Download Ideas to Implementation by Jonathan Chan

From Ideas to Implementation
Part 1: Cathode Rays
Cathode Rays
Cathode ray tubes consist of an evacuated glass tube with almost all gas removed and 2
metal electrodes embedded at each end of the glass tube. It requires low pressure to
minimise collision between air molecules and electrons, and a high voltage so the electric
field is strong enough to accelerate electrons from the cathode to the anode.
Cathode rays fitted the wave model as:
 They travelled in straight lines
 If an opaque object was placed in their path, its shadow would appear
 It could pass through thin metal foils without damaging them
 It causes fluorescence like ultraviolet waves
They also fitted the particle model as:
 The rays left the cathode at right angles to the surface
 They were obviously deflected by magnetic field
 They did not appear to be deflected by electric fields
 Small paddlewheels turned when placed in the path of the rays
 They travelled considerably slower than the speed of light
These inconsistencies arose from inadequacies in experimental design and the knowledge of
the atom. In 1897, J J Thomson showed beyond doubt that the rays were streams of
negatively charged particles that had mass – the electron
A high voltage applied across the electrodes causes cathode rays, streams of negatively
charged particles (electrons) to flow from the cathode towards the anode with little
obstruction from the remaining gas particles. Cathode ray tubes allowed the manipulation
of a stream of charged particles by electrical and magnetic field and solid objects in its path.
Charged Particles in a Magnetic Field
A moving charged particle in a magnetic field experiences a force. If a particle with charge q
is moving with velocity v, perpendicularly to a magnetic field of strength B, at an angle of θ,
the particle will experience a magnetic force given by: 𝐹 = 𝑞𝑣𝐵 sin 𝜃.
Electric Fields
An electric field is a region in which charged particles experience a force. For point charges,
the electric field is strongest closest to the charge and gets weaker with distance (i.e. it is
strongest when the lines are closer together). For pairs of opposite charges, the electric
field is stronger closer to the charges and weaker between them.
Oppositely charged parallel plates that are separated by a small distance generate a uniform
electric field. The strength of this field is the same at all points between the plates and at
right angles to the plate, except near the edges. Quantitatively, the electric field strength
𝑉
due to oppositely charged parallel plates is given by 𝐸 = 𝑑 , where V is the voltage and d is
the separation between the plates.
Thomson’s Experiment
In Thomson’s experiment to measure the charge/mass ratio of an electron, cathode rays
were passed through two narrow slits to make a thin parallel beam aimed at the centre of a
fluorescent screen. Electrodes were placed to create a uniform electric field that exerted a
downward force on the beam. Electromagnets were placed to produce a uniform magnetic
field that exerted an upward force on the beam.
Thomson manipulated the strengths of the two fields so that the deflections produced by
produced by them exactly balanced out, showing that the two forces on the electrons in the
beam were equal and opposite. By equating the expression for these two forces, Thomson
calculated the velocity of these particles.
Thomson 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 calculated that all electrons had the same
charge/mass ratio of 1.76x1011 C kg-1.
Cathode Ray Tubes in Conventional TVs and Oscilloscopes
In the electron gun, the heating filament heats the cathode, releasing electrons by
thermionic emissions. A number of electrodes are used to control the brightness of the
beam, to focus the beam and to accelerate the electrons along the tube.
Two sets of parallel deflecting plates are charged to produce an electric field that can
deflect the beam of electrons separately, up or down and left or right. This is done so that
the electrons can be directed to all points on the fluorescent screen
The screen is coated with layers of fluorescent material, which emits light when high energy
electrons strike it.
Televisions use a cathode ray tube as the output devices. The information is transmitted to
the receiver which directs the appropriate signal to one of three electron guns, with the
picture reconstituted on the screen by an additive process involving the three coloured
phosphors. The phosphorescent paint on the screen converts the energy of the electron
beam into light.
A shadow mask is used to control the brightness of the three sets of phosphors and that it
only hits the corresponding spot on the pixel. After it is struck each pixel continues to shine
for a short period of time so rapid scanning of screen allows image to be displayed on
screen without discernible flickering.
A Cathode Ray Oscilloscope (CRO) uses a cathode ray tube to display a variety of electrical
signals. The horizontal deflection is provided by a time base (or sweep generator), which
allows the voltage to be plotted as a function of time. This enables complex waveforms or
very short pulses to be displayed and measured.
The input waveform enters with one part of the signal amplified and the voltage produced
goes to the vertical deflecting plates. The other part passes to the trigger and time base and
is then passed through to the horizontal deflecting plates. The combination of both signals
produce the waveform displayed on the front screen.
Striation Patterns and Discharge Tubes
An induction coil was adjusted so that a continuous spark was produced. The induction coil
was connected to the discharge tubes of differing pressures one at a time and observations
of the discharge were made.
Pressure (mmHg)
40
10
6
3
0.14
0.03
Striation Pattern Observed
Purple coloured streamers from the cathode to anode
Bright purple at the cathode and anode
Pink stream of light in the body
Purple light at cathode and anode
A dark gap near the cathode
Pink-purple body
Pink anode and purple flashes at the cathode
Pink-orange body with striations
Pink-purple glow at anode and cathode
Purple body and less striations
Glass fluorescing
Tube is dark except for a green glow appearing in the glass at the end
of the discharge tube opposite the cathode
Part 2: The Photoelectric Effect and Black Body Radiation
Hertz and the Photoelectric Effect
In his experiments, Hertz used an induction coil to produce sparks between the spherical
electrodes of a transmitter. He observed that when a small length of wire was bent into a
loop with a small gap and held near the sparking induction coil, a spark would jump across
the gap in the loop also.
Hertz experimented with these waves and showed that they could be reflected, refracted,
polarised and diffracted just like light waves, also discovering that they travelled at the
speed of light. When Hertz placed the detecting loop in a dark case he observed that the
maximum spark length was smaller and it was only when he removed the portion of the
case that screened the detecting loop from the transmitting loop that it returned to normal.
Hertz had discovered the photoelectric effect as illuminating the spark gap in the receiving
loop with UV light from the transmitting gap gave stronger sparks in the receiving loop, but
the glass case blocked the UV and reduced the intensity of the sparking. However, he did
not realise the significance of his discovery and his observation of the effect of a radio wave
on a receiver, failing to investigate it further.
Hertz’s Experiments with Radio Waves
Hertz knew that if he could measure the frequency and wavelength of these radio waves, he
could determine their speed. The frequency was controlled by Hertz and the wavelength
was measured by placing the transmitter loop near a metal plate which reflected the radio
waves which interfered with the transmitted waves.
Using the interference pattern produced, which Hertz detected with his receiver loop and
the varying degrees of sparking, he found half the wavelength by measuring the distance
between 2 non-sparking points (destructive interference).
From this, he found that radio waves travelled at the speed of light and that this radiation
had the properties of light. Hertz concluded and verified Maxwell’s prediction that a whole
spectrum of electromagnetic radiation exists, which all travels at the speed of light.
Planck’s Hypothesis of Quantised Radiation
A black body is one which absorbs all incoming radiation and is a perfect absorber and
emitter of radiation. The classical theory of light predicted that as the wavelength of the
radiation emitted becomes shorter, the radiation intensity would increase without limit,
which would violate the principle of conservation of energy.
Planck proposed that the radiant energy, although exchanged between the particles of the
black body and the radiant energy field in continuous amounts, could be exchanged in
discrete packets of energy or quanta (photons). This became known as the quantisation of
thermal energy.
Einstein and the Quantum Theory
Einstein combined Planck’s Quantum Theory with the
classical wave theory of light to solve the apparent
conflicts. The energy associated with the radiation from
a black body is concentrated in packets of energy called
photons, which is the smallest amount of radiation
energy possible at a given frequency. Light is a wave
and each photon of light has an amount of energy given
by 𝐸 = ℎ𝑓.
When a photon interacts with matter, it can either transfer all its energy or none of it – it
cannot transfer only part of it. In relation to black body radiation, the shorter the
wavelength, the greater the total energy radiated for a given temperature. Einstein also
explained that wave and particle behaviour can coexist as it possesses many wave
properties, but when energy transfers occur, the photons act like little particles
The Particle Model of Light
Some properties of light are best explained if light is considered to consist of a stream of
discrete packages of energy called photons. A photon carries an amount of energy
proportional to the frequency of the radiation. All photons of light of a particular frequency
have precisely the same amount of energy.
The higher the frequency of light, the more energy the photon possesses. More intense light
contains more photons, and all photons have zero rest mass and travel at c in a vacuum.
The energy of a photon is given by 𝐸 = ℎ𝑓 where E
is energy in J, h is Planck’s constant (6.63x10-34) and f
is frequency in Hertz. The speed of light is given
ℎ𝑐
by 𝑐 = 𝑓𝜆. By combining the equations, 𝐸 = 𝜆
results, which is a relationship between energy and
wavelength.
The resulting kinetic energy of the emitted electron
from the photoelectric effect is equal to the energy
of the photon minus the work function.
Ek = hf – 𝜙
Einstein and Planck’s Conflicting Views
Both Einstein and Planck lived in Germany in the early 1900s, with their work being subject
to social and political forces under the Nazi regime. They had both been working in
correspondence with regard to the photoelectric effect.
While Planck often disagreed with German politics, he was a strong patriot, supporting the
use of science to support a social and political agenda as he saw scientific progress as
beneficial to all Germans. In 1914, Planck signed the ‘Manifesto of the 93’, which was a
formal declaration of his support of the acts of the German military. As a result, he began to
devote his research toward the German war effort and also supported the Nazi regime.
However, he was later to suffer from his son’s death in the conflict.
In contrast, Einstein was a pacifist and believed that science should be completely removed
from social and political forces, only being interested in the pursuit of knowledge.
Consequently, he had signed a counter manifesto to show his disapproval of the war effort.
Einstein later migrated to the United States due to the Nazi party’s policy of anti-Semitism
that also denounced his work on relativity.
Einstein’s fears grew in the early 1940s when he realised that his work on the equivalence of
mass and energy was being further researched in Germany. Consequently, he wrote to
President Roosevelt to encourage him to develop the atomic bomb before the Germans,
which became the Manhattan Project, in an effort to end the Nazi atrocities, but later came
to regret his decision.
Photocells and the Photoelectric Effect
Vacuum photocells are used in circuits that require some sort of sensitivity to light (e.g.
burglar alarm, sensor in doorways in shops). The photocell only allows electricity to flow in
the circuit if the cathode is struck by photons above a certain threshold frequency.
A vacuum photocell contains 2 electrodes. The negative cathode is the photo-emissive
surface and is coated with a metal which undergoes photo-emission with low energy light.
For artificial light, caesium on silver oxide is used. When the cell is placed in a circuit in the
dark, it is a barrier to the flow of electricity, but when exposed to photons above the
threshold frequency, current flows in the circuit, which is proportional to the intensity of the
light hitting the cathode.
Part 3: Transistors
Electrons in Solids
In materials that are good insulators, the atoms in the lattice are held by strong covalent
bonds in which electron pairs are shared between atoms, which means that the electrons
are held tightly and are not available to conduct electricity through the lattice.
In some solids, the outer electrons are very loosely bound to particular atoms. These
electrons can therefore move across the entirety of the solid. Metal lattices consist of an
orderly array of positive metal ions permeated by a sea of delocalised electrons. Delocalised
electrons in the metal lattice move randomly between atoms, but under the influence of an
electric field, the electrons begin to have a net drift towards the positive terminal,
producing an electric current.
Conductors, Insulators and Semiconductors
In a conductor, the valence band is only partially filled and since there are numerous energy
levels, the electrons from the partially filled valence energy level are free to move, so the
conduction and valence band overlap in a conductor, giving it a low electrical resistance.
In an insulator, the electrons completely fill the valance band and the gap between it and
the conduction band is large, so the electrons cannot move under the influence of an
electric field unless they are given sufficient energy to cross the large energy gap, giving it a
high electrical resistance.
In a semiconductor, the valence band is almost filled and the gap between the valence and
the conduction band is smaller than in an insulator, so under certain conditions, the
electrons in the valence band can gain sufficient energy to cross the gap, which reduces its
electrical resistance. The resistance in semiconductors decreases when temperature
increases.
Property
Valence band
Conduction
band
Forbidden gap
Resistance
Insulators
Completely
filled
Empty
Very large
High
Semiconductors
Almost filled
Conductors
Partly filled
Empty at absolute zero,
partially occupied at high
temperatures
Small
Reduced, particularly at
high temperatures
Overlaps with the valence
band, so electrons are free
to move between them
None
Very low
Conductors contain high numbers of free electrons in the conduction band. Under normal
conditions, insulators and semiconductors have far fewer free electrons than conductors.
Raising the temperature, using certain lighting conditions or applying a potential difference
can induce electrons in some semiconductors to move into the conduction band.
Holes in Nearly Full Bands
At absolute zero, all of the electrons in a semiconductor occupy the valence band and the
material acts as an insulator. As the temperature of the semiconductor material increases,
thermal energy allows some electrons to cross the gap into the conduction band, leaving the
valence band unfilled or with holes. This means that holes have been created in the valence
band where electrons have left.
These holes act as a positive flow of current moving in the opposite direction to the electron
current flow. Conduction in the conduction band is a flow of electrons and in the valence
band is a flow of positive holes. The hole current flows towards the negative potential while
the electron current flows towards the positive potential, but the speed of the electron
current is much greater.
Germanium in Early Transistors
Germanium was the first Group 4 element that could be sufficiently purified to behave as a
semiconductor. It is relatively rare and is never found uncombined in nature. Early diodes
and transistors were made form germanium because suitable industrial techniques were
developed to purify the germanium to the ultrapure level required for semiconductors
during World War II.
However, a major problem is that when used in electronic components, it gets hot and
becomes a relatively good conductor, allowing too much current to pass through and
possibly damage electronic equipment. The issue was that the resistance to electric current
flow that makes the semiconductor useful in electronic components also generates heat.
Silicon eventually replaced germanium as the desired semiconducting material and it was
very common in the Earth’s crust. However, silicon is more difficult to purify, but it is the
most useful semiconductor for electronics as it is affected less by higher temperatures,
being able to maintain its performance level.
Germanium
 Easy to purify
 Breaks down as
temperature increases
Silicon
 More difficult to purify
 Very common
 Forms self-protective oxide layer
 Retain semiconducting properties at high temperatures
‘Doping’ a Semiconductor
Doping is the addition of a tiny amount of an impurity in the ratio of about one part per
million in an otherwise pure crystal lattice to alter its electrical properties.
This occurs as the dopant atom has a different number of valence electrons from the atom
of the semiconductor it replaces, so extra energy levels can be formed within the energy gap
between the valence and conduction bands, so it is easier for these materials to conduct as
the energy difference between the valence and conduction bands for such dopant atoms is
less.
N-type semiconductors are formed when a Group 5 donor atom (e.g. phosphorous or
arsenic) is substituted into a silicon crystal lattice. A semiconductor doped in this manner
produces an excess of negative charge carriers.
These atoms have 5 electrons in the valence band, while silicon only has 4, so when it is
substituted into the lattice, 4 of the 5 electrons fill the valence band like electrons from a
silicon atom and the extra electron if promoted to the conduction band where it is mobile.
P-type semiconductors are formed when a Group 3 acceptor atom is substituted into the
crystal lattice in place of a Group 4 atom. The Group 3 atom only has 3 valence electrons, as
there is one electron short in the tetrahedral structure, so a hole has been incorporated into
the crystal lattice. The holes act as positive charge carriers that are mobile and carry current
Solid State and Thermionic Devices
Thermionic devices (valves) utilise heated filaments and terminals set in glass vacuum
tubes. The filament in the vacuum valve is heated by an electric current with the thermionic
coating causing it to liberate electrons and act as a cathode. These electrons are then
accelerated by a high potential difference toward an anode.
Solid state devices are made from semiconductor materials, with the interface between a ptype semiconductor and an n-type semiconductor acting as a diode. This combination allows
current to only flow in one direction as electrons move across the junction from the n-type
semiconductor to neutralise available holes in the p-type semiconductor material.
Thermionic devices cannot match the efficiency, cost or reliability of solid states devices.
Appliances using valves had several disadvantages:
 Thermionic devices and appliances were bulky, and valves were fragile and had a
relatively short lifetime
 A large amount of heat was developed by the valves, requiring the protection of
surrounding electronics
 The cathode was coated with a metal that released large numbers of electrons. The
heat produced slowly boiled off the metal coating which reacted with traces of gas
present in the tube

High voltages were required to correctly bias the triodes to amplify signals, while a
silicon transistor uses about 0.6V to do the same job
As such solid state devices were favoured over thermionic devices and have allowed for the
development of smaller, more reliable and efficient pieces of equipment and mobile
technology. Neither of these devices works as conductors when the flow of current is in the
opposite direction.
Modelling P-Type Semiconductors
On a Chinese checker board, fill all spaces in a line except one. As a checker (electron) is
moved into the gap in one direction, the holes (positive holes) move in the opposite
direction.
Set up a scenario where people line up in the waiting room to see the doctor. The electron is
the patient, the hole is an empty chair and is circuit is the path around the room. The
applied electric field is the motivation to see the doctor. The movement of electrons is
represented by the patient’s movement, while the movement of a hole is the moving of
empty seats in the opposite direction.
Invention of the Transistor
Thermionic valves had been widely used in radios and were key components in televisions.
In wars, carrying radios with fragile valves was impractical and led to poor communication.
Valves were also important in switching connections in telephone exchanges as growing
communication demands required automatic dialling and connection technology. However,
the valve-based technology was too slow, unreliable and expensive for the growing
telecommunications industry, and also required a lot of space and maintenance.
These shortcomings in available communication technology led to an increased knowledge
of semiconducting materials through research, and the development of the transistor, using
a sandwich of p-type and n-type semiconductor material. Due to the properties of the
semiconductors, the transistor can replicate the thermionic valve, but:
 Is only a fraction of its size and is cheaper to make
 Only consumes a small amount of electrical power
 Produces virtually no waste heat
 Operates much faster than a valve and is not as fragile
 Does not need to warm up
 Is highly reliable and rarely needs maintenance, lasting for long periods of time
Impact of Transistors on Society
Transistors offered the same functions as thermionic valves, but they were in a tiny solid
state form that made the miniaturisation of electronic circuits possible, especially with the
development of the integrated circuit and microchips. Integrated circuits eliminated the
need for wiring and soldering and increased reliability and speeds of components.
As transistors were more reliable and cheaper to construct than thermionic valves, they
required less maintenance, space and power and as such microchips could be mass-
produced. With the ability to increase the number of components on a microchip,
microprocessors with many microchips connected on an integrated circuit board were soon
developed that could perform multiple complex tasks.
Positive impacts:
The development of transistors has enabled the development of better medical and
industrial devices as well as mobile technology, such as mobile phones and notebooks,
which are core facets of modern day life. It has also revolutionised communication, which is
now faster through the internet and mobile phones.
Electronic transactions are also possible, making it more convenient for individuals. It has
also catalysed the development of new industries, such as IT and internet related
companies, and changed education and the workplace through productivity increasing
devices and programs (e.g. word processing, calculators).
Negative impacts:
New types of entertainment derived from microprocessors may have led to poorer social
interaction between people, through the increase in leisure activities relating to electronic
devices. In some cases, the computer entertainment industry can cause violence and
desensitise children. The use computers itself can be harmful to one’s healthy, possibly
myopia and repetitive strain injury. Many manual jobs have been replaced by automation,
increasing unemployment.
Solar Cells
Initially, the p-type and n-type silicon layers are neutral, but at the p-n junction, free
electrons from the n-type layer move across into the holes on the p-type layer. This occurs
until an equilibrium is reached in terms of charge distribution at the junction, making the ntype layer slightly positive and the p-type layer slightly negative, though collectively it is still
neutral. This field allows electron flow from the p-type side to the n-type side through the
semiconducting material, but not in reverse.
When sunlight is incident on the p-type semiconductor, a photon above the threshold
frequency will be able to excite an electron into the conduction band. The electron moves
across the junction to the n-type layer, while holes migrate to the p-type layer, due to the
electric field. Electrons liberated by the photoelectric effect are then collected by an
electrode on the n-type side and passed through an external circuit and load before it is
returned to the p-type side of the photovoltaic cell to fill holes.
Thus, the potential difference between the p-type and n-type layers causes electron flow in
the external circuit and produces a current that can be used. Solar cells are manufactured to
be thin enough so that most of the incoming light is transmitted to the p-type
semiconductor. An anti-reflection coating on the surface also helps to maximise the number
of incident photons that excite electrons.
Part 4: Superconductivity
The Braggs Method to Determine Crystal Structure
Sir William Henry Bragg and his son Sir William Lawrence Bragg developed an X-ray
spectrometer to systematically study the diffraction of X-Rays from crystal surfaces. They
proposed that due to the short wavelength of X-Rays, they could penetrate the surface of
matter and reflect and scatter from the atomic lattice planes within the crystals.
These X-Rays would result in an interference pattern that could be analysed to give
information about crystal structure. The Bragg experiment used X-Rays reflected from
adjacent atomic planes within the crystal (e.g. sodium chloride), so they interfered
constructively and destructively, producing a pattern, with the measurement of the angles
allowing the spacing and arrangement of the crystal to be determined.
The X-Rays wavelengths are of similar size to the atom separations in the crystal. If the XRays are then projected on a screen then an interference pattern is obtained. This can be
interpreted with the help of Bragg’s diffraction formula (2dsinθ=n𝜆) to determine the
spacing of atoms in the crystal lattice and their pattern.
Conduction and Resistance in Metals
Metal lattices consist of an orderly array of positive metal ions permeated by a sea of
delocalised electrons, which are shared by all the cations in the lattice. In a metal, the
valence electrons are thought of as being shared by all the positive ions, so the electrons are
free to move throughout the crystal lattice. When a potential difference is applied, electrons
experience a net drift towards the positive terminal, which constitutes a current and is
unimpeded by the lattice.
Resistance is the impedance of electron flow through a conductor. Resistance increases as a
direct result of collisions with irregularities in the crystal lattice. Free electron movement is
also impeded by vibrations in the lattice as the vibrating lattice collides with free moving
electrons, deflecting or scattering them from their linear progress through the crystal. As
temperature increases, the lattice vibrations increase and increase the resistance.
Superconductivity
As temperature increases, resistance will also increase. Before superconductivity as
discovered, scientists expected the relationships between temperature and resistance to be
continuous and never reach zero as collisions still occur between the electrons and the
lattice atoms.
However, investigations showed that at a certain low critical temperature, resistance
suddenly dropped to zero. At the critical temperature, it undergoes a phase change into a
new state of superconductivity where electrons are not affected by resistance as it is not
impeded by collisions with lattice atoms.
At temperatures below the critical temperature, lattice effects impeding electron
movement changes dramatically from impeding to assisting electron flow. The assistance
comes about by an effect that pairs electrons and assists them to move freely through the
conductor.
The BCS theory
In 1957, John Bardeen, Leon Cooper and John Schrieffer put forward their theory of why
superconductivity occurs in terms of electron pairs and phonons (sound waves related to
lattice vibrations). The superconducting material is cooled down to or below its critical
temperature, which minimises lattice atom vibrations.
As an electron passes through the lattice, there is significant bending of the lattice inwards
towards the electron due to electrical attraction as the lattice atoms are vibrating slowly
enough. The bending of the lattice releases a packet of sound energy called a phonon,
which is absorbed by a following electron as it is attracted to the increased positive charge
density from the distorted lattice.
The energy gained from the phonon gives the second electron enough kinetic energy to
bind with the first electron despite electrical repulsion, forming a Cooper pair. The
electrons have opposite spins and linear momentum, so the net spin and momentum is
zero. This pairing of electrons can pass through the lattice undisturbed, colliding with no
lattice atoms in their path, thus having a zero resistance.
Examples of Superconductors
Superconductor
Critical Temperature (Tc)
Pure Metals
Below 7K
Alloys
Below 23K
Compound ceramics
Below 133K
Example
Sn (~4K)
NbTi (~10K)
YBCO (~92K)
Type I superconductors are all pure metals, have very low
critical temperature and produce small magnetic fields.
They usually have a sharp critical temperature and
exclude the external magnetic field with circular currents
on its surface.
Type II superconductors are made from alloys or ceramic
compounds, which have a higher critical temperature and
produce more useful magnetic fields. They usually do not
have a distinct critical temperature and exclude the
external magnetic field with circular currents within it.
Coolant
Liquid helium
Liquid helium
Liquid nitrogen
Magnetic Levitation
When in a superconducting state, a material will actively expel all magnetic fields from its
interior, which is called the Meissner Effect. Eddy currents are set up in the YBCO as the
other magnetic field approaches and flow freely, since there is no resistance. These eddy
currents have their own magnetic field, which oppose the magnetic field of the approaching
magnet, thus repelling it.
This produces poles in the superconductor that mirror and oppose the magnet’s poles,
resulting in a repulsive force on the magnet that balances with its weight, allowing it to
hover above the superconducting material.
Advantages and Disadvantages of Using Superconductors
The uses of superconductors have significant advantages, such as:
 Being used as a more efficient means to transporting electricity due to their lack of
resistance and since no electrical energy is lost when superconductors carry
electrical current (e.g. transmission lines)
 The lack of resistance and the Meissner effect, superconductors can be used for
making electromagnets that generate large magnetic fields with no energy loss, with
applications including magnetic levitation, motors and generators
 Electric generators made of superconducting wire is more efficient and about half
the size of conventional generators wound with copper wire
 Since current can flow indefinitely in superconductors, superconducting coils could
be used to store electricity produced in periods of low demand for use in periods of
peak demand
 Particle accelerators using superconducting electromagnets are cheaper to run as
they use less electricity to produce the needed magnetic fields
However, superconductors have their limitations including:
 Needing to be kept at their critical temperature, which requires immersion in liquid
nitrogen and is unfeasible for long wires
 High temperature ceramic superconductors are difficult to produce, brittle, difficult
to make into wires for electricity transmission and chemically unstable in certain
environments
 Metallic superconductors require temperatures close to absolute zero for their
operation, which necessitates the use of very expensive liquid helium as the coolant
 Superconductivity fails when too much current flows through and when a magnet is
brought too close to the superconductor.
Possible Applications of Superconductors
 Computers
o Josephson Junctions which consists of two superconducting materials separated
by a very thin insulator, with its simplest use in high speed switches
o High speed computing requires fast switching between current highways using
transistors, but this can be made even faster through the use of transistors that
employ Josephson junctions
 Motors and Generators



o The use of superconducting electromagnetic coils result in a stronger magnetic
field that makes it much more efficient than current motors and generators
o Motors and generators could be made smaller than they are currently
o Excess generated energy can be stored in coils of superconducting material for
later use
Transmission through the Power Grid
o Very high currents can be transmitted through one wire with zero resistance
o There will be less/no need for transformers in transmission
o Superconducting power lines would exhibit smaller power losses
Magnetic Resonance Imaging (MRI)
o MRI uses powerful magnetic fields and beamed radio waves to sense the
locations of hydrogen atom sin the body
o The spinning protons in the nucleus of hydrogen atoms act as tiny magnets
o In the chamber, a powerful external magnetic field is varied in direction,
interacting with the protons, with some consequently absorbing the beamed
radio waves
o A 3D image of the body is produced, with superconductors being useful in
generating the magnetic field in MRI machines
Maglev Trains
o All maglev trains need 2 sets of magnets: one set for levitation and one set for
propulsion
o Japan is the only country to have a full-size superconducting maglev train
o This uses coils of niobium-titanium alloy cooled by liquid helium, with the current
flow creating intense magnetic fields necessary for levitation
o These interact with levitation and propulsion coils that are located on the walls
of the U-shaped guideway, which are fed with AC in a complex way to levitate
and propel the train