and 'proven' by experiment
prior to 1897.
_____________________________________________________
_____________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
12
From ideas to implementation
2
Complete the table below. List the features observed during
experiments that would suggest that cathode rays were wave like in
nature. Identify any work done that refuted any of the points
suggesting cathode rays were waves.
Features suggesting cathode rays
were waves (before 1897)
Features suggesting cathode rays
were particles (before 1897)
Check your answers.
Complete Exercise 1.2 now.
Repeating Crookes’ experiments
To do this activity you will need access to specialised equipment including
modern day copies of Crookes’ tubes and an induction coil. For that reason
this activity will have to be completed at a practical session with your
teacher.
Ensure you stay clear of the induction coil and bare conductors in this
experiment. The circuit is carrying electrical currents at very high voltages
and can cause injury or painful electric shock. The tube also emits low
levels of X-rays. Stay at least 3 m from the tube if viewing for extended
periods.
Part 1: Discharge tubes
13
Procedure
1
Set up the equipment as shown in the figure below with the Crookes’
tube containing the Maltese cross.
Crookes’ tube containing a Maltese cross anode. (Photo: Ric Morante)
Maltese cross anode
cathode
cathode rays
shadow of the Maltese
cross forms on the end
of the tube
vacuum tube
anode
A sketch showing the details of the vacuum tube containing a Maltese cross
photographed above.
2
Look at the end of the tube that is furthest from the cathode.
Note what you see in the space below.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
3
14
Set up the Crookes’ tube with the glass paddle wheel in it as shown
in the figure following. Do not switch on the electrical power to the
circuit.
From ideas to implementation
high voltage source
anode
cathode
cathode rays hitting the
paddle wheel cause it to
turn and roll up slope
glass runners on a very
slight upward slope
away from the cathode
stand
Crooke’s tube containing a mica paddle wheel.
4
Ensure that the tube is set up on a level surface. This is so that the
cathode rays will hit the paddle wheel such that the only grade the
wheel has to climb is due to the glass rails in the tube.
5
Turn on the electricity and observe the effect of the cathode rays on the
paddle wheel. Describe that effect in the space below.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answer.
6
The deflection of a cathode ray beam by a bar magnet is a simple
process to accomplish. Look at the photographs following.
A cathode ray tube showing a cathode ray beam that is undeflected.
(Photo: Ric Morante)
Part 1: Discharge tubes
15
A cathode ray beam that has been deflected at 90 by the south pole of a bar
magnet when it is placed next to the beam as shown. (Photo: Ric Morante.
Hand: Tim Reid)
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16
From ideas to implementation
Electric and magnetic fields
The electric field surrounding a point charge is a radial electric field.
If the charge is positive the field is, by definition, directed out of the
charge. If the charge is negative, the field is, by definition directed into
the charge. The diagram below shows the electrical fields surrounding
point charges. As you may recall from the module Electrical energy in
the home, like charges repel, unlike charges attract.
An electrical field directed out of a
positive charge.
An electrical field directed into a
negative charge.
Forces produced by fields
You learned in the preliminary topic Electrical energy in the home that a
magnetic field surrounding a magnetised object can be described with
field lines. You also learned in the same module that the magnetic flux
density is represented by the symbol B and has the unit tesla (T). The
magnetic field lines by definition are directed out of the north pole and
into the south pole.
When an electric charge moves through a magnetic field it experiences a
force. A positive charge q moving with a velocity v through a magnetic
field of flux density B will be subjected to a force given by the formula
r
rr
F = qvBsin q
q is the angle between the magnetic field and the velocity of the positive
charge. The direction of the force is most easily determined using the
right hand palm rule. This rule is demonstrated in the figure following.
Part 1: Discharge tubes
17
F
~
B
~
The three vectors
r r
r
F, B, and v are at right angles to each other.
The labels on the hand show the relationship. Knowledge of any two of the
vector directions enables the determination of the third vector direction using
this rule.
F
~
B
~
The relationship of the vectors to each other is as shown in the figure above.
If a moving charge enters a magnetic field at 90° to the magnetic field it
will be subject to a force at 90° to the velocity vector.
Because sin 90° = 1 the formula describing the magnetic force on the
r r r
positive charge is simplified to F = Bqv .
This force will cause the direction of the velocity vector of the charge to
change. As the velocity vector changes the force vector will also change
to a direction at 90° to compensate for the new velocity vector.
The result will be a changing velocity that essentially places the path
followed by the negative charge into circular motion. This is shown in
the figure following.
18
From ideas to implementation
particle path
qv sin
v cos
B
~
v
The path followed by a negative particle entering a magnetic field. Note the
path ends up following a circular spiralling track. Note the direction of the v
must be reversed because this particle is negative. The rule applies to a
positive particle but can be used when dealing with electrons or negative
particles if the v direction is reversed.
particle’s track
F
~
B out of
~
the page
r
q
m
The path a positive particle takes after entering a magnetic field. Note the
direction of the v is as per the rule shown above.
Calculating the force
The force therefore is supplying a centripetal force like you learned in the
preliminary module The cosmic engine. Centripetal force is described by
the formula:
r mv 2
F=
r
2
r
Therefore, mv = Bqvr for the positive particle moving in the magnetic
r
field.
The relevance of this relationship will be addressed in the following
section of this part when looking at Thomson's experiment. Using this
relationship Thomson was able to determine the charge to mass ratio for
an electron.
Part 1: Discharge tubes
19
The other type of field that is relevant to Thomson's experiment is the
field that exists between charged electric plates. You may recall that you
were introduced to the electric field concept in the preliminary module
Electrical energy in the home and may wish to refresh your memory by
referring to that learning material.
Complete the following sentence.
Two oppositely charged plates separated by a gap produce _________
_________________________________________________________
between the two plates.
When two metal plates are arranged in parallel in proximity with a
separation and one plate is carrying a positive charge and the other is
carrying a negative charge as shown in the figure below an electric field,
r
E is produced between the plates.
Note that, as with point charges, the field is directed out of the positive
plate and into the negative plate. If a moving positive charge q travelling
r
at some velocity v is introduced between the plates the charge will
experience a force due to the electric field that is parallel to the electric
field.
source of positive particle
particle
d
V
F
Path of a positive particle in an electric field between two charged plates.
r r
You may recall that force is F = Eq from your learning in the
preliminary module Electrical energy in the home.
This force accelerates the charge in the direction of the electric field.
The initial motion or velocity component in the direction between the
charged plates does not change because there is no force acting on the
charge in that direction. This is similar to the constant horizontal
component of velocity in projectile motion you learned about in the
module Space.
20
From ideas to implementation
The vector addition of the additional velocity component in the direction
perpendicular to the charged plates does, however, result in a deflection
of the moving positively charged particle's motion away from the
positively charged plate toward the negatively charged plate. This
deflection is dependent on the velocity of the charged particle and the
strength of the electric field.
The strengthr of the electric field is directly proportional to the potential
difference V and the distance between the plates.
r
r V
This is represented in the equation E = .
d
d
E
v
The field between charged plates. The plate at the top is positive. The plate at
the bottom is negative.
1
To what is the electric field strength between the two plates due?
Use the terms: point charge, positive charge, negative charge, and
oppositely charged parallel plates in your answer.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
Describe the way an electric field between two oppositely charged
parallel plates will affect a positive point charge introduced into the
gap between the plates.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answers.
Part 1: Discharge tubes
21
Thomson used his knowledge about the behaviour of moving charged
particles in electric and magnetic fields to establish his determination of
the charge to mass ratio of the electron.
Sample problem 1
Two parallel plates having a potential difference of 1000 V are separated
by a gap of 0.02 m. What is the strength of the electric field between the
plates?
Solution
r
r V
E=
d
1000 V
=
0.02 m
= 50 000 NC-1
Sample problem 2
A charge of +6 pC enters an electric field of 50 000 NC-1 acting between
two parallel plates. What is the size of the force acting on the charge.
Solution
r r
F = Eq
= 50 000 NC-1 ¥ 6 ¥ 10 -12 C
= 3 ¥ 10 -7 N
You should now practice using these formulas to solve problems.
1
Two parallel conducting plates separated by 0.1 m have a potential
difference of 100 V between them. What is the strength of the electric
field between the plates?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
22
From ideas to implementation
2
What is the potential difference at a point mid way between the
plates from the question above with respect to the positive plate?
Explain your answer.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
3
In a vacuum tube an electron with a charge of -1.6 ¥ 10-19 C
travelling at 1 ¥ 10-5 ms-1 enters an electric field of 1 NC-1 between
two 0.5 cm long horizontally placed parallel plates. The plates are
1.5 cm apart and the electron enters the gap between the plates at
right angles and at a point midway between the plates.
The mass of an electron is 9 ¥ 10-31 kg.
The arrangement of the equipment is shown in the figure below.
Note that this arrangement is similar to the operating principle of a
working television.
tube edge
screen
electron gun
e
sealed vacuum tube
E
1.5 cm
F
Calculate:
a) The force applied to the electron by the electric field between
the plates.
_________________________________________________
_________________________________________________
_________________________________________________
_________________________________________________
Part 1: Discharge tubes
23
b) The acceleration of the electron due to the force produced by the
electric field.
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
c) The velocity change that will occur to the electron as a result of
the applied force acting over the full length of the plates. (Hint:
calculate the time it takes to travel through the full length of the
plates.)
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
d) The electron continued on its path toward a screen located 0.2 m
from the exit end of the parallel plates. That screen had a
horizontal line drawn on it parallel with the middle of the
separation distance for the two plates. Describe quantitatively
where on the screen it would hit. (Hint: find the vertical
displacement that occurs within the field first, then calculate the
vertical displacement after it leaves the field.)
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
Check your answers.
Complete Exercises 1.5 to 1.9 now.
Now that you have learned about the forces produced on charged particles
by electric and magnetic fields read on to find out how J J Thomson used
that information to determine the charge to mass ratio of the electron.
24
From ideas to implementation
The q/m ratio of the electron
Karl Braun invented a cathode ray tube with a fluorescent screen making
up one end of the tube in 1897. Shortly after this, Thomson used this
technology to perform a series of experiments to determine the true
nature of the cathode rays.
Thomson’s experiment
Thomson's purpose for performing his experiments were clearly outlined
in the scientific paper he wrote in 1897. He wrote:
‘The most diverse opinions are held as to these rays; according to the
almost unanimous opinion of German physicists they are due to some
process in the aether to which – inasmuch as in a uniform magnetic
field their course is circular and not rectilinear – no phenomenon
hitherto observed is analogous: another view of these rays is that, so
far from being wholly aetherial, they are in fact wholly material, and
that they mark the paths of particles of matter charged with negative
electricity.’
Thomson cleared this argument up systematically. He first showed that
the cathode rays carried negative charge. This had been shown
previously by Perrin in 1895 when he placed two coaxial metal cylinders
(one cylinder inside the other) in front of a flat cathode. The cylinders
were insulated from one another but each had a small hole in the side that
the cathode rays could pass through and into the inner cylinder.
The outer metal cylinder was earthed by a wire connection to the outside
of the tube and so would not retain a charge. This is similar in principle
to the electroscope experiment you learned about in the preliminary
module Electrical energy in the home. Earthing the electroscope causes
it to lose its charge.
When the cathode rays were deflected by a magnetic field so that they
passed through the holes and into the inner cylinder an electroscope
connected to it showed the presence of a negative electrical charge.
Part 1: Discharge tubes
25
If the cathode rays were deflected with a magnetic field so that they did
not pass through the holes there was no charge detected by the
electroscope.
J J Thomson was the first person to succeed in deflecting the cathode ray
with an electrical field. This was necessary to show that cathode rays
were themselves negatively charged and not just producing some hitherto
unknown effect of causing objects they struck to become negatively
charged. He did this in 1897 using the apparatus shown in the figure
below.
Thomson’s original drawing of his apparatus. (Thomson, 1897)
As already mentioned, Heinrich Hertz had failed to observe any
deflection of cathode rays caused by an electric field in any of his
experiments. Since Hertz was a significant experimentalist at the time
this was seen to prove cathode rays were not particles carrying negative
charges. Thomson suspected they were. To that end he constructed the
apparatus shown above. The original letters Thomson used on his
diagrams to refer to bits of his apparatus that aided his descriptions are
retained in the discussion that follows.
The idea of Thomson's experiment was that cathode rays from the
cathode C would be accelerated toward, and pass through, a slit in anode
A before continuing through another slit at B. They were then basically a
wide beam that passed between electrically charged plates D and E
before producing a narrow well defined phosphorescent patch at the end
of the tube. Thomson had a scale attached to the end of the tube to
measure any deflection that might occur due to the field from the charged
plates D and E.
When Hertz had performed the experiment with a very similar apparatus
he had found no deflection of the cathode ray beam when a potential
difference was applied across the plates D and E. Hertz logically
concluded that the electrostatic properties of the cathode ray were at best
very weak.
26
From ideas to implementation
When Thomson first performed the experiment he saw no deflection but
Thomson reasoned that this may have been due to the low pressure air in
the tube becoming ionised and hence capable of conducting a charge.
Subsequently he decided to remove as much air from the tube as was
possible to see if he could reduce the conductivity of the air.
The idea was that by reducing the amount of air that could become
ionised and hence conduct electricity, the lower the conductivity that
could be attained by the gas. It turned out Thomson was correct.
Experiments showed that as the vacuum in the tube became greater the
deflection of the cathode rays by the electric field became apparent.
When Thomson produced the experimental apparatus (shown on the
previous page) he was able to prove once and for all the negative nature
of cathode rays. Thomson saw the cathode rays bend toward the positive
plate whether it was set up as the D or E electrode plates in the diagram.
This confirmed that cathode rays were negatively charged. The paddle
wheel experiment had already demonstrated that the cathode rays had
momentum. The conclusion then was that the cathode rays were
negatively charged matter.
Thomson concluded:
ÔAs cathode rays carry a charge of negative electricity, are deflected
by an electrostatic force as if they were negatively electrified, and are
acted on by a magnetic force in just the way in which this force would
act on a negatively electrified body moving along the path of these
rays, I can see no escape from the conclusion that they are charges of
negative electricity carried by particles of matter.Õ (Thomson, 1897)
Thomson was essentially describing the electron for the first time.
Having established that cathode rays were negatively charged particles,
Thomson analysed his data to determine exactly what these particles
were. At this point it is important you understand that no subatomic
particles were known to exist.
To investigate the particles making up the cathode ray Thomson made
measurements of the charge to mass ratio of cathode rays. Thomson's
method used both the electrostatic and magnetic deflection of the cathode
rays to determine the charge to mass ratio for the particles that made up
the cathode rays.
The apparatus shown following could be set up so it also included a
magnetic field that could be created perpendicular to both the electric
field and the trajectory of the cathode rays. That is the magnetic field
was parallel to the two electrode plates D and E. The magnetic field was
produced by current carrying coils on either side of the electric plates D
and E.
Part 1: Discharge tubes
27
The size of the current in the coils and the shape of the coils enabled the
accurate calculation of the size of the magnetic field produced between
the coils.
+
d
c
a
b
e
coils of wire carrying an
electric current placed
outside of the tube
Thomson’s apparatus with the coils to supply the magnetic field.
The method Thomson used to calculate the charge to mass ratio of the
electron was as follows.
•
Thomson assumed the cathode ray to be made up of particles with a
negative charge.
•
He then considered the cathode ray beam to be made up of particles
of mass m and charge e, travelling at velocity v.
•
He then considered what happened when the beam passed through
an electric field in the region between plates D and E, over a length l.
The time for a particle to pass through this region would be equal to
t =
l
v
The force due to the electric field on the particles could
r be calculated
r F
from the known electric field strength because E = .
q
The cathode ray beam's path was still deflected by a magnetic field even
if there was no electric field. When only the magnetic field was turned
r r r
on the particles in the beam experienced a force of: F = Bqv .
Because the force in this situation must be perpendicular to the direction
of the velocity of the beam, the beam was deflected in a near circular
path while it was in the uniform magnetic field. This meant that the
particles making up the beam were experiencing a centripetal force that
r2
r
was equal to F = mv where m was the mass of the particles and r was
r
effectively the distance of deflection of the cathode ray beam from the
centre of the screen at the end of the cathode ray tube.
28
From ideas to implementation
This centripetal force was due to the effect of the magnetic field
r r r
( F = Bqv ). Equating these two forces leads to the relationship:
r r mvr 2
.
Bqv =
r
Rearranging and simplifying the equation lead to the relationship:
r
q
v
= r
m Br
Thomson then set up a situation where the beam of cathode rays
simultaneously passed through both a deflecting force due to the electric
r
r
r
field E and a magnetic field B in the same region. Thomson adjusted B
so that the beam was undeflected. Thus, the magnetic force was equal to
the electrostatic force.
qvB = qE or v = E/B.
Since, both E and B were known or calculable quantities this enabled the
velocity of the beam to be determined.
r
q
v
Each of the quantities v, B and r in the expression:
= r was able to
m Br
be measured or could be calculated so the charge to mass (q/m) or mass
to charge (m/q) ratio could be determined.
The two formulas above could not give either the charge or the mass of
the cathode ray particle that eventually became known as the electron.
All Thomson knew was that the q/m ratio was around 1800 times larger
than the values for hydrogen ions. The only conclusions possible were
that the electron was carrying a charge 1800 times bigger than a
hydrogen ion (proton) or that the electron was around 1800 times less
massive.
Thomson then performed a series of relatively inaccurate experiments to
determine the charge on the electron and found it to be approximately the
same as that on a hydrogen ion. This convinced him that the mass of an
electron was in all likelihood around 1800 times less massive than the
mass of a hydrogen ion.
To try a simulation of ThomsonÕs experiment or view a movie of the
experiment see sites on the physics website page at:
http://www.lmpc.edu.au/science
After reading about Thomson's experiments to determine the charge to mass
ratio of the electron prepare your own summary of what Thomson actually
did as a list of points with subheadings as indicated in the scaffold below.
Part 1: Discharge tubes
29
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30
From ideas to implementation
Summary
Gas discharge tubes were invented when the air pump could evacuate the
air well enough to allow the gas in the tube to conduct an electric current.
The phenomenon that appears in gas discharge tubes is pressure
dependent. Some of the features that occur include:
_________________________________________________________
_________________________________________________________
_________________________________________________________
A systematic study of Crookes’ tubes showed the behaviour of cathode
rays that were evidence that the rays were both of a wave like character
and a particle like character.
The evidence for a wave nature was:
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
The evidence for a particle nature was:
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
The formula to describe the force acting on a moving charge in a
magnetic field is:___________________________________________
The formula to describe the effect of an electric field on a charge
between two parallel charged plates is: _________________________
Part 1: Discharge tubes
31
32
From ideas to implementation
Suggested answers
Observing the phenomena
n
lum
iv
sit
e
iat
str
o
dp
o
ec
Faraday’s
dark space
Crookes’
dark space
cathode glow
cathode
to vacuum pump
negative glow
00
20
Part 1: Discharge tubes
V
33
Discharge tube experiments before 1897
1
There were a number of cathode ray properties determined over the
years by cathode ray tube researchers. They were:
•
It was shown that if an object is placed in the path of the cathode
ray, a shadow of the object was cast on the glowing tube wall at
the end away from the cathode. This showed that the cathode
rays travelled in straight lines.
•
It was shown that cathode rays could push a small paddle wheel
up an incline, against the force of gravity. This demonstrated
and proved that the cathode ray carried energy and could do
work.
•
It was shown that cathode rays were deflected from a straight
line path by a magnetic field, suggesting that the two were
related in some way. This was discovered in 1855
•
It was shown cathode rays were negatively charged by
experiment in 1895.
2
Features suggesting cathodes rays
were waves (before 1897)
Features suggesting cathode rays
were particles (before 1897)
the rays travel in straight lines
the rays left the cathode perpendicular
to the surface
if an opaque object such as a Maltese
cross was placed in the path of the rays
a shadow of that object appeared at the
opposite end of the cathode tube
the cathode rays were deflected by
magnetic fields
the cathode rays did not appear to be
deflected by electric fields.
small paddle wheels turned when
placed in the path of the rays
the rays could pass through thin metal
foils without damaging them
the rays had a velocity that was slower
than that of light
Repeating Crookes’ experiments
34
5
You should have seen the paddle wheel being caused to turn and roll
up slope. This shows the particles making up the cathode rays must
have some momentum to transfer to the paddle wheel.
6
The cathode beam is deflected toward the north magnetic pole of the
bar magnet.
From ideas to implementation
A cathode ray beam being deflected at 90 down by the north pole of a bar
magnet when it is in front as shown. (Photo: Ric Morante. Hand: Tim Reid.)
~
V@7QD6758N9665G<=7@97NC9:;A=7D76@C=D879P7A4DC>=:7NDC@5A<=67NC59C7@9
@4=7:56A4DC>=7@;G=Z675?B=?@59?H77„P@=C7@4=75?B=?@59?79P7@4=7:56A4DC>=
@;G=76A5=?@56@7A9;<:7NC=NDC=7D?:7G;5<:7@4=7@;G=67Q5@47C=<D@5B=7=D6=
D?:79?A=7@4=F7Q=C=76=D<=:7;?:=C7BDA;;87@4=7=JN=C58=?@67A9;<:7G=
;?:=C@DK=?79B=C765>?5P5AD?@7<=?>@4679P7@58=H
Calculating the force
3Q979NN965@=<F7A4DC>=:7N<D@=676=NDCD@=:7GF7D7>DN7NC9:;A=7D?
=<=A@C5A7P5=<:79P7;?5P9C876@C=?>@47=JA=N@7?=DC7@4=7=:>=67G=@Q==?7@4=
@Q97N<D@=6H
‚
„7N965@5B=<F7A4DC>=:7N<D@=76=NDCD@=:7GF7D7>DN7PC987D7?=>D@5B=<F
A4DC>=:7N<D@=H7734=6=79NN965@=<F7A4DC>=:7N<D@=675?:;A=7D?7=<=A@C5A
P5=<:7G=@Q==?7@4=8H77„7N965@5B=<F7A4DC>=:7N95?@7A4DC>=7Q5<<7@=?:7@9
G=76;G€=A@=:7@97D7P9CA=7Q45<=75?7@4=7P5=<:7@4D@7N;64=67@4=7A4DC>=
@9QDC:7@4=7?=>D@5B=7N<D@=H77„7?=>D@5B=<F7A4DC>=:7N95?@7A4DC>=7Q5@4
=JN=C5=?A=7D7P9CA=7N;645?>75@7@9QDC:7@4=7N965@5B=<F7A4DC>=:7N<D@=H
‡
34=7P5=<:7Q5<<7C=6;<@75?7D7P9CA=7@4D@7Q5<<7;?5P9C8<F7DAA=<=CD@=7@4=
N95?@7A4DC>=7@9QDC:7@4=7?=>D@5B=7N<D@=H
L
ƒ
V
d
100
!
0.1
= 1000 NC -1
E =
R5?A=7@4=7=<=A@C5A7P5=<:76@C=?>@47567;?5P9C87G=@Q==?7@4=7N<D@=67ˆ=JA=N@
?=DC7@4=7=:>=6‰7@4=7N9@=?@5D<7:5PP=C=?A=7AD?7G=7AD<A;<D@=:7658N<F7D6
P9<<9Q6H
V = Evd
= 1000 v 0.05
= 50 V
Part 1: Discharge tubes
35
5
a)
F = Eq
= 1 NC-1 ¥ -1.6 ¥ 10 -19 C
= -1.6 ¥ 10 -19 N
F
m
-1.6 ¥ 10 -19
=
6.31 ¥ 10 -31
= -2.54 ¥ 1011 ms-2
b)
a=
c)
The time that the electron is between the plates is
s
v
0.005
=
100000
= 0.00000005 s
t =
The change in velocity is all directed toward the positive plate.
vy = a yt
= - 2.54 ¥ 1010 ¥ 0.00000005
= 1.27 ¥ 10 3 ms-1
d) The velocity of the electron after passing through the plates is
directed toward the positive plate with the initial horizontal
velocity in addition to the changed velocity in the vertical
direction. No further acceleration of the electron occurs after it
leaves the gap between the two plates as no force is acting on
the electron. While between the plates the electron is
accelerating toward the positive plate. The displacement toward
the plate over this interval of time is
1
r = u y + at 2
2
1
= 0 + 2.54 ¥ 1010 ¥ 0.000000052
2
= 0.00031 m
The time the electron takes to reach the screen 0.2 m away is:
0.2
10 0000
= 0.000002 s
t =
In that time the electron will travel toward the positive plate
with a velocity of 1270 ms-1.
r = vt
= 1270ms-1 ¥ 0.000002 s
= 0.00254 m
36
From ideas to implementation
The electron will therefore hit the screen a distance of:
(0.00031 + 0.00254) m = 0.00285 m toward the positive plate
below the horizontal line.
Thomson’s experiment
What did Thomson want to do?
Determine the charge to mass ratio on the particles making up cathode
rays.
How did Thomson carry out his investigations?
He used a discharge tube and equated the effect of external electric and
magnetic fields of known strength to enable him to determine all
unknown quantities he needed to measure in order to determine the q/ m
ratio of cathode ray particles.
What did Thomson eventually find?
The q/m ratio of a particle that was 1800 times larger than that of a
hydrogen nuclei. That particle eventually became known as the electron.
Thomson had discovered the first known subatomic particle.
Part 1: Discharge tubes
37
38
From ideas to implementation
Exercises – Part 1
Exercises 1.1 to 1.10
Name: _________________________________
Exercise 1.1
The figure below shows a discharge tube. Look at the striation patterns
visible in the tube and give an estimate of the pressure within the tube.
Explain your answer by referring to the names of the striations visible in
the sketch of the tube.
n
lum
iv
sit
e
iat
str
o
dp
o
ec
Faraday’s
dark space
Crookes’
dark space
cathode glow
cathode
to vacuum pump
negative glow
00
V
20
Part 1: Discharge tubes
39
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 1.2
a)
List the observed cathode ray behaviour that were typical of particles
and those typical of waves prior to 1897.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
b) Explain why this behaviour would have led to debate as to the nature
of cathode rays.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
Exercise 1.3
What properties of cathode rays are demonstrated by discharge tubes:
a)
containing a Maltese cross
______________________________________________________
______________________________________________________
______________________________________________________
40
From ideas to implementation
b) with a fluorescent display screen.
_____________________________________________________
_____________________________________________________
c)
containing a glass paddle wheel.
_____________________________________________________
_____________________________________________________
_____________________________________________________
Exercise 1.4
Imagine you were to bring the north pole of a bar magnet next to a
cathode ray tube where the cathode was located on the left of the page
and the anode on the right. Which way would the cathode beam deflect?
Explain your answer with a diagram.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 1: Discharge tubes
41
Exercise 1.5
An electron (-1.6 ¥ 10-19 C) is moving between two parallel oppositely
charged plates that create an electric field strength of 100 NC-1. What is
the size of the force acting on the charge and in which direction is the
force acting?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 1.6
Calculate the force acting on a electron entering a magnetic field of 4 T
at 90r to the magnetic field if the velocity of the electron at entry was
2000 ms-1.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 1.7
Describe the path taken by an alpha particle (positively charged) as it
passes between the poles of a large horseshoe shaped magnet. The alpha
particle enters the magnetic field at 90r. Explain your answer with a
diagram.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
42
From ideas to implementation
Exercise 1.8
A proton with a charge of 1.6¥10-19 C travelling with a velocity of
106 ms-1 enters a magnetic field at an angle of 30r to the direction of the
magnetic field. The magnetic field strength is 0.5 T. Calculate the
magnitude of the force acting on the proton.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 1.9
a)
If the direction of the magnetic field for the previous question is
directly down the page in which direction will the force act as the
proton enters the field?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
b) Will the direction of action of the force in the example above remain
constant throughout the passage of the proton through the magnetic
field? Explain your answer.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 1: Discharge tubes
43
Exercise 1.10
Explain why Thomson was able to determine the charge to mass ratio in
his famous experiment using cathode rays but could not measure the
mass nor the charge by themselves accurately.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
44
From ideas to implementation