Transcript Superconductivity Syllabus Col. 3

Core Module 9.4
A word from the creator
This Powerpoint presentation was prepared by
Greg Pitt of Hurlstone Agricultural High School.
Please feel free to use this material as you see fit,
but if you use substantial parts of this presentation,
leave this slide in the presentation.
Share resources with your fellow teachers and students.
*
* and quantitatively (see next section of syllabus) *
Cathode Ray Tubes and Charged Particles
A very simple cathode ray tube
cathode
The development of various types of cathode ray
tubes beginning in the mid-19th century allowed
the manipulation, using electric and magnetic
fields, of streams of charged particles.
Cathode Ray Tubes and Charged Particles
Observations of the behaviour of these charges led to an
increased understanding of matter and the atom. Application of
this understanding led to new technologies in the 20th century
including the development of
• the oscilloscope
• television
• communications technologies
• the electron microscope
• photocopiers and fax machines
Cathode Ray Tubes and Charged Particles
Charged particles entering a
uniform magnetic field with a
velocity at right angles to the
field are deflected along a
circular path.
cathode ray
velocity
magnet
The path is circular because the
force on the moving charge is of
a constant magnitude (qvB) and
it is always perpendicular to the
direction of the velocity, making
it a centripetal force.
The maximum force is exerted
on the charge when its velocity
is perpendicular to the field.
Cathode Ray Tubes and Charged Particles
Factors affecting the radius
• the greater the velocity the
greater the radius
• the greater the particle mass
the greater the radius
• the greater the magnetic field
strength the smaller the radius
This image shows the circular path
of a positively charged particle in a
uniform magnetic field into the page
Animation see: ChargeMotionBfield.avi
• the greater the charge of the
particle the smaller the radius
Application
The mass spectrometer
Cathode Ray Tubes and Charged Particles
A cathode ray tube is a
highly evacuated glass
tube containing a
positive and a negative
electrode.
When a large DC potential difference is applied between
the electrodes, the cathode releases electrons, forming a
beam, which is attracted towards the positively charged
electrode.
This beam of electrons is called a cathode ray.
A cathode ray is an electric current in a vacuum tube.
Cathode Ray Tubes and Charged Particles
The electrons themselves, making up the cathode ray, cannot be seen.
This graphic shows how a cathode ray (electron beam) is made visible
using a curved phosphor coated metal screen.
Electrons travelling through the tube strike the phosphorescent screen,
causing it to emit green light, thus making the path of the electrons visible.
Cathode ray tubes and charged particles
The magnet in this photo is just
behind the cathode ray tube
with one end pointing out of
the page.
What is the polarity of the
magnet’s pole that is visible
near the tube?
It is a south pole.
The cathode ray can be deflected
from a straight-line path by a
magnetic field, suggesting that the
two were related in some way.
The discovery of this effect in 1855
predates by some ten years the
unification of electricity and
magnetism by James Clerk Maxwell.
X
X
X
X
X
X
X
X
X
X
v
–
X X
B X X X
X
X
X
X
X
X
X
X
X
X
F
Cathode Ray Tubes and Charged Particles
A moving charged particle,
such as an electron, can be
deflected by an electric field.
An electric field can be
produced by a potential
difference applied across a
pair of parallel charged
conducting plates.
The electron entering the field
at right angles to the field is
deflected along a parabolic
trajectory in the field.
For animation see: ElectronDeflectionPlates2.mov
Debate Cathode Rays - Particles or Waves?
• Cathode rays – charged particles or em waves - debate in late
1800s
• Similar debate regarding light in the 1600s. Newton argued for
particles. Huygens for waves. Young secured the wave model for
light.
• Lenard predicted that cathode rays would travel with the velocity
of light but Thomson (1884) determined the velocity of cathode
rays to be less than 1/100 of the speed of light
• German and British rivalry between researchers concerning the
nature of cathode rays. Germans  a wave British  particles.
• Ultimately there was to be truth in both.
• J.J. Thomson was awarded the Nobel Prize in Physics in 1906 for
showing the electron to be a particle.
• George Thomson (JJ’s son), was awarded the Nobel Prize in
Physics in 1937 for showing that the electron is a wave!
Striation Patterns in Low Pressure Discharge Tubes
These pictures show the same discharge tube with
different high voltage sources.
Left: Induction coil
Right: Very high voltage transformer from a TV set
first
Maltese Cross Tube
Electrons produced at the
cathode are accelerated
towards the cross,
connected to the anode.
The inertia of the electrons
(due to their mass) carries
them past the cross if they
do not hit the metal cross
itself.
High energy electrons
make the glass at the end
of the tube glow.
first
Identification of Properties
Maltese Cross Tube
If an object is placed in the path of the cathode ray, a shadow of
the object is cast on the glowing tube wall at the end.
This showed that the cathode rays travelled in straight lines.
first
Identification of Properties
Tubes with Electric Plates - Cathode Ray Control
charged plates
A potential difference applied across
the charged electric plates causes the
cathode ray to deflect to the right or
the left, depending on the direction of
the electric field produced.
first
Identification of Properties
Tubes with Electric Plates - Cathode Ray Control
The deflection of
electrons in the
opposite direction to
the electric field
(away from the
negative
plate,
towards the positive
one) shows that
electrons have a
negative charge.
velocity
first
Identification of Properties
Fluorescent materials and cathode rays
These natural minerals fluoresce
due to the absorption of UV
radiation, which is subsequently
emitted as visible light.
Cathode rays cause certain
materials, called phosphors, to
behave in a similar manner. This is
the basis of screens on CROs, TV
and CRT based computer monitors.
This demonstrates that cathode
rays possess energy.
first
Identification of Properties
Paddle Wheel Discharge Tube and Cathode Rays
This cathode ray tube contains a
small paddle wheel free to roll on its
axle along glass tracks.
Cathode rays cause hit the paddle
wheel, causing it to turn and move
along the track.
This demonstrates that cathode
rays have momentum.
first
Identification of Properties
Cathode Rays
(1) If an object is placed in the path of the cathode ray, a shadow of the object is cast
on the glowing tube wall at the end. This showed that the cathode rays travelled in
straight lines.
(2) The cathode ray can push a small paddle wheel up an incline, against the force of
gravity. This showed that the cathode ray carried energy and could do work.
(3) The cathode ray can be deflected from a straight-line path by a magnetic field,
suggesting that the two were related in some way. The discovery of this effect in
1855 predates by some ten years the unification of electricity and magnetism by
James Clerk Maxwell.
(4) Cathode rays cause phosphorescent materials to give off light. This also shows
that the cathode ray carries energy and can do work.
(5) Although there was some speculation that the cathode rays were negatively
charged, it is not shown to be true by experiment until 1895, just two years before
Thomson announced the discovery of the electron.
(6) J.J. Thomson is the first individual to succeed in deflecting the cathode ray with an
electrical field. He did so in 1897. The cathode rays bend toward the positive pole,
confirming that cathode rays are negatively charged.
first
Force on charged particle in magnetic field
• Electric charges experience no
force if they are stationary in a
uniform magnetic field
• Electric charges experience no
force if they move with a
velocity parallel to a uniform
magnetic field
• Electric charges experience a
maximum force when they move
with a velocity perpendicular to
a magnetic field
• The direction of the force is
perpendicular to the velocity
and the magnetic field direction
Review
Quiz - force on charged particle in magnetic field
What is the direction of the force acting on the moving
charged particles X and Y, both of which have the same
magnitude charge?
The force acting particle X is perpendicular out of the page
The force acting particle Y is perpendicular out of the page
Both the charge and the direction of the velocity are opposite to X
What is the direction of the
force acting on the moving
charged particle Z?
The force acting particle Y is
perpendicular out of the page
Its magnitude is less than the
force on X or Y.
X
Y
Z
Force on Charged Particle in Magnetic Field
The magnitude of the force (F) acting on a charged particle
moving with velocity (v) in a magnetic field (B) is given by
F  qvBSin( )
The force (F) is measured in newtons (N)
The velocity (v) is measured in metres/second (ms–1)
The magnetic field (B) is measured in teslas (T)
Force on Charged Particle in Magnetic Field
Q1. An electron (mass 9.1 x 10-31 kg) moves with a velocity of 3 x 107 m/s
perpendicular to a magnetic field of 2 teslas. The charge on an
electron is -1.6 x 10-19 coulombs. What would be the force on the
electron?
Q2. If a proton (mass 9.1 x 10-31 kg) entered the same field at the same
speed of 3 x 107 m/s, compare its behaviour with that of the electron.
Q3. A magnetic field and an electric field are arranged perpendicular to
each other. A stream of charged particles moving perpendicular to
both fields remains undeflected when the electric field has a strength
of 5 x 103 NC-1 and the magnetic field is 2 x 10-2 T. What is the
speed of the particles? Explain how this is independent of the mass.
Q4. Compare (numerically) the mass to charge ratio of a beta particle
and an alpha particle. State whether the mass or the charge of these
particles has the greater effect on the radius of curvature of the
particle in a magnetic field. Explain your answer.
Solving Problems
F  qvBSin( )
Electric Field Strength - Point Charges
• Electric fields are represented
arrows to indicate the direction of
the field.
• The closer the lines, the stronger the
field represented.
• These diagrams show the electric
fields surrounding positive and
negative charges.
• The direction of an electric field is
the direction of the force that it
produces on a positive charge.
Qualitative description
Quiz - Electric fields and point charges
Qualitatively describe the electric field
surrounding a point positive charge.
The electric field surrounding an isolated
point positive charge is radial. The field
lines point away from the positive charge.
The strength of the field decreases with
distance from the charge.
A diagram can be used to augment the description.
Qualitatively describe the electric field
surrounding a point negative charge.
Qualitative description
Electric Field Strength - Parallel Plates
The electric field between two parallel
plates is uniform and has a direction
from the positive to the negative plate.
The field becomes less uniform as the
distance between the plates increases.
The field near the edges of the plates is
non-uniform.
The field direction is the direction of the
force that would act on a positive charge
placed in the field.
Qualitative description
–
+
+
–
Quiz - Electric field between parallel plates
Draw the electric field between the
pair of square parallel plates seen
edge on in the adjacent diagram.
E
Under what conditions is the field
between the plates uniform?
The field is uniform providing the plates are parallel to each other
and that the separation between the plates is small compared with
the size (length of the sides) of the plate
Solving Problems
Charged Plates and Electric Field Production
The magnitude of the electric field (E)
between two parallel plates is
• proportional to the potential difference
(V) between the plates
• inversely proportional to the separation
(d) between them
V
E
d
Potential difference is measured in volts (V)
Distance is measured in metres (m)
Electric field strength is thus measured in volts/metre* (Vm–1)
*the alternative unit newton/coulomb is identical
Quantitative description
Quiz - Electric field strength
The distance between the two
square plates shown edge on in
the adjacent diagram is 2 mm.
The potential difference applied
across the plates is 12 volts. What
is the electric field strength?
V
E
d
12
E
2  10–3
The electric field strength is 6000 volts/metre
Solving Problems
Thomson’s Experiment - Properties of Electrons
• JJ Thomson used the vacuum tube above to determine
the charge/mass ratio of the electron
• He adjusted electric and magnetic fields so that the
forces they produced on the electrons cancelled each
other [Electric F = qE, Magnetic F = qvB]
+++++
–
v
–––––
E
An electron moves to the right
An electric field is applied
Which way must a magnetic field be applied to
counter the force produced by the electric field?
Charge to mass ratio of electron
Thomson’s Experiment - Properties of Electrons
Deduce the direction of the
magnetic field required to make
the electrons travel in a straight
line in the tube.
Answer: into the page
Charge to mass ratio of electron
QuickTi me™ a nd a
Cinep ak decompre ssor
are need ed to se e th is p icture.
Cathode Ray Tube - The Electron Gun
The electron gun in a cathode ray tube produces electrons.
• The cathode is heated by an electric current
• Thermal vibrations give the electrons energy, releasing them
from the metal surface
Cathode Ray Tube - The Electron Gun
An electric field accelerates and focuses the electron beam
• What is the direction of the electric field in space in which the
electrons are being accelerated?
• The electric field is to the left (positive to negative)
Cathode Ray Tube Operation
A stronger electric field, produced by the high voltage of the
accelerating anode, accelerate the electron beam further
The kinetic energy of the
fast moving electrons
causes the screen
phosphor to give off light.
Movement of the electron
beam in a vertical and
horizontal direction is
controlled by a
combination of magnetic
and electric fields.
Cathode Ray Tube - Electric Field
Charged particles experience a force due to an electric field
E
+
force
force
–
Electric field direction is by convention the direction of the
force on a positive charge
Electrons are therefore accelerated in the opposite direction
to the electric field
Cathode Ray Tube - Fluorescent Screen
Electrons with high kinetic energies hit the phosphorescent
screen, causing the atoms in the phosphor to produce light
Cathode Ray Tube and the Oscilloscope
Oscilloscopes have played
a key role in scientific
research.
The oscilloscope is effectively a voltmeter capable of displaying
variations in voltage (vertical axis) against time (horizontal axis).
Any variable that can be measured electronically and converted to
a voltage can be analysed with an oscilloscope. Many
oscilloscopes now use computers to display the data rather than a
dedicated instrument.
Application
Cathode Ray Tube and the Oscilloscope
Oscilloscopes contain a cathode ray tube that
is less complex than a television picture tube
Application
Cathode Ray Tube and the Electron Microscope
Cathode ray tubes are the basis
of the electron microscope.
The electron microscope uses
the wave properties of electrons
to produce images of objects.
The small wavelength permits a
much greater resolution and
hence magnification than a light
microscope.
Magnetic fields are used to focus
and control the electron beam in
the electron microscope.
Image: Desktop electron microscope
Application
Cathode Ray Tube and the Electron Microscope
Image: Electron microscope
Application
Cathode Ray Tube and the Television Set
Application
Magnetic Fields and the Television Set
Application
The Oscilloscope and Experimental Physics
Image: cathode ray oscilloscope
• Cathode ray tubes are the basis of many cathode ray oscilloscopes.
• Computer based oscilloscopes are now common, as is LCD screen use.
• The development of the oscilloscope as a key tool for measurement propelled
research in many fields of science. Any variable that could be converted to a
voltage could be displayed as a function of time on the oscilloscope screen.
• Such variables include electronic voltages, sound levels, light intensities,
biological signals such as heart and brain activity.
Discussion of development
The Photocopy Machine
Discussion of application - see separate ppt on photocopier
Lightning Conductors
Lightning conductors
Discussion of application
Lightning conductors are used
on many buildings, towers and
high voltage power lines.
Lightning Conductors
See article “Lightning misses point” for an interesting example of how science
can sometimes take things for granted - and get it wrong in the process!
[Copied to notes page below]
Discussion of application
Hertz and the Speed of Radio Waves
Hertz used interference between radio waves following two different
measured paths to determine the wavelength of the waves. He knew the
frequency and so was able to calculate the speed v = fl, confirming that they
were indeed electromagnetic waves.
Radio waves and electromagnetic radiation
Hertz and the Photoelectric Effect
See notes page below
A missed opportunity!
Production and Reception of Radio Waves
first
A First-hand Investigation
Production and Reception of Radio Waves
• If a charge is moved, ,a disturbance is created in the
electric field lines associated with that charge
• The disturbance in the electric field propagates outward
at the speed of light
Propagating disturbance
moving to the right
References: Electromagnetic Waves, emwavgeneration.mov, emwavepropagating.mov
Production and Reception of Radio Waves
• A simplified 2-D animation of the disturbance in the field
is shown in this movie…
QuickTime™ and a
Animation decompressor
are needed to see this picture.
References: Electromagnetic Waves, emwavgeneration.mov, emwavepropagating.mov
Production and Reception of Radio Waves
• If the charge oscillates, ,a field disturbance having wave
properties is created
• The disturbance propagates outward at the speed of light
References: Electromagnetic Waves, emwavgeneration.mov, emwavepropagating.mov
Production and Reception of Radio Waves
An electromagnetic wave is created by
an oscillating dipole.
A dipole is a pair of oppositely charged
particles.
An alternating dipole produces an
electromagnetic wave with a frequency
equal to that of the oscillations of the
dipole.
An AC voltage applied to a length of
conductor becomes a dipole aerial.
References: Electromagnetic Waves, emwavgeneration.mov, emwavepropagating.mov
Production and Reception of Radio Waves
An alternating current
in an aerial creates an
electromagnetic wave
consisting of electric
and magnetic fields
perpendicular to each
other.
References: Electromagnetic Waves, emwavgeneration.mov, emwavepropagating.mov
Production and Reception of Radio Waves
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
This animation represents a propagating electromagnetic wave
References: Electromagnetic Waves, emwavgeneration.mov, emwavepropagating.mov
Production and Reception of Radio Waves
Radio waves are a part of the electromagnetic spectrum
Production and Reception of Radio Waves
The output terminals of an induction coil act as a simple dipole
transmitter, producing electromagnetic waves, including radio waves
over a wide frequency range.
References: Electromagnetic Waves, emwavgeneration.mov, emwavepropagating.mov
Production and Reception of Radio Waves
The radio waves can be detected with a radio receiver tuned off the
station (so that the station signal does not swamp the weak radio
waves from the induction coil)
References: Electromagnetic Waves, emwavgeneration.mov, emwavepropagating.mov
Production and Reception of Radio Waves
Current in
antenna
Receiver
• Reception of radio waves is dependent on the energy carried by the radio
wave fields producing a current in a receiving antenna.
• Induction of a current occurs because the antenna is a conductor.
• The antenna may be a simple length of wire, or sometimes a coil of wire
around an iron core, which can be moved by a tuning dial to alter the
response of the antenna so that waves of different frequency can be
received.
• The length of the antenna determines which frequencies are best received.
Production and Reception of Radio Waves
• Radio waves can be transmitted most easily along a line-of-sight path
• Communication using microwaves requires line-of-sight repeater transmitters
• The Earth’s ionosphere can be used to reflect the waves
Production and Reception of Radio Waves
• Almost all electronic devices produce electromagnetic radiation.
• calculators, computers, cell phones, TVs, radios etc…
• Legislation requires computers to be shielded to prevent radio
interference
• the shielding is usually a metal case enclosing the computer
Production and Reception of Radio Waves
Attenuation
Radio waves, like all electromagnetic waves, obey the inverse square law.The
further the receiver is from the source, the weaker the radio signal.
References: Electromagnetic Waves, emwavgeneration.mov, emwavepropagating.mov
Production and Reception of Radio Waves
1 km
transmitter
Attenuation
The intensity of the radio wave
decreases as the square of the
distance from the source.
2 km
3 km
4 km
The Photoelectric Effect
UV
Demonstration
Two negatively charged electroscopes
with polished zinc plates on top of them
Observation and Investigation
Ultraviolet light shone onto one
of the metal plates (the other
one is the control) causes the
electroscope it is resting on to
lose its charge more rapidly than
the control.
Why is a control necessary?
A control is necessary because
both electroscopes lose their
charge. The important fact is
that the UV light causes one to
lose charge more rapidly.
first
The Photoelectric Effect
UV e e e
e
Photoelectrons are ordinary electrons
Ultraviolet light produces photoelectrons
These are ejected from the zinc surface
Observation and Investigation
Explanation
The UV light causes electrons in
the zinc to be ejected from the
metal surface, resulting in a loss
of negative charge from the
metal, causing electrons to
move from the electroscope to
the zinc.
The electroscope is thus
discharged - this is the
photoelectric effect
first
The Photoelectric Effect
An excellent quantitative simulation of the photoelectric effect
http://home.a-city.de/walter.fendt/phe/photoeffect.htm
first
Observation and Investigation
stopping voltage
The Photoelectric Effect
The photoelectric effect
•
•
•
•
•
The filter can be changed to select a specific wavelength from the source
Light above the threshold frequency will eject electrons from the cathode
A variable stopping voltage (note polarity) is adjusted to reduce the current to zero
The stopping voltage is proportional to the incident light frequency
At a given frequency light, the current is proportional to the light intensity
first
Observation and Investigation
The Photoelectric Effect
The photoelectric effect and
Albert Einstein
Einstein was awarded the Nobel
Prize for his explanation of the
photoelectric effect (not for
relativity!)
He proposed that
• light consisted of quanta (later
called photons)
• either ALL of a photon’s energy
was absorbed by an electron or
NONE of it was
The Photoelectric Effect and Breathalysers
Alcohol reacts with chemicals in
the breathalyser, causing a
colour change dependent on the
amount of alcohol in the sample.
The chemicals absorb varying
amounts of the IR light,
depending on their colour.
• A lamp (A) produces a broadband
(multiple-wavelength) IR beam.
• The IR beam passes through the
sample chamber (D) and is focussed
by a lens (E) onto a filter wheel (F).
• The filter wheel contains narrow band
filters specific for the wavelengths of
the bonds in ethanol. The light
passing through each filter is detected
by the photocell (G), where it is
converted to an electrical pulse.
• The electrical pulse is relayed to a
microprocessor, which interprets the
pulses and calculates the BAC based
on the absorption of infrared light.
Application (ref: http://www.howstuffworks.com/breathalyzer3.htm)
The Photoelectric Effect and Solar Cells
• A photon enters the semiconductor
• It may be absorbed and raise an electron
from the valence to the conduction band
• The absorption process generates
electron-hole pairs
• Immediately after their creation, the
electron and hole decay to energy states
near the edges of their respective bands
A “solar cell” is more correctly called a photovoltaic cell
A photovoltaic cell converts light energy to electrical energy
Application of photoelectric effect
The Photoelectric Effect and Photovoltaic Cells
Contact between p and n
type semiconductors
produces an electric field
at the junction region.
The direction of this field
is from the n-layer to the
p-layer.
Electric field region
Application of photoelectric effect
The Photoelectric Effect and Photovoltaic Cells
• At the p-n junction, conduction band
electrons move from the n-layer to
holes in the p-layer, creating a field
• Light falling on a p-n junction device,
disturbs the electric field equilibrium
• Light energy produces free electrons
in the p-layer allowing current to flow,
establishing a voltage at the external
terminals
• Thus light energy has produced a
voltage providing electrical energy
Application of photoelectric effect
The Photoelectric Effect and Photovoltaic Cells
An alternative explanation
Application of photoelectric effect
The Photoelectric Effect and Photovoltaic Cells
An alternative explanation
Application of photoelectric effect
The Photoelectric Effect and Photovoltaic Cells
An alternative explanation
Application of photoelectric effect
The Photoelectric Effect and Photovoltaic Cells
An alternative explanation
Application of photoelectric effect
The Photoelectric Effect and Photovoltaic Cells
An alternative explanation
Application of photoelectric effect
The Photoelectric Effect and Photocells
A “photocell” is probably better called a “photoconductive cell”
A photoconductive cell is also known as a photoresistor it is the earliest photoelectric device
developed.
Photoconductive cells are used to turn street lights on and off automatically, as counting
devices on production lines, in various alarm systems, and in supermarkets as the sensor that
scans codes on grocery items at checkout counters and in photography as the light meters used
to measure the intensity of illumination.
Energy of light in modern photoconductive cell is used to free electrons from their valence
bonds in a semiconductor material. At room temperature, 21°C (294.16K), the number of free
charges in a semiconductor is relatively limited. Light-released electrons raises its conductivity.
The resistance may change from several hundred thousand ohms in the dark to a few hundred
ohms in sunlight. To increase the dark resistance and reduce the dark current, the conducting
path is often laid down in a zigzag manner on a ceramic wafer.
A number of substances are photoconductive. Some of which are lead sulfide (PbS), lead
selenide (PbSe), and lead telluride (PbTe) are sensitive to infrared radiation, whereas cadmium
sulfide (CdS) has sensitivity to light in the visual range.
Application
The Blackbody Radiation Model
In the second half of the 19th century, the
study of heat lead to major scientific and
industrial developments.
Scientists were interested in understanding
the nature of radiation emitted by hot
objects.
The dependence of the radiation on the
temperatures of the objects was investigated
and the amount of radiation emitted at
different wavelengths for objects at different
temperatures was determined...
Sun 15th Aug 2002
Blackbody Radiation – Planck and Einstein
These graphs show the radiation
emission curves for hot objects at
different temperatures.
Note that…
• as the temperature increases,
the peak wavelength emitted by
the black body decreases
• as temperature increases, the
total energy emitted increases,
because the total area under the
curve increases
Black body radiation curves at
various temperatures
The concept of a blackbody radiator was first defined by Kirchoff in 1860
The Blackbody Radiation Model
An ideal blackbody radiator has these properties... It
• is a perfect absorber and emitter of radiation
• re-emits all radiation incident on the blackbody
• emits radiation with characteristics dependant only on the
temperature of the object
Examples closely approximating
ideal blackbody radiators include...
• the inside of a furnace (such as a
brick kiln or pottery kiln
• the surface of the Sun
• tiles on the space shuttle during
re-entry into the atmosphere
A blackbody was first defined by Kirchhoff in 1859-60 as an object that re-emits all of the
radiant energy incident upon it. i.e., it is a perfect emitter and absorber of radiation.
(http://astro.estec.esa.nl/SA-general/Projects/Planck/mplanck/mplanck.html)
Blackbody Radiation – Planck and Einstein
Max Planck
Planck was able to account for the shape of the blackbody radiation curves by
postulating that energy was emitted in discrete packets (“quanta”), rather than
continuously as was the classical theory.
This novel concept was to usher in quantum mechanics, the triumph of
physics in the 20th century.
Einstein was soon to use the quantum concept in explaining the photoelectric
effect - but he never accepted the uncertainty inherent in quantum mechanics.
Blackbody Radiation – Planck and Einstein
The solid curve shows
the blackbody spectrum
of an object at 5000 K
The surface of the Sun
is an approximation of
such a blackbody
emitter.
The dotted line shows
the intensity / frequency
The classical theory* was unable to account relationship predicted
for the observed distribution of radiation by the classical theory
intensity at different wavelengths.
* A key concept of classical physics was that quantities
were continuous and could take on any value
Assess and Analyse
Blackbody Radiation – Planck and Einstein
UV e e e
e
E  hf
In his 1905 explanation of the photoelectric effect,
Einstein concluded that
• electromagnetic radiation must be absorbed by
electrons in the metal in discrete packets (quanta)
• electromagnetic radiation is quantised
• an oscillating charge (e.g. electron) can accept or
lose energy in small discrete amounts with the
quantum having an energy hf*
* hf = Plank’s constant x frequency
h = 6.6 x 10–34 Js
Blackbody Radiation – Planck and Einstein
Therefore Einstein no longer thought of radiation as being
continuous, as demanded by the wave model. He said it
consisted of a series of "packets" of energy.
This meant that radiation was being thought of as a
"packet of energy" but also as a wave because it had a
frequency. These later became known as photons.
Planck had had no explanation for his quantum model for
blackbody radiation but he knew it was necessary and
that it worked - this is called an empirical result.
Einstein’s quantum explanation of the
photoelectric effect thus provided an
explanation for Planck’s quantum
analysis of blackbody radiation.
E  hf
UV e e e
e
The Particle Theory of Light
Calculate the energy of a photon of red light having a
wavelength of 700 nm. [700 nm = 700 x 10–9 m]
Calculate the frequency of the light
c  fl
3  10  f  700 x 10
8
f  4.29  10
14
9
Hz
Hence calculate the energy of the light photon
E  hf
E  6.626  10
34
 4.49  10
E  2.84  10 19 J
Photon energy and frequency
14
The Particle Theory of Light
E  hf
c  fl
Solving Problems
The Einstein - Planck Debate
There was no direct debate between Einstein and Planck.
The intention of the syllabus outcome is unclear
It may refer to the different views Planck and Einstein took about
scientists remaining in Germany during the WWI Nazi era and
continuing to do scientific research.
Planck, along with nearly a hundred leading German intellectuals
signed a manifesto defending Germany’s war actions. Planck
stayed on and directed the Kaiser Wilhelm Institute.
Einstein left Germany. Although there was no direct
correspondence between Einstein and Planck, consideration of the
actions of each provides a case study of the complexity of
evaluating the moral responsibility of science to social orders.
See MS word article “Planck_Einstein_Debate.doc” for further comment.
The nature of science research - social and political forces
Describing the de Broglie Model
De Broglie hypothesised that electrons might have
wave properties.
He reasoned that if photons have an equivalent mass
(based on Einstein’s relativity theory relating mass and
energy), and they had wave properties, that electrons,
with a small mass, would conversely have wave
properties.
His hypothesis was confirmed by experiments
demonstrating that an electron beam produced a
diffraction pattern, very similar to x-ray diffraction, when
passed through crystalline materials.
X-ray diffraction produced by potassium sulfate crystals
[right]
Describing the de Broglie Model
• De Broglie predicted that moving electrons would have wave properties
• Wave properties restrict the orbits of electrons to specific, discrete values
• Orbits not involving a whole number of electron wavelengths would
produce destructive interference
A possible electron orbit
Electron orbits
An impossible orbit
Describing the de Broglie Model
De Broglie related work done by
Planck and Einstein, producing a
result that correctly predicted the
wave properties of electrons and
their associated wavelengths.
Electron orbits - background
Electrons in Solids - Insulators
The electrons in insulators are all held firmly
in chemical bonds (“shared between atoms” syllabus)
The electrons are unable to move, hence
these materials do not conduct electricity.
Diamonds are a good example of an electrical
insulator.
Covalent compounds (for the benefit of
chemistry students), or substances made of a
mixture of such compounds, do not conduct
electricity well - they are insulators. e.g.
sugar, alcohol, wood, paper, glass, ceramics
(except for superconducting ceramics).
Electrons in Solids - Insulators
C
C
C
C
C
C
In insulators such as diamond, the outer electrons are used in
pairs to form chemical bonds between atoms and so these
electrons are not free to move through the material
Electrons in Solids - Conductors
Electron conduction
through metals under
the action of an
electric field.
Solids, that are conductors, have electrons in them that can readily move
through the material. Most metals are very good electrical conductors
because the outer electrons of the atoms are free to move from atom to
atom. Even the colour of metals is due to this “sea of electrons”.
Electrons in Solids - Conductors
Metals have a lattice structure of immovable positive ions in a
sea of electrons free to move through the structure
Electrons in insulators cannot move through the material
There is no sharp division between conductors and insulators
Electrons in Solids - Semiconductors
A pure single crystal of silicon
Sliced into disks for IC production
Becomes an integrated circuit (P2)
The Periodic Table - Semiconductors
14
Si
32
Ge
Comparison of number of charge carriers
Number of charge carriers
conductors have
many charge carriers
insulators have
almost no free
charge carriers
insulators
Qualitative only
Semiconductors
have much fewer
charge carriers than
conductors, but
more that insulators
conductors semiconductors
Electrons in Solids - Semiconductors
Electrons in Solids - Semiconductors
Outer electrons in metals are already in the conduction band.
It takes some energy to move electrons from the valence to
the conduction band in semiconductors.
Larger band gaps result in greater resistances of materials.
Band structure and electrical resistance
Electrons in Solids - Semiconductors
Band structure and electrical resistance
Electrons in Solids - Semiconductors
Explanation
Atomic vibration excites some
electrons across the band gap into
the conduction band.
Corresponding holes are created in
the valence band.
Both holes and electrons act as
charge carriers.
As the temperature increases, the
number of electron-hole pairs
increases and the semiconductor
conductivity increases.
In metals, conductivity decreases
with increasing temperature.
Band structure and electrical resistance
Electrical Conduction in Semiconductors
Pure semiconductors are called
intrinsic semiconductors
Intrinsic semiconductors are
insulators at 0 K (absolute zero)
Above absolute zero the
conductivity of intrinsic
semiconductors increases with
increasing temperature
14
Si
32
Ge
Electrical Conduction in Semiconductors
–
–
–
+
+
+
electron
hole
Electrons and holes exist in equal numbers
When an electric field is applied…
electrons and holes move in opposite directions
Investigation - electrons, holes and conduction when an electric field is applied
Electrical Conduction in Semiconductors
–
–
–
+
+
+
This model applies to intrinsic semiconductors
Investigation - electrons, holes and conduction when an electric field is applied
The invention of the transistor
By the late 1950s, electrical engineers were
aware of the potential of digital electronics.
The first digital computers had been built
already, using vacuum tube technology.
Circuits being designed required exponentially
increasing numbers of components. The first
digital computer had over 18000 valves which
meant there was a very short time between
breakdowns.
This was mediated against by the physical
limitations of assembling such large numbers of
components together.
How shortcomings in available technology lead to the invention of the transistor
Invention of the Transistor - Role of Germanium
The first transistors were made of germanium, rather than silicon, because
• germanium could be purified and worked at significantly lower temperatures
• it was too difficult to produce pure crystals of silicon for semiconductor use
The first [germanium] transistor
An early germanium transistor
Germanium use related to lack of ability to produce [silicon] of suitable purity
Germanium was the First Semiconductor Used
Germanium transistors had disadvantages including
• undesirable variation in performance with
temperature increase
• their low power outputs (limited by temperature
constraints)
• the relative rarity and hence expense of germanium
• mechanical “point contact” between doped
semiconductors, resulted in a lack of ruggedness
and reliability
Why silicon became the preferred material for the transistor
Silicon and the transistor
The later use of silicon in transistors
• resulted in an increase in the operating
temperature of semiconductor devices
• improved semiconductor power handling ability
(because of higher Toperating)
• grown junctions later used in silicon transistors
increased ruggedness and reliability
Why silicon became the preferred material for the transistor
Doping of semiconductors
• The electrical properties of semiconductors can
be changed by the addition of minute quantities of
other elements into the crystal lattice
• This process is called “doping”
• Elements used for doping typically have either 3
or 5 outer electrons, and similar size atoms to the
semiconductor material so that they substitute
readily into the semiconductor crystal lattice
• The electrical properties change due
to the creation of additional charge
carriers - doped semiconductors are
better conductors than intrinsic
semiconductors because they contain
more charge carriers per unit volume
How doping changes electrical properties
5
B
15
P
Doping of semiconductors
• The addition of dopants containing 5 outer electrons produces a
semiconductor having more free electrons in the conduction band
than an intrinsic semiconductor. There are more negative charge
carriers than holes.
P-type and N-type semiconductors and relative numbers of charge carriers
Electrical Conduction in Semiconductors
n-type semiconductor
1 atom in 200 000 substituted
The old “group V” is now called the group 15 elements
Intrinsic, p-type and n-type semiconductors
Doping of semiconductors
• The addition of dopants containing 3 outer electrons produces a
semiconductor having holes than there are electrons in the
conduction band. There are more positive charge carriers than
electrons free to flow as current.
P-type and N-type semiconductors and relative numbers of charge carriers
Electrical Conduction in Semiconductors
The old “group III” is now called the group 13 elements
Intrinsic, p-type and n-type semiconductors
Differences between thermionic and solid state
Solid state devices have generally replaced thermionic devices
• Small size
• Better economy of operation - lower power consumption
• Greater speed
• Reliability is much better (more rugged, lower temperatures)
• Economical to produce
• Mass-production techniques are possible
Why solid state replaced thermionic devices (vacuum tubes)
Solar cells and the photoelectric effect
Click here for link
Semiconductors, electric fields and current in solar cells
9.4.4 Investigations into the electrical properties of
particular metals at different temperatures led to
the identification of superconductivity and the
exploration of possible applications...
William and Lawrence
Bragg (father and son)
were awarded the 1915
Nobel Prize for “their
services in the analysis
of crystal structure by
means of X-rays”
They were Australian
scientists working in
Britain.
William and Lawrence Bragg used a
collimated beam of x-rays to
investigate the crystal structure of
materials, including metals.
•
•
•
•
The x-rays were reflected from layers within the crystal.
Reflected x-rays produced an interference pattern.
The pattern was recorded on photographic film.
Mathematical analysis of the diffraction pattern
permitted the crystal structure to be deduced.
The pattern produced depended on
• The wavelength of the x-rays
• The distance between planes
of atoms in the crystal
• The angle of incidence of the
x-rays on the crystal planes
X-ray crystallography apparatus - schematic
A typical x-ray crystallography diffraction interference pattern
[potassium sulfate]
The Impact of the Braggs’ Contribution to
Understanding Crystal Structure
The work of William and Lawrence Bragg was a key to
understanding the crystal structure of
• Metals
• Inorganic compounds (e.g. sodium chloride)
Later, the techniques developed by William and Lawrence
Bragg were used to investigate the structure of
• Ceramics
• DNA (by Rosalind Franklin, Crick and Watson)
Knowledge of the structure of metals and ceramics lead to
the development of theoretical models for superconductivity
and the discovery of high temperature superconductors.
Question
• William and Lawrence Bragg developed the Bragg
spectrometer
• (a) State which component of the electromagnetic
spectrum the Bragg spectrometer used. (1 mark)
• (b) State specifically what their device was used to
study and why visible light was not used for their
spectrometer.
(2 marks)
• Clarify the difference in behaviour of electrons when
they flow in a metal at room temperature and when
they flow in a superconductor below its critical
(transition) temperature.
(3 marks)
William and Lawrence
Bragg pioneered x-ray
diffraction techniques
that allowed the crystal
structure of metals to be
investigated
1. Electrons interact with the
lattice atoms when a
collision occurs, and then
the velocity of the electron
abruptly and randomly
changes direction as a
result of collision and some
energy is transferred to the
lattice atoms, causing them
to vibrate.
–
+
–
+
–
+
–
+
2. Thermal equilibrium exists throughout the conductor.
3. Conduction electrons do not interact with each other.
• Metals have a large number of
electrons in the conduction band
relative to other materials.
• Electrons in the conduction band
can move freely when an electric
field is applied.
+
+
+
+
–
–
Lattice of positive ions
–
electron
Electric field
–
A potential difference
producing an electric
field to the left…
will cause the electrons
to drift to the right.
Random changes of
direction of the motion of
individual electrons is a
result of collisions with
vibrating atoms in the
metallic crystal lattice.
–
+
–
+
–
+
–
+
Individual electrons move through a metal conductor at an
average speed of a few centimetres per minute.
They are said to “drift” through the metal.
The average rate at which electrons move through the
conductor is called the drift velocity.
Collisions of the conduction
electrons with the atoms in
the lattice cause the atoms
to vibrate more.
Vibration of atoms is heat
energy.
Increasing the current
through a metal results in
more collisions and hence
greater vibration of the
atoms in the lattice.
Hence the temperature of a metallic conductor increases
as the current increases.
If a metal conductor is cooled, there is less vibration and
fewer collisions with the conduction electrons.
This causes the resistance to decrease.
This animation
models the drift of
electrons through a
conductor and the
collisions of the
electrons with the
lattice ions
Alt. Ref: normalcurrent.mov
+
+
–
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
–
+
–
+
–
* This is hardly a “discuss” outcome!
but that’s what the syllabus says...
For any given current, the
electron drift velocity is
 inversely proportional to the
density of electrons
 inversely proportional to the
cross sectional area of wire
 inversely proportional to the
electronic charge
*
Review of band gaps and
conduction
Metals have very small band gaps compared with
insulators and semiconductors
Superconductivity
Syllabus 9.4.4 Column 2
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Pair formation (avi)
Superconductors are materials which,
below a particular temperature called
the transition temperature allow
electrons to move through the crystal
lattice with no loss of energy to the
lattice - i.e. no heat loss.
Superconductivity
Syllabus 9.4.4 Column 2
Bardeen, Cooper and Schrieffer
proposed a theory [Now called the
BCS Theory] of superconductivity in
which electron pairs interact with the
superconducting lattice in a process
called electron-phonon interaction.
Superconductivity
Syllabus 9.4.4 Column 2
Bardeen
Cooper
Superconductivity
Syllabus 9.4.4 Column 2
BCS Theory
Bardeen
• In 1957 a remarkably successful theory
was developed to explain superconductivity
• The theory explains superconductivity as a
result of electrons interacting and travelling
in pairs within the crystal lattice
• This theory was developed by Bardeen,
Cooper, and Schrieffer at the University of
Illinois 46 years after the discovery of
superconductivity
• They were awarded the Nobel Prize in
1972 (Bardeen also won the Nobel Prize in
1956 for inventing the transistor)
Cooper
Superconductivity involves pairs of electrons
 The electron creates a
phonon, a wave
disturbance, carrying
momentum through the
lattice as if it were a particle
travelling through the lattice
 A second electron passing
by the moving region of
positive charge density
experiences an attractive
electrostatic force causing it
to increase its momentum
i.e. it absorbs the phonon
Superconductivity involves pairs of electrons
 The electrons exchange
some momentum with
each other through the
phonon interaction
 The
second
electron
effectively travels freely in
the virtual lattice wake of
the leading electron
 BCS theory predicts that
under certain conditions,
the attraction between the
two electrons due to
phonon exchanges can be
slightly greater than the
electrostatic repulsion
between them
 The effect of this attraction
is that the electrons will be
weakly bound together
what is called a Cooper
pair
 pairs interact with the
superconducting lattice in a
process called electronphonon interaction.
Conditions Needed for Superconductivity
The conditions for production of superconductivity are
1. The temperature of the material must be low
enough so that the number of random thermal
phonons be small
2. The interaction between an electron and phonon
must be large
3. The number of electrons in energy states capable
of forming Cooper pairs must be large
4. The two electrons have antiparallel spins enabling
them to form a pair
The Mechanism for Superconductivity
 Cooper pairs are constantly forming and breaking apart
 Many Cooper pairs occupy overlapping spaces within the
lattice (this is possible because as a pair, they have zero spin)
permitting the interaction of many electrons in the pairing
process
 All Cooper pairs have the same quantum state and in the
ground state, all electrons form bound pairs
 All 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 locked to that of
every other pair so that none are involved in the random
scattering within the lattice, which gives rise to electrical
resistance
Strong External Magnetic Fields Destroy
Superconductivity
 An external magnetic field
interacts with the opposite spins
of the Cooper pair, raising the
energy of one and lowering the
energy of the other
 If the external field is large
enough, both electrons will point
in
the
same
direction,
destroying the Cooper pairs and
hence the superconductivity of
the material
Superconductivity Syllabus Col. 3
Superconductivity Syllabus Col. 3
Superconductivity of Elements
• At temperatures approaching absolute zero, the
resistance of many metals suddenly drops to zero
• The temperature at which this occurs is called the
transition temperature
• They are said to have become superconductors
• The temperature at which the transition to the
superconducting state is different for different
metallic elements
• These materials are classified as low-temperature
superconductors
Material
Transition Temp
(K)
Ir
Ru
Cd
Zn
Ga
Al
Sn
Hg
Pb
Nb
0.1
0.50
0.56
0.85
1.08
1.20
3.72
4.15
7.19
9.46
Superconductivity Syllabus Col. 3
Superconductivity of Alloys and Ceramics
• Some alloys have higher transition temperatures
than the constituent elements
• Ceramic materials have been made with even higher
transition temperatures than the metal alloys - these
are called high-temperature superconductors
• There is a special interest in high-temperature
superconductors, because the transition
temperature can be attained using liquid nitrogen at
a temperature of 77 K
Material
Transition Temp (K)
Material
Transition Temp
(K)
Ni-Sn
17.9
La-Ba-Cu-oxide
30
Y-Ba-Cu-oxide
92
Tl-Ba-Cu-oxide
125
Ir
Ru
Cd
Zn
Ga
Al
Sn
Hg
Pb
Nb
0.1
0.50
0.56
0.85
1.08
1.20
3.72
4.15
7.19
9.46
Superconductivity Syllabus Col. 3
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Superconductivity Syllabus Col. 3
A small, strong
permanent magnet, such
as a neodymium magnet,
placed above a
superconductor hovers
above the surface of the
superconductor due to a
force of repulsion.
The force is produced by
the magnetic fields
produced by the electric
currents induced in the
superconductor. The
induced currents produce
magnetic poles that mirror
the levitated magnet’s
poles.
Superconductivity Syllabus Col. 3
Superconductors
Magnetic Levitation
magnet
Superconductor
Superconductivity Syllabus Col. 3
Superconductors
Magnetic Levitation
Question
In your course, you performed
an investigation to observe
magnetic levitation and the way
in which a magnet is held in
position by a superconducting
material.
Recount how you performed
this investigation.
Answer
Superconductivity Syllabus Col. 3
Superconductors - Magnetic Levitation
Question
In the physics course, you performed an investigation to observe
magnetic levitation and the way in which a magnet is held in position
by a superconducting material. With reference to specific hazards,
clarify procedures you put in place to minimise these hazards when
carrying out this investigation.
Answer
Superconductivity Syllabus Col. 3
Superconductors
Magnetic Levitation
Question
Explain why a magnet is able to hover above a
superconducting material that has reached the
temperature at which it is superconducting.
Answer
Superconductivity Syllabus Col. 3
A small, strong permanent neodymium magnet (gold
colour), hovers above the surface of the dark coloured
superconducting disk immersed in liquid nitrogen at 77
kelvins. The magnet remains suspended, even when
stationary (no flux change) because the current in the
superconductor keeps flowing once it is induced.
Superconductivity Syllabus Col. 3
Superconductivity Syllabus Col. 3
A Japanese designed and built magnetically levitated train has
travelled at speeds exceeding 520 kmh–1 using superconducting
magnets on-board the train.
The magnets
induce currents in
the rails below
them, causing a
repulsion, which
balances the force
of gravity, allowing
the train to move
without direct
contact with the
track.
Superconductivity Syllabus Col. 3
Superconductivity and Maglev Trains
Superconductivity Syllabus Col. 3
Superconductivity Syllabus Col. 3
Superconductivity and Maglev Trains
Superconductivity Syllabus Col. 3
Key points
Maglev has had
slow development
because of high
infrastructure costs
Running costs are
expected to be
about a third that of
traditional rail
transport and 20%
that of an aircraft
Commercial maglev
due to begin
operation in 2003
from Shanghai to its
airport (33 km link)
Maglev trip MelbSyd would be 8
hours
[Copy/paste and print this
article for clearer reading!]
Superconductivity Syllabus Col. 3
Key points
Transrapid says
maglev would be
faster than the
French TGV
(300 km/h)
(it’s a business - they
WOULD say that!)
Two maglevs being
considered in
Germany - short
distances only
[no air competition!]
Two projects
receiving funding in
US
Netherlands also
considering maglev
[Copy/paste and print this
article for clearer reading!]
Superconductivity Syllabus Col. 3
Superconductivity Syllabus Col. 3
Applications of Superconductivity
• Electrical Grids - for the generation, transmission and
storage of power, improving power supply quality (in
generators, cables, transformers)
• Motors - DC and AC electric motors, propulsion systems
• Bearings - for frictionless motion (flywheels, magnetic
levitation)
• Magnet Systems - for current leads, MRI, beam focusing
magnets, very high field research magnets
Superconductivity Syllabus Col. 3
Superconducting Transmission Lines
Since 10% to 15% of generated electricity is dissipated as heat due to
resistive losses in transmission lines, the prospect of zero loss
superconducting transmission lines is appealing.
A prototype superconducting
transmission line was able to
1000 MW of power within a
conductor of diameter 40 cm.
This is equivalent to the
transmission of the entire output
of a large electrical power station
on a single conducting
transmission line.
Superconductivity Syllabus Col. 3
Superconducting Motors
The world's first 1000 hp HTS
industrial motor was
successfully tested in July
2000 by Rockwell Automation
Power Systems and
American Superconductor
http://www.amsuper.com/navyupdate.htm
Superconductivity Syllabus Col. 3
Superconducting Transmission Lines
Superconducting wires would have the advantage of allowing energy to
be transmitted using low voltage DC.
High voltages are used to transmit energy to minimise resistive heat
losses in the transmission lines.
The use of superconductors would remove the need for large
transformer banks and multiple high voltage AC transmission lines on
towers as is used in the conventional electricity grid.
Efforts are being made to develop practical
high temperature superconductors with
transition temperatures higher than the
boiling point of nitrogen to permit the
transmission of energy at temperatures
realistically and economically achievable on
a large scale.
Superconductivity Syllabus Col. 3
Superconductivity is Used to Make Super-strong Magnets
Electromagnets using superconducting coils are used to produce very
strong magnetic fields used in particle accelerators such as Fermilab
Superconductivity Syllabus Col. 3
Superconductivity is Used to Make Super-strong Magnets
Electromagnets using superconducting coils are used to produce very
strong magnetic fields used in particle accelerators such as Fermilab
Superconductivity Syllabus Col. 3
Transition Temperatures [or Critical Temperatures]
Historical Advance of Tc
Useful reference: http://superconductors.org/
Superconductivity Syllabus Col. 3
Transition Temperatures
Historical Advance of Tc
The discovery of hightemperature
superconductors,
significantly changed the
potential for applications
of superconductivity.
Such materials are
superconducting above
the temperature of liquid
nitrogen - a temperature
readily achievable.
Superconductivity Syllabus Col. 3
Transition Temperatures
Historical Perspective on Superconducting Transition Temperatures
Critical temperature
(kelvins)
Superconductivity Syllabus Col. 3
Resistivity Drops to Zero at the Transition Temperature
Resistance characteristics of a high temperature superconductor
The resistance of superconductors becomes zero abruptly at Tc
Superconductivity Syllabus Col. 3
Resistance of a Superconductor
Real data - experimental error occurs in real science!
Superconductivity Syllabus Col. 3
Application of Superconductors to Power Transmission
Superconductivity Syllabus Col. 3
Superconductors are Used in MRI Machines
Electromagnets using superconducting coils are used to produce very
strong magnetic fields used in medical magnetic resonance imaging
Superconductivity Syllabus Col. 3
Are Superconductors a New State of Matter?
Macroscopic Quantum State
• All electron pairs in a superconductor move through the lattice in unison.
• The Cooper Electron Pairs retain this ordered structure while moving through the
crystal lattice in the presence of an external electric field. Thus each pair becomes
locked into its position with others pairs, and as a result no random scattering of
electron pairs may occur due to crystal imperfections (caused by a low degree of
vibrational energy resulting from the low temperature).
• Zero resistivity may be defined as the absence of electron scattering from the crystal
lattice and hence the superconductor now demonstrates zero resistivity.
• All Cooper Electron Pairs throughout the material effectively condense to a single
quantum state, and the entire structure behaves as though an enormous quantum
mechanical system. On the macroscopic level, the system is quantised and may be
represented by a single wave function extending the entire material.
• It could be argued that because the electron pairs throughout the
material are in a single quantum state, unlike any other state of
matter - solid, liquid or gas - that superconductors could be
considered a separate state of matter.
Superconductivity Syllabus Col. 3
Are Superconductors a New State of Matter?
Superconductivity is a Phenomenon Needing a Quantum
Mechanical Model for its Explanation and as such it could be
considered a new state of matter. It is called a Bose-Einstein
condensate. Such a state is generally agreed to have properties
justifying its being classified as a separate state of matter.
Phenomena requiring a quantum model for their explanation…
• Lasers
• Superfluidity - liquid helium
• Superconductivity
• Bose-Einstein condensates
[includes superconductors]
Superconductivity Syllabus Col. 3
Are Superconductors a New State of Matter?
Phenomena Needing a Quantum Mechanical Model
Examples of BECs
• Helium atoms: superfluidity
• Cooper pairs: superconductivity
• Super-atoms at extremely low temperatures
(0.000 000 1 Kelvin)
Superconductivity Syllabus Col. 3
Scientists Have Created Other Bose-Einstein Condensates
2000 Rubidium Atoms Forming a BEC
END
Good Luck with your HSC
Question
Recount* how you performed this investigation.
Return
* Retell a series of events
• a high temperature superconducting disk
was placed in a container and the disk
was just covered with liquid nitrogen
• the superconductor was left for a short
time until it cooled to the temperature at
which its resistance dropped to zero
• a strong magnet was then carefully
placed, using plastic tweezers, just above
the superconducting disk
• The magnet hovered above the HTS disk
Liquid N2
HTS disk
Liquid N2
magnet
HTS disk
* Clarify: make clear or plain
Return
Question
… clarify procedures you put in place to minimise these hazards…
• Wear safety glasses to protect eyes from being splashed
with liquid nitrogen or being exposed to freezing vapours
• Wear thick cotton gloves to protect hands in the case of
spills, and to prevent their contact with very low
temperature (cryogenic) materials in the experiment
• Handle the magnet with plastic tweezers when placing it
near the cryogenic materials - this keeps hands at a safe
distance and the plastic does not conduct heat well
• Wear thick, closed shoes to prevent spills coming into
contact with the feet. Long pants should cover shoes.
Return
Question
Explain* why a magnet is able to hover above a
superconducting material that has reached the
temperature at which it is superconducting. * Relate cause and
The effect of magnetic levitation is caused
by induced magnetic poles on the surface
of the superconductor repelling the
magnet, causing it to hover above the HTS
surface
effect; make the
relationships
between things
evident; provide why
and/or how
magnet
The magnetic poles on the superconductor
Liquid N2
are produced by large currents induced on
N opposing
the superconductor surface by the flux
N poles
changes produced by the magnet above.
HTS disk
The currents are large because the HTS has
no resistance at temperatures below Tc
Light and Colour
Production and Perception
Light and Colour
Visible light consists of light of wavelengths ranging from 400 to 700 nm
If all colours are present at equal intensities, the result we see is white
Light and Colour
Production and Perception
A balance of red, green, and blue is also perceived as white
If the colours are present at equal intensities, the result we see is white
Production and Perception
Light and Colour
A CRT or LCD can display all colours using just three coloured pixels
The reason this is possible is because the eye has cells that respond to
red, green and blue wavelengths.
When we perceive a colour such as yellow, it is because the red and
green sensing cells are being equally stimulated.
Production and Perception
Light and Colour
Hot objects emit light over a range of frequencies
warm
hot
very hot
ouch
If the colours are present at equal intensities, the result we see is white
Production and Perception
Light and Colour
An object at 5000 K emits light over all visible wavelengths, as well as
longer and shorter wavelengths than visible light.
The balance of wavelengths present favours the longer parts of the
spectrum resulting in a perceived colour that is yellow rather than white.