From Ideas to
Implementation
Core Module 9.4
Introduction
A new dawn in physics had begun at the start of the
twentieth century…
…it appeared that the stage had been reached when all the
disparate elements of scientific knowledge seemed to be
coming together…
…many scientists felt that the total knowledge about the
universe lay just beyond their horizon…
…future generations of scientists, it was believed, would
have nothing to discover…
…this idea was about to be challenged.
Until now, physicists believed that all phenomena could be
broken down into fundamental interactions that were
capable of being described by the laws of physics.
However, early in the twentieth century, physics, or
classical physics as it is now known was to be shaken to
the core by two remarkable new insights about the
universe.
The first of these insights was was made by Albert Einstein
when he proposed his now famous Theory of Special
Relativity in which he showed that space and time were
interrelated.
The second insight came when Max Planck realised that in
spite of his best efforts to make the laws of classical
physics fit natural phenomena on the atomic scale, these
laws were simply incapable of resulting in an explanation.
These insights lead to the development of quantum physics,
which was primarily developed to explain atomic
phenomena and eventually proved to be more fundamental
than classical physics.
By extending the laws of quantum physics to large masses,
distances and times, it was shown that it was possible to
derive the laws of classical physics.
In this unit, we will investigate some of the fundamental
ideas of quantum physics.
We start with the discovery and identification of cathode
rays and how this ultimately led to modern television.
We then look at how black body radiation and the
photoelectric effect led to a new view of the nature of
electromagnetic radiation with the quantum theory of
Max Planck.
We conclude by looking at how the study of sub-atomic
particles led to the invention of the transistor and to the
discovery of superconductivity.
Students learn to:
1. Increased
understandings of
cathode rays led to the
development of
television
Students:
•explain why the apparent inconsistent
behaviour of cathode rays caused debate as to
whether they were charged particles or
electromagnetic waves
•perform an investigation and gather first-hand
information to observe the occurrence of
different striation patterns for different
pressures in discharge tubes
•explain that cathode ray tubes allowed the
manipulation of a stream of charged particles
•perform an investigation to demonstrate and
identify properties of cathode rays using
discharge tubes:
•identify that moving charged particles in a
magnetic field experience a force
– containing a maltese cross
•identify that charged plates produce an electric
field
– containing electric plates
•describe quantitatively the force acting on a
charge moving through a magnetic field
– containing a glass wheel
– with a fluorescent display screen
•discuss qualitatively the electric field strength
due to a point charge, positive and negative
charges and oppositely charged parallel plates
– analyse the information gathered to
determine the sign of the charge on cathode
rays
•describe quantitatively the electric field due to
oppositely charged parallel plates
•solve problem and analyse information using:
•outline Thomson’s experiment to measure the
charge/mass ratio of
an electron
•outline the role of:
F  qvB sin 
F  qE
and

– electrodes in the electron gun
– the deflection plates or coils
– the fluorescent screen in the cathode ray tube
of conventional TV displays and oscilloscopes

E
V
d
Discharge Tubes
By the 1850’s, much was known about which solids and
liquids were electric conductors or insulators, and it was
thought that all gases were electric insulators.
In 1855, Heinrich Geissler (a glass blower
and maker of scientific equipment) had
refined a vacuum pump that could
evacuate a glass tube to within 0.1% of
normal air pressure.
Julius Plucker (a colleague of Geissler’s)
sealed a wire in each end of a glass tube
and connected it to a high voltage power
supply. He then steadily removed the air
from inside using Geissler’s pump. To
their surprise, they found that at reduced
pressures, air (and all other gases)
conduct electricity!
Plucker showed that as the pressure is reduced in the
discharge tube (as it is now known), a series of changes
progressively take place. We will examine these in the
following first hand investigation:
ACTIVITIES
•
Experiment 20A: Discharge tubes
Discharge Tube Sample observations:
As the pressure of the gas inside the tube was reduced, both
the anode and cathode were surrounded by a luminous
glow. As the pressure was reduced further, the positive glow
extended about halfway along the tube (between the
positive glow and the negative glow is an area called the
Faraday dark space). At even lower pressure, the positive
glow broke up into columns called striations. Lower still,
an area called the Crooke’s dark space filled the entire tube
and a green glow appeared in the glass at the end of the
discharge tube opposite the cathode.
-
+
Cathode
Anode
Negative
glow
Crooke’s dark
space
Positive striations
Faraday’s dark
space
Crookes Tubes
The discharge coming from the cathode had by now been named
cathode rays.
By 1875 Sir William Crookes had made further
discoveries. He built and experimented with a variety
of different tubes (which he called Crookes tubes variations of the tubes used by Geissler and Plucker).
The Crookes tubes had structures built into or around them to allow the
cathode rays to be manipulated. For example, electrodes were built
into the cathode ray tube to create an electric field to attempt to change
the path of the cathode rays. Magnetic fields were applied to the
cathode rays through the glass from outside the tube, and solid objects
were placed inside the tube to block the path of the rays.
ACTIVITIES
•
Prepare a Risk Assessment for the following experiment
•
Experiment 20B: Properties of Cathode Rays
Sample observations:
A fluorescent screen shows that cathode rays
can cause fluorescence. This demonstrates that
cathode rays have energy. A fluorescent screen
can also be used to trace the path of cathode
rays being manipulated by other means.
A Maltese cross placed in the path of the cathode rays
causes a clearly defined shadow at the end of the tube.
This effect is used to infer that cathode rays travel in
straight lines and are blocked by solid objects.
Pairs of electric plates
cause the cathode rays to
bend towards the positive
plate. This shows that
cathode rays have
negative charges.
Charged plates
Note that Crookes believed
that cathode rays could be
deflected by an electric
field but never succeeded
in demonstrating this
experimentally.
A lightweight glass paddle wheel, able to rotate freely, is
placed in the path of the cathode rays. The cathode rays
cause the wheel to spin and move away from the cathode.
This demonstrates that the cathode rays have momentum,
and therefore mass, and that they are emitted from the
cathode.
first
A cathode ray in the
presence of a magnetic field
will deflect (bend). As we
already know, moving
charges experience a force
in a magnetic field (RHPR).
This shows that cathode
rays carry a negative
electrical charge.
Use the diagrams
to show that the
cathode rays are
negatively charged
These (and other) experiments showed that cathode rays:
•
always have the same properties regardless of the metal in
the cathode
•
travel in straight lines
•
carry energy
•
have mass
•
cause a green luminescence when they strike glass
•
change the colour of some silver salts (i.e. expose
photographic film)
•
pass straight through thin metal foil without damaging it
•
travel considerably more slowly than light
•
are deflected by magnetic fields
•
are deflected by electric fields (we’ll come back to this
one!!)
The cathode ray debate – waves or particles?
The controversy as to whether cathode rays were streams
of charged particles or electromagnetic waves is a great
example of the nature of scientific development. This
particular debate raged for years.
Some of the properties of cathode rays suggested to
physicists that cathode rays were a wave similar to light.
For instance:
•
they produced fluorescence
•
they travelled in straight lines
•
they pass straight through thin metal foil
•
they produced similar chemical reactions on
photographic film to those produced by light
•
they were not deflected by electric fields (or so it
seemed).
Yet cathode rays were deflected by a magnetic field as if
they were negatively charged particles. This apparently
inconsistent behaviour of cathode rays led to much
controversy over the nature of cathode rays.
The debate was finally resolved in 1897
by J.J. Thompson.
The reason the debate took so long to clear
up was that initially the cathode rays could
not be deflected by an electric field.
J.J. Thompson was able to make a cathode ray tube with
sufficiently low pressure and using a very high voltage
supply he was able to deflect the cathode rays with an
electric field. This showed conclusively that cathode rays
were beyond doubt streams of charged particles. We now
call these charged particles electrons!
Before we look at J.J. Thompson’s experiment, we need to
look at the effects of charged particles moving in electric
and magnetic fields.
ACTIVITIES
• Read 10.1
• Complete 1 – CRTs
• Complete 2 – Properties of Cathode Rays
• Complete Assignment 1
Electric fields
An electric field is said to exist in any region in which an
electrically charged object experiences a force. An electric
field has both strength and direction.
The strength of the electric field due to
a point charge diminishes with
distance from it. The direction of the
field around a point charge:
• is directed radially away from a
positive point charge; and
• is directed radially towards a
negative point charge.
The number of lines drawn to
represent a field at any point
indicates the electric field strength
at that point. The stronger the field,
the more lines are drawn in a given
space.
The electric field between two
oppositely charged parallel plates is
uniform in strength and direction.
The field direction is at right angles
to the plates and away from the
positive plate.
The electric field due to charged parallel plates
It should be noted that the electric field is not uniform
at the edges of the parallel plates:
Electric field between two parallel charged plates
The electric field strength, E, between two oppositely
charged parallel plates is:
• proportional to the potential difference, V, between
the plates;
• inversely proportional to the separation, d, between
the plates;
Hence,
E = V/d
Where:
Note: the syllabus
expects you to be
able to solve
problems using
this equation
E is electric field strength (V/m)
V is the potential difference between the plates (V)
d is the distance between the plates (m)
Practice Questions
Example:
In a glass tube from which air has been removed, electrons
are accelerated by a uniform electric field between a pair
of charged parallel plates, separated by 5 cm. If an
electrical potential difference of 4 kV is applied across the
plates, determine the electric field strength half way
between the plates.
Solution:
This is a trick question. The field is UNIFORM, hence the field
strength is the same in all areas between the plates:
E = V/d
E = 4000 / 0.05
E = 8 x 104 V/m (N/C)
A charged object placed in an electric field will experience
a force. The magnitude of the force acting on this charged
object is given by:
F = qE
Where:
Note: the syllabus
expects you to be
able to solve
problems using
this equation
F is the force acting on the charged object (N)
q is the magnitude of the charge of the object (C)
E is the electric field strength (N/C or V/m)
Example:
A particle carries a charge of -3.2x10-19C. It is placed in a
uniform electric field of strength 5000 V/m.
(a) Where might such a uniform field exist?
(b) Determine the electrostatic force on the particle.
Solution:
(a) Between two oppositely charged parallel plates
(b) F = Eq
F = 5000 x (-3.2x10-19)
F = -1.6x10-15 N
(the negative reminds us that the force is in the
opposite direction to the electric field)
Moving charges in magnetic field experience a force
As we saw in the topic ‘Motors and Generators’, a magnetic
field exerts a force on electric currents – that is, on moving
charged particles.
The direction of the force can be determined using the right
hand palm rule. Remember that current is in the opposite
direction to the flow of electrons.
The magnitude of the force is a maximum when the charge
moves perpendicular to the magnetic field:
X
X
BX
X
X
X
IX
X
X
X
X X
X X
+
X X
X X
F
X X
X
X
X
X
X
A constant magnitude force,
which is perpendicular to the
velocity of a particle, results
in the particle travelling in a
circular path.
Can you explain why?
The force, F, on a charge moving through a magnetic field
is:
• proportional to the size of the charge, q;
• proportional to the velocity of the charge, v;
• proportional to the magnetic induction, B; and
• proportional to the sine of the angle, 
between the velocity and the magnetic field
lines (being a maximum when it is 90o, and
zero when the velocity is parallel to the field).
F = qvB sin
Note: the syllabus
expects you to be
able to solve
problems using
this equation
F is the force on the charged object (N)
q is the charge of the object (C)
v is the velocity of the charged object (m s-1)
B is the magnetic field flux density (T)
is the angle between the velocity and the magnetic field.
Example 1:
A proton travelling at 5.0x103 m s-1 enters a magnetic field
of strength 1.0 Tesla at 90o. Determine the magnitude of
the force experienced by the proton.
Solution:
Example 2:
The path of a helium nucleus, travelling at 3.0x103 m s-1,
makes an angle of 90o to a magnetic field. The electron
experiences a force of 1.2x10-15 N while in the field.
Calculate the strength of the field.
Solution:
Example 3:
A charge of +5.25 mC moving with a velocity of 300 m/s
north east, enters a uniform magnetic field of 0.310 T
directed from left to right across the page. Calculate the
magnetic force acting on the charge.
Solution:
F  qvB sin 
F  (5.25 10 3 )  300  0.310  sin 45
F  0.345 N into the page
ACTIVITIES:
• Read 10.2 – 10.3
• Complete Assignment 2
• Complete 4 – Electric Fields
• Complete 5 – Charges in Magnetic Fields 1
• Complete 7 – Charges in Magnetic Fields 3
J.J. Thompson’s charge to mass ratio experiment
With the knowledge of how charges
are influenced by electric and
magnetic fields, we can return to the
cathode ray controversy.
Remember that it was J.J Thomson
who finally established the true nature
of cathode rays – they were negatively
charged particles.
In order to do so he carried out an astonishing experiment.
He subjected beams of cathode rays to deflection by electric
and magnetic fields set at right angles to each other. By
doing so, he was able to measure the charge to mass ratio of
cathode rays.
In his experiment, cathode rays were passed through two
narrow slits to make a thin parallel beam aimed at the
centre of a fluorescent screen. Charged parallel plates were
placed to create a uniform electric field that exerted a
upwards force on the beam. An external electromagnet
(called a Helmholtz coil) was placed to produce a uniform
magnetic field that exerted a downward force on the beam.
Thomson manipulated the strengths of the two fields until
the beam passed through both fields undeflected. This
occurred when the two forces on the particles in the beam
were equal and opposite. By equating the expressions for
these two forces, Thomson calculated the velocity of the
particles.
Mathematically:
FB = FE
therefore,
hence,
qvB = qE
v = E/B
Note: the syllabus does
not expect you to be able
to solve problems using
these equations, but you
can use them when you
explain Thompson’s
experiment.
Thomson then removed the electric field and calculated the
radius of the circular path followed by the particles in the
uniform magnetic field alone. By equating the force due to
the magnetic field to the centripetal force, he was able to
calculate that all cathode ray particles had the same charge
to mass ratio of 1.76 x 1011 C kg-1 – this was 1800 times
larger than the q/m value for a hydrogen ion (a proton).
Mathematically:
FB = FC
therefore,
hence,
qvB = mv2/r
q/m = v/Br
Note: the syllabus does
not expect you to be able
to solve problems using
these equations, but you
can use them when you
explain Thompson’s
experiment.
In other experiments, Thomson showed that the charge
on the cathode ray particles was the same magnitude as
the charge on a hydrogen ion (a proton). This combined
with the fact that the q/m ratio for cathode rays was 1800
times larger than that for the hydrogen ion meant that the
mass of the cathode ray particles had to be 1800 times
smaller than that of the proton.
On the basis of all these results, Thomson suggested that
the cathode ray particle was a fundamental constituent of
the atom (i.e. a sub-atomic particle). Although he
originally referred to the particles as ‘corpuscles’, the
name electron slowly became accepted as the official
name.
The cathode ray TUBE
A cathode ray tube (CRT) consists of three main
components:
• A fluorescent screen
• An electron gun
• The deflection system
fluorescent screen
The Fluorescent Screen:
The end of the CRT is coated on the inside surface with
some fluorescent material such as zinc sulfide (ZnS).
When an electron strikes the end of the tube, the material
fluoresces, that is gives off light. This enables a spot of
light to appear wherever an electron (from the electron
gun) strikes the end of the tube.
electron
beam
deflection
plates
The Electron Gun:
This produces a narrow beam of electrons. It consists of a filament
enclosed in a cylindrical cathode electrode, a ring-shaped electrode
called the grid and two cylindrical anode electrodes. The filament is
heated by passing electric current through it. Electrons are then
emitted due to thermionic emission from the heated filament. These
electrons are accelerated towards the anodes by the electric field set
up between the cathode and anodes.
The dual anode system
helps to focus the
electron beam into a
single beam.
The ‘grid’ is negatively charged,
and its magnitude can be varied
easily. What effect would it have on
the brightness of the screen? Why?
electron
beam
deflection
plates
The grid is placed between the cathode and anodes and
constantly varies its negative charge. This enables the
intensity of the electron beam to be controlled – the more
negative the grid, the fewer electrons are permitted to
leave the electron gun and ultimately the spot on the end
of the tube is less bright.
The Deflection System:
This allows the electron beam to be deflected from the
straight-line trajectory with which it leaves the electron gun.
The deflection system usually consists of two sets of parallel
plates, one set in the horizontal plane, and the other in the
vertical plane. When potential differences are applied
between each set of plates, electric fields are set up between
the plates.
The electrons in the beam then experience forces vertically
while passing between the horizontal plates and horizontally
while passing through the vertical plates. Thus, by applying
appropriate voltages to the deflection plates, the position of
the spot on the end of the screen can be controlled.
Both sets of plates are uncharged, and the cathode
ray passes through undeflected.
The horizontal plates are charged and the cathode ray is
deflected vertically.
The vertical plates are now charged, the cathode ray
is deflected horizontally.
Vertical plates
Horizontal plates
Cathode Ray Oscilloscope (CRO)
A cathode ray oscilloscope (CRO) uses cathode rays
to form an image on a fluorescent screen of the
waveform of a periodically varying voltage. This
allows the waveform to be analysed for frequency,
amplitude and irregularities, etc.
An electric field is used to make the beam sweep
horizontally from left to right, then rapidly back to the
start. The periodically varying voltage of the input
signal makes the beam move up and down with the
same frequency as the input signal. The glowing trace
on the fluorescent screen forms a visual representation
of the waveform of the input signal.
Television – a household particle accelerator!
A television picture tube is a cathode ray tube in which the
cathode rays (beams of electrons) are focused onto a
fluorescent screen to form a moving picture. The picture is
formed by combining and synchronising the actions of
horizontal and vertical electric fields (or sometimes
electromagnetic coils), in response to signals from the
television signal.
The horizontal field causes the beam to sweep uniformly
across the screen from one side to the other, then rapidly back
to the start. The vertical field moves the beam down slightly
for each successive sweep, filling the screen with a series of
horizontal lines, until it reaches the bottom, then returning
rapidly to the top. This process is repeated 50 or 60 times a
second.
The intensity of the electron beam is
modulated by the input signal (via the
grid) to cause lighter and darker
spots along on the fluorescent screen,
forming the detail of the picture. The
picture is constantly refreshed, giving
the appearance of smooth motion.
Looking at the electron gun
A television picture tube
Close up, you can see the filament in the
center of the gold colored electron gun.
ACTIVITIES
• Read 10.4 – 10.6
• Assignment 3
Students learn to:
2. The reconceptualisation of the
model of light led
to an
understanding of
the photoelectric
effect and black
body radiation
• describe Hertz’s observation of the
effect of a radio wave on a receiver and
the photoelectric effect he produced
but failed to investigate
Students:
• perform an investigation to demonstrate
the production and reception of radio
waves
• outline qualitatively Hertz’s
experiments in measuring the speed of
radio waves and how they relate to
light waves
• identify data sources, gather, process
and analyse information and use
available evidence to assess Einstein’s
contribution to quantum theory and its
relation to black body radiation
• identify Planck’s hypothesis that
radiation emitted and absorbed by the
walls of a black body cavity is
quantised
• identify data sources gather, process
and present information to summarise
the use of the photoelectric effect in
photocells
• identify Einstein’s contribution to
quantum theory and its relation to black
body radiation
• solve problems and analyse information
using:
• explain the particle model of light in
terms of photons with particular energy
and frequency
• identify the relationships between
photon energy, frequency, speed of
light and wavelength:
E hf
and
c f l
E hf
and
c f l
• process information to discuss Einstein
and Planck’s differing views about
whether science research is removed
from social and political forces
Background Information
In 1865 James Maxwell predicted
the existence of electromagnetic
waves. He derived four highly
complex mathematical equations
that suggested that an accelerating
charge would produce a changing
electric field that would in turn
produce a changing magnetic field.
By Faraday’s Law, this uniformly changing magnetic
field would in turn produce a changing electric field and
so on.
These fields will propagate outwards from the source
through space as a wave with a speed of 3 x 108 m/s. This
speed agreed so closely with values of the speed of light
measured by Fizeau in 1849 and Foucault in 1862 that
Maxwell became convinced that light was a form of
electromagnetic wave. But there was no way to prove it.
Hertz discovers Radio waves
Heinrich Hertz, a German physicist,
achieved the first experimental evidence of
the existence of electromagnetic waves in
1887. Hertz used an induction coil to
produce oscillating electric sparks between
two brass spheres separated by a small
distance. He used a small loop of wire with
a tiny gap in it as the receiver.
Spark gaps
Induction coil
Receiver
Brass sphere
As sparks jumped across the gap between the spheres,
sparks were also observed jumping the gap in the receiver.
Hertz reasoned that the spark discharge oscillating
backwards and forwards between the brass spheres set up
changing electric and magnetic fields that propagated as an
electromagnetic wave, as postulated by Maxwell. When
these waves arrived at the receiver loop, the changing
electric field caused charges in the loop to oscillate, thus
producing the spark across the gap in the receiver.
Spark gaps
Induction coil
Receiver
Transmitter
Brass sphere
EXPERIMENT: Radio waves
AIM:
To demonstrate the production and transmission of radio waves.
MATERIALS: Power pack, induction coil, radio, wires.
RISK ASSESSMENT: The spark across the gap of an induction coil generates
all wavelengths of EMR – including X-rays and short wavelength
UV. These are potentially dangerous.
METHOD: 1.
Adjust the gap on the induction coil to about 5mm and
set the power pack to 6V DC
2.
Adjust the tuner on the radio so that it receives a signal
from a radio station. Set the radio at the back of the laboratory.
3.
Record any observations when the induction coil is turned on
then off.
Hertz conducted several experiments on his waves (he
called them radio waves) and showed that they:
•
travel in straight lines
•
can be reflected by a metal sheet
•
can be refracted
•
can be diffracted
•
can be polarised
•
travel at the speed of light (we will see how
Hertz calculated the speed shortly)
These properties as you may recall are the same as the
properties of light – Hertz had shown that his radio waves
and light waves are part of the same family – (we now call
it the electromagnetic spectrum). It wasn’t long before the
other EMR waves were discovered.
Hertz measures speed of Radio waves
Hertz was able to calculate the velocity of his radio waves
by reflecting the generated waves off a metal sheet and
measuring the wavelength of the standing wave set up by
interference. He the substituted this wavelength and the
frequency of the waves (i.e. the frequency of the induction
coil) into the general wave equation, v = f λ. Hertz then
calculated the wave speed at 3 x 108 ms-1 – the same as
that estimated by Maxwell and measured by Fizeau!
Thus, Hertz’s experiment confirmed Maxwell’s prediction
of the existence of EM waves and provided strong
experimental support for the idea that light was a form of
transverse EM wave.
Hertz and the Photoelectric effect
While conducting his initial experiments, Hertz often
placed the receiver in a darkened box to make it easier to
see the tiny sparks in the gap. He noticed that the sparks
across the gap in the receiver were distinctly weaker when
the receiver was in the box.
After much tinkering, Hertz discovered that the sparks
jumping the gap in the receiver were greater if ultraviolet
light from the initial spark was shone onto the gap.
Although this was a most amazing discovery, Hertz did
not further investigate this phenomenon but confined
his research to the production and study of EM waves.
Other scientists including J.J. Thompson found that the UV light itself
was causing electrons to be emitted from the surface of the receiver.
This effect is known as the Photoelectric effect and we will examine it
shortly, but first, we need to look at black body radiation…
These diagrams show
some experiments
carried out by Hertz.
Remember that he
only had a passing
interest in this effect,
and never actually
investigated it.
Can you describe
what he would have
seen?
ACTIVITIES
• Read 11.1 – 11.3
• Assignment 4
Black body radiation and the Ultraviolet Catastrophe
When objects are heated, they begin to give off light - first red,
then yellow, and finally white.
The dominant wavelength (colour) of a hot object is directly
related to the temperature of the object.
To understand exactly how the wavelength of the radiation
varies with temperature, experiments were carried out using a
‘black body’.
A black body is a perfect emitter and absorber of energy. An
example is a completely sealed oven with a small hole drilled
in one side. At a temperature of say 6000 degrees, the inside
walls of the oven will emit all types of radiation (including
visible light, UV, IR), but will peak in intensity at a particular
frequency (e.g. red – it will therefore appear red to our eyes).
All light that enters the hole will be absorbed by the walls
of the oven, the oven will increase in temperature, and the
walls will emit radiation that peaks at a shorter wavelength
(higher frequency) e.g. blue. Some of the emitted radiation
will escape through the hole and can be analysed using a
spectroscope.
This emitted radiation is called black body radiation,
and when it is plotted on a graph we get a black body
curve.
The diagram below shows the black body curves
(obtained through experiment) of the radiation emitted
at different temperatures:
visible light (VIBGYOR)
Note: all
wavelengths are
emitted, but intensity
peaks at a certain
wavelength that is
dependent upon the
temperature.
Intensity
(energy
radiated)
Wavelength (x10-6m)
The shape of the curves presented a problem for scientists
trying to explain the intensity and wavelength variations.
Classical ‘wave theory’ states that light is a wave. It
predicted that as the black body became hotter, the
wavelength of the peak emitted radiation would become
shorter, and the intensity (amount) of this peak wavelength
would increase (i.e. as the oven becomes hotter it will
produce more blue light than the other wavelengths and
therefore appear blue).
This in itself is not a problem (after all it explains what we
actually see), but when the predicted curve is plotted we
get the following:
Classical (wave)
theory of light
predicted this curve
Experimental
results gave
this curve
UV
What happens at
the wavelength of
UV to the amount of
energy released (i.e.
the intensity) by this
object according to
classical (wave)
theory?
Visible
Infra-red
The classical curve shows that as the energy decreased in
wavelength from the visible into the UV portion of the
spectrum, the intensity of the radiation emitted from the
hole in the black box would approach infinity! This
increase in energy would violate the law of conservation
of energy!! This was called the Ultraviolet catastrophe.
Experimental results clearly show the radiation intensity
curve corresponding to a particular temperature having a
definite peak and then declining – take another look at
the black body radiation curves:
visible light (VIBGYOR)
Intensity
(energy
radiated)
Note: all
wavelengths are
emitted, but
intensity peaks at a
certain wavelength
that is dependent
upon the
temperature.
It was becoming apparent that light might
not be a wave! Clearly, a new approach
was needed. This came in 1900 when a
German physicist, Max Planck, suggested
a revolutionary idea.
Planck suggested that all EMR was
emitted or absorbed by a black body in
discrete quanta (packets of energy) rather
than continuously, as suggested by
classical physics. This daring hypothesis
led to the successful explanation of the shape of the black body
radiation curves. The big problem was that this explanation was
based purely on mathematical equations, and the only way it
could work was if Planck introduced a ‘fudge’ constant, h,
(6.626 x 10-34) into his calculations.
Unfortunately, Planck had no real direct experimental
evidence for his theory. So he put it aside and forgot about
it!!
With time and the contributions of many physicists – most
notably Albert Einstein (who was now on the threshold of
obtaining the experimental evidence needed to support
Planck’s quantum theory), Planck’s hypothesis that all EMR
is quantised eventually was proven to be correct and
ultimately led to the development of a whole new branch of
Physics called Quantum Physics. But in order to see how
this occurred, we need to return to the photoelectric
effect…
ACTIVITIES
• Read 11.4 – 11.5
• Read Black Body Radiation and Quanta handouts
• Complete Assignment 5
• Complete 17 – Planck and Black Body Radiation
The Photoelectric effect
As mentioned previously, Hertz stumbled across a curious
effect of light when conducting his EM wave
experiments. Sparks jumping the gap in his receiver were
more vigorous when the receiver was exposed to
ultraviolet light. Because both light and electricity were
involved in this phenomenon, it was called the
photoelectric effect.
In 1900, Philip Lenard (Planck’s student) showed that the
photoelectric effect is actually the emission of electrons
from the surface of material when the material is
illuminated by light of high frequency.
EXPERIMENT:
The photoelectric effect.
AIM:
To determine whether altering the intensity and frequency of
the incident light on a metal surface will have an effect on the
number of electrons ejected and the kinetic energy of these
ejected electrons
BACKGROUND: The photoelectric induced current is a measure of the number
of electrons released. The applied voltage (stopping voltage)
needed to stop the photoelectric induced current is a measure
of the electrons Ek
MATERIALS:
Photoelectric apparatus.
METHOD:
…
-
Our experiment showed that:
The number of electrons released (the photocurrent)
is proportional to the light intensity.
-
The emission of photoelectrons was virtually
instantaneous (if it occurred at all).
-
Emission was frequency dependent. There is a certain
threshold frequency below which no photoelectrons
were emitted.
-
As the intensity of the light increased, the maximum
kinetic energy of emitted electrons remained
constant. The maximum kinetic energy of emitted
electrons was found to depend on the frequency of the
light used.
Refer to the following diagrams…
maximum Ek of ejected electrons (J)
The energy required to
release an electron from
the surface of the
cathode is called the
work function (j)
frequency of light (Hz)
j
threshold frequency f0
The gradient of the line is a
constant for all types of metals
used as the cathode. This gradient
is is equal to 6.626 x 10-34, and is
exactly the same value
hypothesised by Planck – it is
called Planck’s constant, h.
The photoelectric effect didn’t make sense!
The last three of our experimental results could not be
explained by classical wave theory.
Classical theory for instance predicted that electrons in any
surface that absorbed low intensity radiation of any
frequency should accumulate this energy over time and
eventually have sufficient energy to be ejected.
The stage had now well and truly been set for a radical
overhaul in our thinking about the nature of light –
classical theory had to go!
Einstein and the Photoelectric effect
In order to explain the photoelectric effect, Albert Einstein
proposed that light energy was transmitted in discrete
packets of energy rather than as a wave (this is exactly
what Planck had hypothesised when he tried to explain
black body curves!!).
The amount of energy in each packet is a quantum (a
discrete amount) and represents the quantity of energy
associated with particular frequency.
Einstein gave the name photon to a quantum of energy.
The energy of a photon can be calculated using the
following equation:
E = hf
where:
Note: the syllabus
expects you to be
able to solve
problems using
this equation
E = energy of the photon (J)
h = Planck’s constant (6.626 x 10-34 J/ s)
f = frequency (Hz)
Note that h is the gradient of the K.E./frequency graph
for the photoelectric effect.
This model of light is referred to as the particle model.
Einstein used his particle model of light to explain the
photoelectric effect in the following way:
 Light striking a surface consists of photons.
 Each photon carries an energy (hf) into the surface.
 Each photon gives up all of its energy to a single
electron.
 Part of that energy (j) is used in causing the electron
to pass through the metal surface.
 It follows that if the photon energy (hf), exceeds the work
function (j) , then the photoelectric effect occurs and the
electron escapes with the ‘leftover’ KE.
This explanation is summarised in Einstein’s photoelectric
equation:
KEmax= (hf - j)
where j is the work function of the metal surface and is
the minimum energy required to remove an electron from
the surface.
and since,
j = hfo
where fo is the threshold frequency below which no
photoelectrons are emitted. Then, the equation above
Note: the syllabus does not
becomes:
expect you to be able to
KEmax = h (f - fo)
solve problems using this
equation, but you can use it
when you explain Einstein’s
particle model of light
(quantum theory).
Einstein’s experimental evidence based on the photoelectric effect
had now confirmed Planck’s hypothesis that the light emitted in
the black body experiments was quantised. Remember that this
was the only way that Planck could explain BBR curves.
Planck’s ‘fudge’ constant used in his mathematical explanation of
BBR curves was validated by Einstein – after all, he obtained the
same value experimentally in his explanation of the photoelectric
effect!!
Planck’s Quantum Theory had now been established – all EMR
was quantised, and not simple waves as classical theory had
suggested.
Today, we view light as having a dual character. Light behaves
like a wave under some circumstances and like a particle, or
photon, under others. This is known as the wave-particle duality
of light (and all EMR).
Thus, all EMR have both wave and particle characteristics –
they are basically bundles or packets of wave – NOT a
continuous wave as classical theory suggested. This is the basis
of Quantum Theory.
For each photon, its energy is related to its frequency, and as
we know, frequency is related to both the speed of light and
wavelength. We can see the connection in the two equations
below:
c=fl
and
Note: the syllabus
expects you to be
able to solve
problems using
these equations
E=hf
Remember that h = Planck’s constant (6.626 x 10-34 J/ s)
Example 1:
What energy would be associated with a photon of red
light of frequency 3.85x 1014 Hz?
Solution:
E = hf
E = (6.626 x 10-34) x (3.85x 1014)
E = 2.55 x 10-19 J
Example 2:
If a photon has an energy of 1.0 J, what will its
frequency be?
Solution:
E = hf
f = E/h
f = 1.0 / (6.626 x 10-34)
f = 1.6 x 10-33 Hz
Example 3:
At what velocity would a 1.75 x 10-6 g particle have the
same energy as light of wavelength 4.62 x 10-13 m?
Solution:
c = fl
KE = ½ mv2
f = c/l
f = (3.0 x 108) / (4.62 x 10-13)
f = 6.49 x 1020
Now,
E = hf
E = (6.626 x 10-34) x (6.49 x 1020)
E = 4.3 x 10-13 J
v
2 KE
m
2  (4.3 x 10-13 )
v
1.75 x 10-6
v = 2.22 x 10-13 m/s
ACTIVITIES
• Watch the Photoelectric Effect DVD
•Read 11.5
• Read Black Body Radiation
• Read The Photoelectric Effect
• Complete 18 – The Photoelectric Effect
Uses of the Photoelectric effect
Photocells:
The electrons that initiate an electric current are produced
by the photoelectric effect. They also contain
semiconductor material discussed in the next section.
They are used as ‘electric eyes’ (to open doors
automatically), radiation detectors and light meters.
You have used them when you used photogates. They
turn street lights on and off by detecting the amount of
light.
Because photocells can detect the amount of light,
they can also be used to measure the concentration of
particles in liquids and gases, as these particles scatter
light. The more particles, the more light is scattered.
Therefore they can also be used as breathalysers, to
measure the purity of drinks and to determine the
level of pollution.
Einstein vs Planck: The clash of values
Einstein and Planck were great friends – both were
German and shared many of the same laboratories.
They collaborated on many scientific works, yet they
had very differing views regarding whether their work
should be used for the pursuit of scientific knowledge
or the evils of war.
ACTIVITIES
• Read 11.6
• Read Source 1 and Source 2
• Complete Assignment 6
3. Limitations of past
technologies and
increased research
into the structure of
the atom resulted in
the invention of
transistors
Students learn to:
Students:
• identify that some electrons in solids
are shared between atoms and move
freely
• perform an investigation to model the
behaviour of semiconductors, including the
creation of a hole or positive charge on the
atom that has lost the electron and the
movement of electrons and holes in opposite
directions when an electric field is applied
across the semiconductor
• describe the difference between
conductors, insulators and
semiconductors in terms of band
structures and relative electrical
resistance
• identify absences of electrons in a
nearly full band as holes, and
recognise that both electrons and
holes help to carry current
• compare qualitatively the relative
number of free electrons that can drift
from atom to atom in conductors,
semiconductors and insulators
• identify that the use of germanium in
early transistors is related to lack of
ability to produce other materials of
suitable purity
• describe how ‘doping’ a
semiconductor can change its
electrical properties
• identify differences in p and n-type
semiconductors in terms of the
relative number of negative charge
carriers and positive holes
• describe differences between solid
state and thermionic devices and
discuss why solid state devices
replaced thermionic devices
• gather, process and present secondary
information to discuss how shortcomings in
available communication technology lead to
an increased knowledge of the properties of
materials with particular reference to the
invention of the transistor
• identify data sources, gather, process,
analyse information and use available
evidence to assess the impact of the
invention of transistors on society with
particular reference to their use in microchips
and microprocessors
• identify data sources, gather, process and
present information to summarise the effect
of light on semiconductors in solar cells
A new model of the atom
Einstein and Planck had challenged the way that we look
at EMR. Previous models visualised light as waves.
Physicists now described light (and all EMR) as
photons. In doing so they pioneered a new way of
describing both matter and energy. This is the waveparticle duality of matter and energy. This then, meant
that the model of the atom had to be adjusted!
Neils Bohr presented a new particle model
of the atom, where the electrons orbits
were quantised:
Background information
Nucleus
lowest
energy
level (n=1)
n =2
diagram 1
The electrons were only allowed to occupy discrete energy
levels (the orbits).
The lowest energy state of an atom is called the ground
state. This is when the electrons fill up all of the energy
levels as close to the nucleus as they can.
Background information
Wolfgang Pauli in his ‘Pauli exclusion
principle’ worked out that only 2 electrons
could exist in the lowest energy level and 8
electrons were permitted to exist in the
second energy level at any one time.
Using Pauli’s work, Prince de Broglie
came up with a new model of the atom.
He suggested that:
- electrons certainly have mass, but
should not be considered as tiny ball
satellites orbiting the nucleus.
- electrons should be considered as
standing waves with whole number
wavelengths.
Background information
The waves (electrons) are therefore quantised, meaning
that they are allowed to occupy only certain positions out
from the nucleus.
de Broglie could now explain that when electrons jump
down a single energy level, they emit a photon and lose
one whole wavelength from their shape (therefore this
photon has a particular frequency!!). If electrons absorb
energy, they move out from the nucleus to the next
quantised energy level.
Background information
Conductors, Insulators and Semiconductors Band Structures
A normal atom at absolute zero temperature has an
electron occupying every one of the lower energy levels,
starting outward from the nucleus and continuing until all
electrons have been placed.
Quantum theory shows that when two atoms are brought
close enough together, each of the allowed electron
energy levels within the atoms splits into two distinct but
closely spaced energy levels as they ‘overlap’ to form the
two-atom system.
In a six-atom system for example, each allowed electron
energy level is split into a set of six separate but closely
spaced energy levels.
Clearly then, as we pack more and more atoms closely
together each set of split energy levels contains more and
more levels spread over the same energy range allowed at
that particular radius from the nuclei.
So, in a crystalline solid, such as a metal or a diamond,
where billions upon billions of atoms are packed closely
together, the energy levels within each set become so
closely spaced in energy that they form a practically
continuous energy band.
In solids, the bands of permissible energy levels are
called the allowed bands. These may either be filled
with electrons or empty depending on whether they
correspond to filled or empty energy levels in the
isolated atoms.
The spaces between these allowed bands are called
forbidden energy gaps. These forbidden energy gaps
correspond to the gaps between electron energy levels
within the isolated atoms.
The highest energy band that is occupied by electrons is
called the valence band. The very next empty energy
band is called the conduction band.
If an electron can get into the conduction band, it is
called a free electron and can drift from atom to atom
in the solid. In other words, some electrons are shared
between atoms.
This means that the solid becomes a conductor of
electricity.
The wider the forbidden energy gap between the
valence and conduction bands, the more likely the
solid is to be an insulator.
The concept of overlapping energy levels (bands)
and forbidden energy gaps in solids can best be
described with a diagram:
In an insulator, there is a substantial gap between the
valence band and the conduction band. You need to
supply a lot of energy to an insulator in order to get
electrons to jump up to the conduction band. Since there
are no free electrons in the conduction band, the solid,
as expected, acts as an insulator – it has a high electrical
resistance.
Conduction Band
BIG energy gap
Valence Band
A lower band
This is empty
This is at least
partially full
This is full
In a conductor (eg a metal), the valence band and the
conduction band actually overlap. This means that there
are many free electrons in the conduction band. The
valence band is also the conduction band!! It has a low
electrical resistance.
Conduction Band
Valence Band
A lower band
The valence band is full,
and the next energy level
(the conduction band) is
partly full. This is because
the valence band and the
conduction band overlap.
This is full
The band diagram for a semiconductor is similar to that of an
insulator, except that there is a smaller forbidden energy gap
between the valence band and the conduction band than with
an insulator. They are therefore insulators, but not quite as
insulating as insulators. They only need a bit of energy to
excite the electrons up to the conducting band, but once they
are there, they are free to move around. They have electrical
resistance between those of conductors and insulators.
Conduction Band
Smallish energy gap
This is empty
This is full
Valence Band
A lower band
This is full
Here is another band diagram showing all the solids
together.
Semiconductors can have HOLES!
At low temperatures, all electrons in a semiconductor fill
the valence band and it is therefore an insulator. But, as the
temperature increases, thermal energy allows some
electrons to jump the small gap into the conduction band.
This leaves the valence band unfilled.
This means that a‘hole’ has been created in the valence
band where an electron has left.
Since this hole represents the absence of an electron, it can
be treated as a positive charge.
A nearby electron then moves into the hole, creating a new
hole at the position from which it moved, and so on.
In essence, these holes act as a positive flow of current,
moving in the opposite direction to the electron current
flow.
As such, current flow is now possible in both the valence
band (as a flow of positive holes) and in the conduction
band (as a flow of electrons).
In a pure semiconductor the number of holes in the
valence band is exactly equal to the number of electrons
in the conduction band. The conduction of a piece of
pure silicon (a semiconductor) is very low compared with
copper, because there are relatively few free electrons.
Who cares !?!?
You should! Semiconductors have led to the invention of
an incredibly important piece of technology – the
transistor. With its development, unprecedented progress
has taken place – particularly in the field of computers.
Enter the silicon chip! We’ll look at this stuff shortly.
But first… a very very silly experiment…
EXPERIMENT:
Modeling the behaviour of semiconductors.
AIM:
To model the behaviour of semiconductors, including the
creation of a hole and the movement of holes and electrons in
opposite directions when an electric field is applied across the
semiconductor
MATERIALS:
20 tennis balls, 10 students.
METHOD:
…
ACTITIVES
•
Read 12.1 – 12.3
Germanium,Silicon and Transistors
Early devices made of semiconductor materials included
diodes and transistors. Diodes are rectifiers – they block
the backward and forward movement of electric current in
a circuit (i.e. they act as a one way traffic cop – consider
their effect on AC current!). Transistors are manufactured
to act as switches and amplifiers in integrated circuits.
The most widely used semiconductor materials are made
from crystals of elements of Group IV of the periodic
table.
Group IV atoms have 4 electrons in their outer (i.e.
valence) energy level. This means that they have a half
complete valence shell. This is a great source of atomic
unhappiness and they desperately want another 4
electrons in order to fill it.
In order to see how this can happen, we will look at the
example of Germanium (symbol Ge) – a Group IV
element.
Ge is very rare on Earth and is quite brittle. It only became
economically significant after 1945 when its
semiconducting properties were first recognised as being
useful in electronics. The first transistor was made of
germanium.
In order to fill its valence shell, a Ge atom shares a
valence electron with each of four adjacent Ge atoms.
Each of these atoms contributes an electron to the
partnership, forming a pair of electrons which act to bond
the atoms together – this is a covalent bond.
This occurs over and over with many individual atoms –
eventually forming a crystal of Germanium.
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Here we can
see the regular
arrangement
of atoms – this
is called a
crystal lattice.
Ge crystals could be grown with tremendous purity. Purity
is important as contaminants as little as one part per
billion will upset the electrical properties.
Despite its rarity and fragile nature, Ge was the
semiconductor of choice – for one simple reason – no
other Group 4 element was capable of being produced at
a suitable purity.
Silicon (Si) is also a group IV element and is the second
most abundant element (by weight) in the Earth’s crust.
Single crystals are grown slowly from molten silicon.
This is a difficult process, and it was not until the late
1950’s that silicon crystals of suitable purity could be
produced.
Germanium use was surpassed by silicon in the 1960’s,
when it became the new semiconductor of choice for use
in solid state devices.
Both Ge and Si do not conduct electricity at room
temperature. In fact they are unable to conduct
electricity unless the bonds are broken by the application
of heat. This is undesirable, as the crystal lattice would
break up!
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Enter the dope…
Ge
Ge
Ge
Ge
Ge
Doping semiconductors
Adding an impurity to the crystal lattice of a semi-conductor can
actually be a good thing – only if done properly!! Adding an
impurity is called doping.
We can dope Si by melting a part of the crystal and adding small
quantities of either group III atoms, or group V atoms. It can also
be done using transmutation in a nuclear reactor.
A group III element has only 3 electrons in it valence shell e.g.
boron, aluminium, gallium and indium.
A group V element has 5 electrons in it valence shell e.g. nitrogen,
phosphorous, arsenic and antimony.
By doping a semiconductor we can radically change its electrical
properties!
p-type Semiconductors
When a dope group III atom replaces a silicon atom, an
electron is missing from the crystal lattice. There is an
empty quantum position in the valence band. This is a
positive hole!! This is called a p-type semiconductor.
p-type semiconductors have positive holes.
p-Type Semiconductor
Indium has
3 valence
electrons
NOT 4
Si
Si
Si
Si
Si
Si
In
Si
Si
Si
Si
Si
Si
In
Si
Positive holes
in crystal
structure
where an
electron is
missing
diagram 7
Such p-type materials are inclined to grab electrons
from outside to repair the defect holes. It is important to
note that the overall charge of the semiconductor is
neutral, because the number of protons in the crystal
exactly balance the number of electrons, so they often
become negative when brought in contact with other
materials.
The diagram below shows how p-type semiconductors
conduct electricity readily under the application of an
external voltage:
p-type semiconductors conduct electricity by the
migration of positive holes.
n-type Semiconductors
n-type semiconductors conduct electricity by the
migration of electrons, which are surplus to the crystal
structure.
The electrons sit in energy levels above the valence band
(i.e they are in the conduction band).
They are called n-type because negative charges move.
To make an n-type, you have to melt the silicon (or Ge) and
add small amounts of group V elements.
An n-type semiconductor is shown below:
Arsenic has
5 valence
electrons
NOT 4
Si
Si
Si
Si
Si
Si
As
Si
Si
Si
Si
Si
Si
As
Si
Electrons
surplus to
crystal
structure
The following diagram shows how n-type semiconductors
conduct electricity readily under the application of an
external voltage:
p-type and n-type semiconductors differ in terms of the
relative number of negative charge carriers (electrons)
and positive holes.
n-type obviously have a large number of negative charge
carriers (i.e. free electrons) because they have been
doped with Group V elements.
p-type have a large number of positive holes because
they have been doped with Group III elements.
In comparison, undoped semiconductors have no free
electrons or positive holes (unless they are heated up).
Thermionic devices
Thermionic emission is the spontaneous emission of
electrons from solids (and liquids) when they are heated
to high temperatures. (Remember the filament inside the
electron gun in TV tubes??)
The first application of semiconductors was in radio
technology - they replaced the old valve radios.
These valve wirelesses were the first amplifying radios
capable of driving a speaker. The valve was a thermionic
device called a triode.
In order for a radio to work, the very faint radio wave has
to be made not so faint!
You will recall that radio waves set up a current in an
antennae (remember Hertz’s experiment) – but this current
is very very small. It has to be boosted (amplified)
somehow. A triode could do this very effectively. The
diagram below shows how a triode works:
A cathode is heated by connecting it to a high voltage circuit,
it then emits electrons thermionically and a current flows
towards the anode. This current is used to make the speaker
work. In between the two electrodes is a third (hence the name
triode) electrode called the grid.
The grid becomes negatively charged because it is connected
directly to the antennae, and it varies in charge constantly.
This causes the electrons that are trying to pass through the
grid from the cathode to slow down or even stop. This has the
effect of constantly varying the current that arrives at the
speaker – sometimes lots of current (a loud sound) and
sometimes no current (no sound).
In this way the current produced by the radio wave is
amplified!
EXPERIMENT:
Thermionic devices.
AIM:
To examine a triode in order to identify the cathode, anode
and grid, and to determine how this device can act as an
amplifier of radio waves.
MATERIALS:
Triode.
METHOD:
…
first
Solid State devices replace Thermionic devices
Thermionic devices dominated the war years, and the allies
secret weapon was RADAR.
The race was on to build better valves for higher frequency
RADAR’s that could detect smaller targets. Eventually, the
scientists realised that there was a frequency limit above
which thermionic valve devices could not detect.
This current technology had shortcomings! Research into
an alternative technology to replace thermionic devices
began.
The search for a replacement technology lead to an
increased knowledge about the properties of many
materials – especially elements that were known as
semiconductors.
It soon became apparent that semiconductors could
easily, efficiently and cheaply replace valves. All
electronic devices made of semiconductors are called
‘solid-state’ devices.
The first solid-state device was called the transistor.
Transistors replaced valves, since they did the same job –
only better.
The advantages of solid state devices over thermionic
devices are:
• solid state devices are much smaller than valves. Huge
numbers of transistors can be printed onto microchips.
• solid state devices require tiny voltages and current to
operate. Valves used the entire mains voltages, took
time to warm up, and wasted huge amounts of energy
as heat.
• because of the low current use in solid state devices,
other circuit components could be miniaturised.
• the huge power use caused valves to burn out quickly,
like light bulbs.
• solid state devices can be made robotically and
cheaply from simple raw materials like sand. Valves
were difficult to manufacture.
• there was a frequency limit beyond which valve
devices would not operate – this limited their use in
such things as RADAR and long distance phone
calls.
Solid State devices change the world
Transistors are solid state devices that allow the current in
one circuit to vary the current in a second circuit, just like
the old valves (triodes) used to do).
They are particularly useful in computers because they act
as ‘gates’ in microchips, where they control the flow of
charge. The gates are either open or closed (1 or 0). Other
transistors acts as amplifiers when information is being
retrieved. Miniature transistors pack modern integrated
circuits (microprocessors).
How solid state devices work
e.g.1 The semiconductor diode:
These consists of a piece of n-type material in contact
with p-type material:
When the voltage is connected as shown in the diagram,
electrons on the n-side and holes on the p-side migrate
towards the junction and current flows in the circuit (i.e.
it is a conductor). If you reverse the voltage, the
electrons and holes migrate away from the junction so
that current flow stops. The diode is now an insulator.
The diode therefore prevents backward motion in AC
currents.
e.g.2 The pnp transistor:
These amplify the AC current that is induced in radio
antennae. They transfer the AC current from the
antennae to a mains (high voltage) circuit (they were
originally called ‘transfer resistors’ – hence
‘transistor’).
A pnp transistor has 3 slices: the base, collector and
emitter. It performs exactly the same function as the old
triode valves. It takes a small AC current from the
antennae and ‘imprints’ it onto the mains current. This
varies the amount of current fed into the speaker of the
radio – in this sense, the current coming from the
antennae has been ‘amplified’. Examine the following
diagram:
Arrows show
electron flow
P (collector)
N (base)
Mains
voltage +
P (emitter)
+
speaker
Antennae
voltage
Solar cells
Solar cells, or photovoltaic cells, are used to power lights,
telephones, radio beacons, calculators and other household
devices .
Solar cells are made up of a number of layers. The antireflective coating reduces reflection losses and the glass
cover provides protection from the weather while
allowing light through.
n and p semiconductors are layered together and are
electrically neutral. Electrons from the n-layer migrate into
the p-layer until an equilibrium is formed, with the p-layer
negatively charged and the n-layer positively charged. This
is called a depletion layer, which is an electric field.
Light photons provide energy for electrons to move from
the valence band to the conduction band and so move
freely. Under the influence of the electric field (depletion
layer) the electrons will move to the n-layer. The electrons
are collected by a metal contact on the n-layer side and
moved around a circuit (where they do work) back to the
p-layer side.
ACTIVITIES
• Read 12.4 – 12.9
• Research task - Assess the impact of the invention
of transistors on society with particular reference to
their use in microchips and microprocessors
• Read Microchip and The Impact of Transistors
• Complete Assignment 7
Students learn to:
4. Investigations into
the electrical
properties of
particular metals at
different
temperatures led to
the identification of
superconductivity
and the exploration
of possible
applications
• outline the methods used by the Braggs
to determine crystal structure
• identify that metals possess a crystal
lattice structure
Students:
• process information to identify some of
the metals, metal alloys and compounds
that have been identified as exhibiting
the property of superconductivity and
their critical temperatures
• describe conduction in metals as a free
movement of electrons unimpeded by
the lattice
• perform an investigation to demonstrate
magnetic levitation
• identify that resistance in metals is
increased by the presence of impurities
and scattering of electrons by lattice
vibrations
• analyse information to explain why a
magnet is able to hover above a
superconducting material that has
reached the temperature at which it is
superconducting
• describe the occurrence in
superconductors below their critical
temperature of a population of electron
pairs unaffected by electrical resistance
• discuss the BCS theory
• discuss the advantages of using
superconductors and identify limitations
to their use
• gather and process information to
describe how superconductors and the
effects of magnetic fields have been
applied to develop a maglev train
• process information to discuss possible
applications of superconductivity and the
effects of those applications on
computers, generators and motors and
transmission of electricity through power
grids
Diffraction
Diffraction occurs when waves pass through an opening.
If the width of the opening is of a similar dimension to the
wavelength, maximum diffraction occurs:
Background information
Why is it that person A,
standing beside an open
doorway, can be heard inside
the room by person B but
cannot be seen by her?
Background information
With two openings, the diffracted waves interfere (either
constructively or destructively) with each other, and we
get an interference pattern:
1. What type of
interference is
occurring at A?
2. What type of
interference is
occurring at B?
3. If the waves were
red light waves,
draw what you
would see on a
screen held along
the line X-Y.
Background information
The Braggs – Crystal structure
William Henry Bragg (1862–1942) and his son, William
Lawrence Bragg (1890–1971), used the principle of
diffraction to study crystals – in particular to measure the
spacing between atoms in crystals. They used X-rays
because their wavelength (10-10) is similar to the separation
between the atoms. In 1915 they won the Nobel Prize in
physics for their work. They were Australian.
The X-rays were reflected from layers within the crystal.
The reflected X-rays produced an interference pattern,
and the pattern was recorded on photographic film.
Subsequent mathematical analysis of the diffraction
pattern permitted the crystal structure to be deduced.
The diagram shows X-rays being reflected from adjacent
atomic planes (i.e. rows in the crystal lattice). The
reflected X-rays then interfered constructively and
destructively producing the familiar interference pattern
on the photographic plate.
Incoming X-rays
of wavelength l

d
ions in crystal lattice
Reflected
X-rays
By measuring the angles, the distance to the screen and
the separation between the bright and dark patches on the
interference pattern, the spacing of atoms (d) and their
arrangement within the crystal was calculated.
They showed that metal crystals are composed of an
orderly arrangement of atoms. This is called a crystal
lattice.
A typical x-ray crystallography diffraction interference
pattern
Knowledge of the lattice structure of metals led to the
development of models to explain why metals are such
good conductors.
This eventually resulted in theoretical suggestions that
superconductors should exist and finally led the discovery
of superconductors themselves!
But before we can look at superconductors, we must first
examine the conduction of electricity in metals…
Conduction in metals
As we already know from the study of band structures,
metals are good conductors because their valence band
overlaps with their conduction band, resulting in the
presence of many free electrons in the conduction band.
Once there are free electrons, they can move around from
atom to atom in the crystal lattice. (Recall that the
movement of electrons is defined as a current).
Notice that there is no net movement of electrons in any
one direction.
Under the influence of an applied potential difference
however, the electrons drift in a net direction. This is called
an electric current.
–
+
–
+
–
+
–
+
The instantaneous speed of the electrons is approximately
1.6x106 m/s, but the average drift velocity is only 1.0x10-5 m/s.
Suggest a reason for this!
Answer:
Since the electrons are moving enormously fast, they
undergo random collisions with other electrons, with
impurities in the lattice and imperfections in the crystal
lattice.
Although drift velocity is small, the speed at which the
influence of the current moves is close to the speed of
light. Can you explain why? Background information
Metals have resistance
As we saw earlier, conduction electrons undergo collisions
with the atoms in the lattice – this causes the electrons to
scatter. The collisions cause the atoms in the lattice to
vibrate, and the conductor becomes hotter. As the
temperature increases, the atoms in the lattice vibrate more,
increasing the chance of collision (and scattering) and so
resistance increases further…
Resistance
Temperature
The resistance of metals is also increased by the presence
of impurities in the lattice.
These impurities are usually other atoms that ‘corrupt’
the crystal lattice. This then increases the chance of
collision between the free electrons and the lattice,
hence the resistance increases.
It follows that if a temperature increase causes an increase
in resistance, then if a metal conductor is cooled, there
would be less vibration and fewer collisions. This would
cause the resistance to decrease.
Think carefully about this!!
…if the resistance of a conductor decreases as
temperature decreases…
…and if you could reduce the temperature of the
conductor low enough…
…then theoretically you should reach a stage where it
has zero resistance…
…and a conductor like that would be SUPER!
Superconductors
Superconductors are materials which, below a particular
temperature called the critical temperature allow
electrons to move through the crystal lattice unimpeded.
This means that these materials have lost all resistance to
the flow of electricity.
The first superconducting material
was discovered by Heike Onnes, in
1911. His results are shown on the
next page:
Resistance curve for Mercury
0.16
0.12
0.08
0.04
-273
-271
-269
Temperature (oC)
-267
Examples of Superconductors
Since Onnes discovered
the superconducting
property of mercury,
many more materials have
been shown to have the
ability to superconduct.
You can see that the
critical temperature at
which the transition to the
superconducting state
occurs is different for
different materials.
Some alloys have higher critical temperatures than the
constituent elements.
Ceramic materials and other compounds have been made
with even higher critical temperatures than the metal alloys
- these are called high-temperature superconductors.
Why do you think there is a special interest in hightemperature superconductors?
BCS theory
In 1957, Bardeen (B), Cooper (C) and Schrieffer (S)
developed a theory (known as BCS theory) to explain how
superconductors work. Their theory described the formation
of pairs of electrons in materials when they are below their
critical temperature. These pairs of electrons were called
‘Cooper pairs’.
Realising that it was the interaction of electrons with the
vibrations of the crystal lattice that produced
superconductivity, they proposed that forcing electrons to
pair up into ‘Cooper pairs’ would result in them being able to
bypass the obstacles in the crystal lattice which were
responsible for electrical resistance.
Here is what they proposed…
1.
An electron moves through the lattice. As it moves it
causes the lattice to distort (because it attracts the
positively charged lattice ions). This sets up a
disturbance (called a phonon) in the lattice:
2.
There is now an increased positive charge density
(caused by the lattice distortion) directly behind the
electron. This causes a second electron to be
attracted to the first in what is called a phononmediated attraction, and travel through the crystal
as a pair. This pair of electrons is called a ‘Cooper
pair’.
3.
All of the Cooper pairs represent a highly ordered
system and when an external electric field is applied,
the pairs move through the lattice with each pair’s
motion synchronised to that of every other pair so
that none are involved in the random scattering
within the lattice (which would otherwise give rise
to electrical resistance).
+
+
+
Cooper pair
+
+
+
Obviously superconductivity only occurs if the material is
cooled to the correct critical temperature. If the
temperature rises above the critical temperature, the lattice
vibrations increase and break up the Cooper pairs.
Superconductivity is lost.
Advantages and applications of Superconductors
APPLICATION
Computers
ADVANTAGES
The speed and further miniaturisation
of computer chips is limited by heat
generation. Super-conductive film can
be used. Switching devices (Josephson
junctions) are being tested for use in
high-speed computers, they are around
10 times faster than the semiconductor
switches currently used.
APPLICATION
ADVANTAGES
Generators
Superconducting magnets would not
need an iron core – therefore they
would be much smaller and lighter.
Higher efficiency in generation results
in less energy wasted, so less fossil
fuels would be burnt - BIG positives
for the environment.
Motors
Increased efficiency results in less
demand for electricity (see above).
Electricity
Transmission
No electrical loss therefore thin wires
could carry very large currents.
Limitations of Superconductors
•
Superconductors have a critical temperature, above
which it returns to a normal conductor. Metal superconductors require temperatures a few degrees above
absolute zero!! It is incredibly difficult to do this on a
large scale, and prohibitively expensive to cool them.
•
Modern ceramic superconductors operate at higher
temperatures, but they are brittle – they lack the
flexibility of metals. Also it is very difficult to make
them into wires.
•
Some superconducting compounds are chemically
unstable – this limits their application.
Magnetic levitation – the Meissner effect
In 1933, W. Meissner discovered that a superconductor
does not allow an external magnetic field to penetrate its
interior. The exclusion of a magnetic field is called the
Meissner effect. A consequence of this is that a
superconductor in its superconducting state can cause an
external magnet to levitate above it!!
This is how it works:
When a superconducting material in its normal state is
placed in a magnetic field, the magnetic field strength
inside the material is almost the same as outside. This is
true of all metals.
But, when the material is cooled so that it is in its superconducting state, electric currents begin to flow through
it, and as we know, these currents will produce their own
magnetic field.
These induced magnetic fields cancel out the applied
magnetic field inside the super-conductor. That is, the
magnetic field inside the superconductor is zero.
The magnetic field caused by the flowing currents not only
cancels out the external magnetic field but also extends
outside the superconductor itself.
If a magnet is bought close to a superconducting
superconductor, the currents set up magnetic fields with
poles in such a direction that cause repulsion between the
magnet and the superconductor (remember Lenz’s Law??).
This is the cause of magnetic levitation.
Currents produced
in superconductor
extending outside.
Magnet
N
Superconductor
N
S
EXPERIMENT:
Magnetic levitation.
AIM:
To observe the Meissner effect – the levitation of a magnet
above a superconductor.
MATERIALS:
Superconductor, small neodymium magnet, liquid nitrogen,
petri dish, styrofoam cup, plastic tweezers, insulated gloves
METHOD:
1. Carefully fill the styrofoam cup with liquid nitrogen.
Place the petri dish on top of the cup, and pour in enough
liquid nitrogen until it is about 1cm deep. Wait until the
boiling subsides.
2. Using the tweezers, carefully place the superconductor in
the petri dish. Again wait until the boiling subsides.
3. Using the tweezers, carefully place the small magnet
about 2 mm above the centre of the superconductor.
Release the magnet.
4. While the magnet is levitating, gently rotate
the magnet using the tweezers.
If this is not possible refer to the next page…
first
Maglev Trains
The magnetic levitation described above suspends an
object so that it is free of contact with any surface – this
provides a frictionless surface – particularly appropriate
for high-speed trains.
In Japan, levitation is achieved by using helium cooled
super-conducting magnets on the train, which interact with
the coils in the ‘guideway’ (or track) in such a way that
repulsion between the ‘like poles’ causes the train to
levitate. Another set of coils along the sides of the
guideway propel the suspended train forwards.
The onboard
superconductors
interact with the
the coil in the
walls of the
guideway. This
raises the train
and keeps it
centred.
Floating
Propulsion
In addition to the levitation coils, there are also propulsion
coils. These push the suspended train forward.
The main obstacle to higher speeds is air resistance.
Maximum speeds of around 520 km/h have been
achieved.
The advantages of Maglev trains are very high speed,
reliability, safety, minimum maintenance and low
environmental impact.
ACTIVITIES
• Read Chapter 13
• Complete Assignment 8
THE END!