Mark Caetano
Ideas to Implementation
Photoelectric effect and solar cells:
a) What is the photoelectric effect?
The photoelectric effect describes the phenomenon in which electrons are emitted from matter
and as a consequence, they absorb energy from EM radiation of a very short wavelength such as
visible and UV light.
b) Describe Hertz’s observation of the effect on a receiver and the photoelectric effect he
produced.
In 1887, Hertz first observed the photoelectric effect and its production and reception of
electromagnetic waves.
The apparatus consisted of a coil that had a spark gap, where a spark would be produced upon
the detection of EM waves. To better observe these sparks, he placed this apparatus on a dark
box, however he noticed that the spark length was reduced.
He also found that when cancelling out UV radiation, the spark length was also reduced.
c) Describe how solar cells work.
Solar cells convert sunlight into electrical energy. The photons
from the sunlight hit the solar panel and are the absorbed by a
semiconductor such as silicon
Electrons, knocked loose from their atoms are able to flow
freely, they then flow through this semiconductor, producing
electrical energy.
Multiple solar cells are often arranged in arrays to produce a
usable amount of DC energy.
d) Outline qualitatively Hertz’s experiments on measuring the speed of radio waves.
Hertz aimed to prove that waves can travel a long distance through air.
Hertz used an induction coil to do this. Sparks were made between a small
gap, which then induced more sparks in a detecting loop that was
positioned around 12 metres away. Hertz believed this spark was evidence
for EM waves being able to travel through space from an induction coil to
a detecting loop over a distance. He then measured how long it would take
to travel through a wire and noted the difference in travel times.
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e) How did this relate to light waves?
In his later experiments, Hertz measured the velocity of electromagnetic radiation and that it
was the same as the velocity of light. He also managed to show that the reflective and refractive
properties of light was the same as those of radio waves and established the fact that light is a
form of electromagnetic radiation, in keeping with Maxwell’s Equation.
Black Bodies and Quantum Physics:
a) What is a black body?
A black body is something which absorbs all electromagnetic radiation falling on it. They absorb
and then re-emit this radiation in a certain way, called a spectrum. The object appears black
when it is cold because no light is refracted or reflected.
b) What was Planck’s hypothesis?
Planck proposed the idea that energy was only able to be radiated or absorbed by the back body
in small amounts, referred to as ‘quanta’ which are now called photons. The size of each of
these photons is relational to the
c) Briefly describe Einstein’s contribution.
Einstein believed that the energy associated with the radiation of a black body is concentrated
into small packets of energy called photons, which is the smallest amount of energy that is
possible at a given frequency.
The amount of energy carried in a photon is proportional to its frequency and the amount of
light is then proportional to the number of photons. The energy in a photon is then proportional
to its emitted frequency, thus the observation that a shorter frequency will radiate more energy
in total.
d) Explain the particle model.
A proton carries a certain amount of energy which it proportional to the frequency of the
radiation emitted. All photons of a particular frequency have the same amount of energy in
them.
As the frequency gets higher, so does the amount of energy possessed in a photon, thus, a
photon with UV light has more energy than that of a blue or red light photon.
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e) Determine the frequency and energy of each of the following:
a) Frequency:
Energy:
𝑐=𝑓× λ
𝐸 =ℎ×𝑓
𝑐
∴ 𝑓=λ
𝑓=
3×108 m s−1
2
7
𝐸 = 6.6 × 10−34 × 15 × 107
𝐸 = 9.9 × 10−26 𝑗
𝑓 = 15 × 10 𝐻𝑧
b) Frequency:
𝑐=𝑓× λ
𝑐
∴ 𝑓=λ
𝑓=
3×108 m s−1
4.5×10−7
14
Energy:
𝐸 =ℎ×𝑓
𝐸 = 6.6 × 10−34 × 6.67 × 1014
𝐸 = 4.4 × 10−19 𝑗
𝑓 = 6.67 × 10 𝐻𝑧
c) Frequency:
𝑐=𝑓× λ
𝑐
∴ 𝑓=λ
𝑓=
3×108 m s−1
2.0×10−11
19
Energy:
𝐸 =ℎ×𝑓
𝐸 = 6.6 × 10−34 × 1.5 × 1019
𝐸 = 3.3 × 10−15 𝑗
𝑓 = 1.5 × 10 𝐻𝑧
d) Frequency:
𝑐=𝑓× λ
𝑐
∴ 𝑓=λ
𝑓=
3×108 m s−1
4.0×10−1
8
Energy:
𝐸 =ℎ×𝑓
𝐸 = 6.6 × 10−34 × 7.5 × 108
𝐸 = 5.0 × 10−25 𝑗
𝑓 = 7.5 × 10 𝐻𝑧
f) Discuss Einstein and Planck’s views about whether science research is removed from
social and political forces.
As always, science and politics find a way to meet. The views of these two scientists were both
different and the same. As they both studied in the midst of WWII and were both German, they
had implications of war. Einstein did not agree with the German focus and rejected his
nationality, moving to America.
Planck, however believed in his country, but after a constant distrust between the two sides, he
slowly turned against them which led to the execution of his son.
Although they had differing beliefs in this sense, they both believed that science should not be
influenced or marred by politics and that it should be Einstein believed that rather than science
being for the benefit of any one person, it should be for the benefit of the world.
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Band Structure:
a) Describe the difference between energy levels and energy bands.
Energy levels are varying amounts of energy carried
by an atom, whereas energy bands are two definite
positions, whereby the energy levels break up and can
be represented as being in the conduction or the
valence band.
b) Compare the resistance properties of conductors, insulators and semiconductors.
Conductors do not have an energy gap. Because of this it takes only a small amount of energy to
move electrons over to the conduction band. Because of this, conductors pass electrons with
minimal effort.
The energy diagram for an insulator shows it as having a very wide gap. The larger this gap is,
the harder it is to move energy from the valence to the conduction band. As a result, an
insulator requires a much larger amount of energy to
obtain a very small amount of current passing between.
The semiconductor has a much smaller forbidden band
and needs less energy to move an electron, but more
than that of a conductor to flow between the valence
and conduction band.
c) Describe what is meant by a ‘hole’ and outline its properties.
In a semiconductor, when electrons move into the conduction band, it leaves what is known as a
‘hole.’ This is an atom with one less valence electron than beforehand. The nearest available
valence electron will them move to fill this hole left by the previous electron which then creates
another hole, a repeating process. This is the equivalent to an electric current in a
semiconductor.
d) Describe how behaviors of semiconductors depend on density of electrons or holes.
The behavior of a semiconductor will always be in relation to the way the semiconductor has
been structured. If there is a high conductivity, there will be a greater amount of holes. Because
of the larger amount of free holes, the valence electrons can move to a closer hole much quicker
than having to move to one further away. This creates a higher rate of current flow due to the
density of electrons and holes being higher.
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Doping and n-type & p-type semiconductors:
a) Describe how doping a semiconductor changes its electrical properties.
Doping a semiconductor is adding an extremely miniscule amount of an impurity to a
semiconductor. By doing this, the conductivity of the semiconductor is increased due to the
extra electrons or holes which now act as additional charge carriers for the valence electrons.
b) Identify differences in p and n type semiconductors in terms of the number of holes
and charge carriers.
In a p-type semiconductor, there are more positive holes than negative charge carriers. Doping
agents such as aluminium which has 3 valence electrons are mixed with the silicon to produce a
p-type.
A p-type is similar; however it features more negative charge carriers than positive holes. In ntypes, phosphor and arsenic with 5 valence electrons will be used as a doping agent along with
the silicon.
c) Describe the operation and electrical properties of a p-n junction.
When P and N type silicon are placed close together, it forms what
is called a P-N junction. A P-N junction at its simplest is a diode
that allows current to flow in only one direction. The electrons will
move into the vacant holes in the P half of the junction, leaving a
‘depletion zone’ which acts as a type of insulator which prevents
other free electrons from the N-type half combining.
Thermionic devices:
a) Describe thermionic emission.
At its simplest, thermionic emission is the emission of electrons from very hot substances. A
thermionic device is one which emits electrons only when it reaches a very high temperature.
b) Compare the efficiency of thermionic devices and solid-state devices.
Thermionic devices have many disadvantages in comparison to solid-state devices. As they require
heat to operate, a thermionic needs to reach a certain temperature before it will operate, thus, it
needs a heating coil, taking more space, requiring more power and giving off more excess heat. A
solid state device works instantly and does not require heat to work due to use of semiconductors.
A near-vacuum is essential in the operation of a thermionic tube to allow for the electron flow
between the electrodes, making them more fragile and susceptible to breakage. Solid state devices
can work in any degree of air pressures and do not need to be in a glass tube, only some form of
insulator, usually a plastic.
Solid state devices also take a fraction of the space used by thermionic, allowing for miniaturisation
of equipment. These combined advantages make solid state devices much more attractive.
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c) What prompted scientists to look for a replacement for valves?
As outlined in the above question, the use of thermionic devices, namely vacuum tubes in
electrical equipment was proving to be inefficient, outdated and a constraint on further
developing new technologies such as smaller and more
powerful devices like the computer. For example, in the
Colossus2, around 2000 vacuum tubes were used, taking up
large amounts of space. The machines used 15Kw/h with
almost all of this being used solely on the tube heaters.
When they had discovered transistors, all of the above
problems had been answered by a device that took a
fraction of the space and power
d) Describe the essential design of a diode valve.
A current is passed through the heating filament which then heats the cathode.
This heat causes thermionic emission of electrons into the near-vacuum of the
tube. A metal electrode surrounds it and is positively charged so that it attracts
these electros emitted from the cathode. The electrons do not easily flow back
from the unheated anode when the polarity is reversed so any reverse flow is
very little or none.
e) Identify the essential difference between a diode and a triode. What is each used for?
A diode will only allow current to flow in one direction only, this is generally used to prevent
current from blowing backwards through a circuit which may cause damage or prevent correct
operation.
A triode is an electrical amplification device with three elements, as opposed to the two in a
diode. The triode will take the incoming electrical impulses, usually quite weak and add a higher
amount of power onto them, acting as an amplifier, such as that of a guitar.
f) The theory for transistors was known 22 years beforehand, what was the problem?
Even though they knew how to build the transistors and miniaturise all components, they still
lacked the understanding of electron mobility in semiconductors. After gaining a better
understanding of this, they discovered that getting the crystal used a semiconductor to a small
size would be very difficult as the holes would be very large and require a high amount of
current passing through to start with, making it useless as an amplifier.
g) Identify and explain the use of germanium in early semiconductor devices.
Germanium was used in early electronics and has made a comeback due to its relatively high
number of valence electrons can withstand very high voltages and temperatures in comparison
to the other metals being used at the time.
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h) Outline why transistors replaced thermionic valves in electrical appliances.
The transistor was much more superior to the vacuum tube which largely dominated electronics
at the time. They could be made at a faster speed for less money, reducing the price of
appliances. They could withstand more shock and were more robust, while
using much less power, giving way to the transistor radio. They did not
require pre-heating so devices could work almost instantly, meaning in
applications such as computers, operations would take place at a fraction of
the speed of vacuum tubes did.
i) Compare the structure and operation of a diode and triode with their solid state
equivalents.
The operation of these devices whether they are in a vacuum tube form or embedded onto a
piece of silicon as a transistor is much the same. The diode still only allows current to flow in one
direction and the triode or a transistor in its solid state form still acts as an amplifier.
The main difference is that the earlier thermionic counterparts pass electric currents through a
void or a vacuum to achieve the effect, whereas the solid state versions use semiconductors and
energy bands to achieve the effect.
Social Impact:
a) Briefly assess the impact of the invention of transistors on society.
With the invention of the transistor, the world of miniature electronics and circuits was opened
up to the masses. Manufacturers could now create devices that needed less power, space and
were much more reliable than their thermionic counterparts. They could now be produced
more sophisticated and much cheaper than before.
Computers and communication devices have brought the world closer in ways that would not
have been imagined before and opened up technologies like communication which have
increased in practicality and decreased in size and price such as mobiles and GPS which would
have been impossible without solid-state transistors.
Computers now no longer take up whole rooms and require copious amounts of power just to
run a simple task. WWII code breaking machines that used almost 2,000 valves and tubes have
now been condensed into a device that can fit on a person’s thumbnail and are available for less
than a fraction of the original cost.
Semiconductors have also led to the development of alternative energy sources such as solar
panels. Energy can now be sourced from the sun, an endless supply of energy with no
environmental effects. As transistors require much less power to run, pollution can be
decreased or even nulled due to the use of solar power, such as in calculators.
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b) Describe the construction of integrated circuits and microchips.
The first step starts with a wafer of 99.99% pure silicon, made from sand and consisting of 4
layers. A mask or stencil in the shape of the circuit desired. UV light is passed through this mask
and reacts with the layers on the wafer. Metal impurities are then layered onto these areas,
known as doping.
Electrical connections are then made by etching parts out of certain layers to provide links
between them. The finished chip is then placed within a protective plastic or ceramic case with
wires and contacts that connect it to the circuit board.
c) Summarise the effect of light on semiconductors in solar cells.
When a certain wavelength of light strikes a semiconductor metal, it is absorbed into the
material. The absorbed energy, in the form of photos is transferred through to the
semiconductor and is converted to electrons and positive holes moving across a P-N junction in
opposite directions. A metal grid placed on either side of the solar cells allows the electrons to
be collected and when connected to a circuit, used to power devices.
Braggs:
a) Outline the methods used by Braggs to determine crystal structure.
He initially studied crystals using x-rays and examined the patterns produced when the x-rays
passed though and stuck a photographic screen to determine their structure. He calculated the
angles between bright and dark spots on the screen to determine the internal structure.
b) What is meant by crystal lattice structure?
The atoms within a crystal arranged in a set repeating pattern, called a lattice, usually displayed
as a 3 dimensional unit. The basic structure of these is known as the unit cell which is repeated
over many times and interconnected with other unit cells to form a lattice.
c) What is the impact of the presence of impurities in the lattice on the passage of
electrons through the lattice?
Chemical and sometimes structural impurities cam impede the flow of electrons though the
lattice, preventing the free movement of valence electrons through the lattice structure to other
electrons.
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Definitions:
a) Define superconductivity.
Superconductivity is when the electrical resistance in a material disappears are very low
temperatures.
b) Compare type 1 and type 2 superconductors.
Type 1 superconductors were the first to be discovered and are mainly metals that show some
conductivity at room temperatures. They need to be at very cold temperatures to slow down
vibrations and give an unimpeded flow.
Type 2 mainly consist of metals and alloys which can yield a higher throughput without lowering
the temperature to a level as extreme as that of type 2 superconductors.
c) Account for the limited use of superconductors at this time.
There are many limitations to their use such as the difficulty of keeping the materials under such
a low temperature to sustain the superconductivity. The materials used are often expensive,
hard to draw into a wire and become brittle at the extremely low temperatures required.
d) Construct a timeline for the discovery and development of superconductors.
1911 - Superconductivity first observed in mercury by Dutch physicist Heike Kamerlingh Onnes
1933 - German researchers Meissner and Ochsenfeld discovered that a superconducting
material will repel a magnetic field
1941 - Niobium-nitride was found to superconduct at 16 K
1953 - Vanadium-silicon displayed superconductive properties at 17.5 K
1957 - First widely-accepted theoretical understanding of superconductivity was advanced by
American physicists John Bardeen, Leon Cooper, and John Schrieffer (above)
1962 - Scientists at Westinghouse developed the first commercial superconducting wire, an alloy
of niobium and titanium (NbTi).
1962 - Brian D. Josephson predicted that electrical current would flow between 2
superconducting materials - even when they are separated by a non-superconductor or
insulator
1964 - Bill Little of Stanford University had suggested the possibility of organic (carbon-based)
superconductors
1980 – First organic superconductor produced by Danish researcher Klaus Bechgaard
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1986 - Researchers at the IBM Research Switzerland, created a brittle ceramic compound that
superconducted at the highest temperature then known: 30 K
1987 - A research team at the University of Alabama-Huntsville substituted Yttrium for
Lanthanum in the Müller and Bednorz molecule and achieved 92 K
1993 - The first synthesis of mercuric-Cuprites was achieved at the University of Colorado
1997 - Researchers found that at a temperature very near absolute zero an alloy of gold and
indium was both a superconductor and a natural magnet.
2001 - Discovery of the first high-temperature superconductor that does NOT contain any
copper
2001 - Japanese researchers measured the transition temperature of magnesium Diboride at 39
Kelvin
2006 - High-Tc copper-oxides discovered with Tc's over 50K
e) Explain the BCS theory of superconductivity.
The BCS theory states that in a superconductor, the atoms do not interact destructively; rather,
they actually interact in a constructive manner. It argues that an attraction exists between
electrons and the crystal lattice.
An electron within the lattice will create an increase in the positive charges that surround it and
will attract another electron, becoming what is known as a Cooper Pair. If the energy needed to
bind these together is less than the energy from the thermal vibrations, they will remain bound,
explaining why superconductors require low temperatures to operate.
f) Explain the Meissner effect and why a magnet can hover over a superconductor.
Superconductors can repel magnets due to it being structurally
different to that of a normal conductor. When it reaches a
temperature below its critical temperature (Tc) it will not allow
any magnetic field to enter it. This induced field repels the
magnetic field which interacts with that of the superconductor.
The magnet will levitate above the superconductor at a distance proportional to the amount of
current flowing through the superconductor and the strength of the magnet.
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g) Define Coopers Pairs and a phonon. Discuss their roles in superconductors.
Cooper pairs are the name given to two electrons that are bound together at low temperatures
in a superconductor. The electrons are repelled from others due to the negative charge and
attract positive ions which make up the lattice structure of the metal.
A phonon is a unit of energy that results from oscillating atoms within a crystal. They behave as
if they were attached to a spring with their own thermal energy or other influences causing
them to vibrate. The energy from these oscillations attracts electrons together, forming
Coopers Pairs which then carry the current through the superconductor.
h) Explain Tc.
Tc or Critical Temperature is a point where a metal, depending on its density reaches zero
resistance, thus making it a superconductor.
When reaching this temperature, the scattering of electrons through the lattice structure of the
crystal is stopped, allowing the electrons in Cooper pairs to move freely through the structure
without being impeded.
i) Identify two element/alloy superconductors and two metal oxide ceramic conductors
and state their critical temperatures.
Element/Alloy:
Metal oxide ceramic:
Niobium/Tin - 18.1k
Yttrium barium copper oxide - 90k
Cupric sulphide - 1.6k
Titanium Barium Copper Oxide – 125k
Definitions:
a) Outline the use of superconductors in the area of medicine
Superconductivity is used mainly in MRI (magnetic resonance imaging) to see inside a person’s
body through use of extremely accurately directed magnetic fields that resonate back and forth
between emitter and receiver.
b) Explain MRI
Through use of large solenoids, MRI produces a strong magnetic field. RF waves ate then used to
produce photons with similar energy levels to those of the human body. The signal produced as
a result is an indicator of the concentration of hydrogen atoms. From this data, a 3 dimensional
representation of the patient’s tissue can be generated.
c) Explain how SQUID’s are used in medical imaging.
A SQUID measures tiny magnetic flux lines which lower the voltage carried through a
superconductor in the device. This voltage drop is measured and is used to determine the
magnetic field strength.
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d) Explain the benefits and limitations of using superconductors in medicine.
The advantages of superconductors in medicine are that they have an extremely high level of
precision and accuracy and no energy loss. However, there are limitations, such as the very high
price tag attached which can be prohibitive. They also need to be kept at a constant, very low
temperature.
e) Outline the use of superconductors in transportation.
Superconducting magnets are used in Maglev trains to create a
levitating effect which creates a frictionless contact with the
ground, utilising repulsion and attraction for propulsion.
f) Explain maglev trains.
Maglev trains utilize the repulsion of same pole magnets to
suspend the train above the ground, eliminating friction with
magnets under the train and along the ‘track.’ By changing the
polarity of the magnets, the train can be attracted and repelled
to move along the track.
g) Explain the benefits and limitations of maglev trains using superconductors.
By utilising superconductors in maglev trains, it is possible to create a faster, safer and smoother
ride as there is no physical contact with the tracks, however they require low temperatures to
work which restricts it to a small geographical area.
h) Outline the proposed uses of superconductors in transportation.
As well as Maglev trains, concepts for using superconductors in motors to power cruise and
defense ships as well as buses are being developed.
i) Outline the use of superconductors in computer development.
Superconductors are recently being used in processors for petaflop speed computers, such as
Roadrunner, the fastest today.
j) Outline the proposed use of superconductors in computer development.
It has been proposed that superconductors can be used for storing and retrieving digital
information by penetrating certain Type 2 superconductors with magnetic fields.
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k) Outline the proposed use of superconductors in energy generation, transmission and
storage.
For transmission, superconducting wire can be used to greatly minimize energy loss and heat
generation; superconducting wire can carry up to five times as much current than a regular
transmission line.
For power generation, the development of superconducting magnets has resulted in magnets
which do not need an iron core, resulting in smaller, lighter and much more efficient generators.
Superconducting fault timers and switches are being developed that will control power faults
and surges
For power storage; the current technologies do not allow energy to be stored easily or without
loss. SMES or Superconducting magnetic energy storage has been proposed as a solution.
Consisting of a large ring or a HTS (high temperature superconductor) it can store DC currents
with no energy loss. By using SMESes, power stations and even homes can store large amounts
of energy for extended periods for when demand is less, saving energy, fuel and money.
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Photoelectric effect: http://en.wikipedia.org/wiki/Photoelectric_effect#Hertz.27s_spark_gaps
Hertz Experiment: http://www.juliantrubin.com/bigten/hertzexperiment.html
Solar cell image: http://www.esru.strath.ac.uk/Courseware/Class-16110/Images/pv1.jpg
http://www.hsc.csu.edu.au/physics/core/implementation/9_4_2/942net.html
www.sparknotes.com/biography/planck/section10.rhtml
http://www.hscphysics.edu.au/resource/PlaEin.flv
http://www.tpub.com/neets/book7/24c.htm
http://www.hsc.csu.edu.au/physics/core/implementation/9_4_3/943net.html
http://en.wikipedia.org/wiki/P-n_junction
http://en.wikipedia.org/wiki/Diode
http://www-03.ibm.com/ibm/history/exhibits/vintage/vintage_4506VV2124.html
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/sili.html
http://wiki.answers.com/Q/How_a_microchip_is_made
http://www.superconductors.org/History.htm
http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/T.J_Barry/bcstheory.html
http://www.users.qwest.net/~csconductor/Experiment_Guide/Meissner%20Effect.htm
http://www.britannica.com/EBchecked/topic/457336/phonon
http://teachers.web.cern.ch/teachers/archiv/HST2001/accelerators/superconductivity/supercondu
ctivity.htm
http://www.ornl.gov/info/reports/m/ornlm3063r1/pt4.html
http://www.physicscentral.org/explore/action/super-train.cfm
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http://www.amsc.com/products/applications/transportation/index.html
http://www.superconductors.org/Uses.htm
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