Topic 3: Ideas to Implementation
22.214.171.124.1 Explain why the apparent inconsistent behaviour of cathode rays caused debate as to whether they were charged particles or electromagnetic waves
Travelled in straight lines
Rays left the cathode at right angles to the surface
Shadow of opaque objects were projected (Maltese Cross)
Deflected by magnetic fields
Could pass through thin foils without damaging them (suggesting they were massless)
Travelled considerably more slowly than light.
Small paddlewheels turned when placed in the path of the rays
126.96.36.199.2 Explain that cathode ray tubes allowed the manipulation of a stream of charged particles
- A Cathode Ray Tube (or a Discharge Tube) is an evacuated glass tube with electrodes at the ends of the tube.
- Geissler and Plucker discovered that an evacuated tube could conduct electricity with a high potential difference in 1855.
- They also found that, the glass at the positive end (anode) glowed a green colour.
- What happened was that:
- As a high Voltage is inputted, a filament in the cathode gets heated up and this excess of energy is enough to set the electrons free. (thermionic emission)
- The free electrons are then attracted to the positive anode on the other end of the tube.
- This stream of electrons flowing is the cathode ray.
- These electrons can be manipulated with electric and magnetic fields as they are (obviously) electrically negatively charged.
188.8.131.52.3 Identify that moving charged particles in a magnetic field experience a force
184.108.40.206.5 Describe quantitatively the force acting on a charge through a magnetic field
- A current passes through a magnetic field it experiences a force. (motor effect)
- As current is just charge over time (I=qt), a charge moving in a magnetic field would also experience a force.
- This force's direction can be found by the Right-Hand rule.
- As such, only the perpendicular component of the particle's motion is taken into account.
220.127.116.11.4 Identify that charged plates produce an electric field
18.104.22.168.7 Describe quantitatively the electric field due to oppositely charged parallel plates
- Two parallel plates that have been oppositely charged produce a uniform electric field.
- This means that any charge at any place between the two plates has the exact same force acting upon it.
(From f = eq and W= qV and W = fd)
22.214.171.124.6 Discuss qualitatively the electric field strength due to a point charge, positive and negative charges and oppositely charged parallel plates
126.96.36.199.8 Outline Thomson’s experiment to measure the charge/mass ratio of an electron
- Thomson built a cathode ray tube with charged parallel plates (capacitor plates) to provide a uniform electric field as well as a source of a uniform magnetic field orientated perpendicular the electric field
- He varied the magnetic and electric fields until their opposing forces cancelled, leaving the cathode rays unaffected.
- By equating the force equations, Thomson was able to calculate the velocity of the particles.
- He then applied the same strength magnetic field (with no electric field) and determined the radius of the circular path travelled by the charged particle in the magnetic field.
- qVB = mv2/r
- q/m = v/Br
- (subbing the first equation in)
- By combining the results he could find the charge to mass ratio (q/m)
- Charge to mass ratio did not change when there were different gasses in the tube
- Charge to mass ratio did not change with different electrode elements
- Was something more fundamental
- 1800 times greater ratio than hydrogen atom
- These charges are subatomic and form part of the atom
188.8.131.52.9 Outline the role of: In the cathode ray tube of conventional TV displays and oscilloscopes
- Electrodes in the electron gun
- The role of this section is to produce electrons at a high, fixed, velocity.
- This is done through thermionic emission.
- A filament in the cathode is heated to the point where its electrons become loose.
- An anode with a high voltage applied to it accelerates the electrons towards the screen due to electrostatic attraction.
- On the way, the electrons pass through a series of control grids which control the brightness of the image produced.
- The more negative the grid, the darker the image and vice versa.
- 3 guns in TVs are used (RGB) which scan the TV faster than the human eye can detect.
- The deflection plates or coils
- The role of the deflection system is to control the image produced by controlling the position that the electrons hit the screen.
- It consists of two sets of Electric/Magnetic fields that are perpendicular.
- This allows control over both horizontal and vertical axes.
- By controlling the voltage applied to the fields, it is possible to vary the deflection through Electrostatic force/Motor effect.
- The role of the screen is to display where the electrons are hitting the CRT and in a TV produce the image.
- It is a screen coated with a material that emits light when struck by electrons.
- Zinc sulfide or Phosphorus are two commonly used materials.
Televisions use magnetic deflection as more deflection can be created in a shorter space
CROs use electric deflection because the beam has to move quicker
Self-induction becomes a problem in magnetic coils if the field is changed rapidly
Cro screens are longer (deeper) to account for the smaller deflection
TVs only operate at 50Hz
184.108.40.206.1 Perform an investigation to observe the occurrence of different striation patterns for different pressures in discharge tubes.
- Apply a voltage across a sealed glass tube where most of the air has been removed and has a cathode and anode inside it
- This will allow the current to flow as the cathode emits electrons (cathode rays) which ionise atoms of gas in their path releasing ions and more electrons.
- The pressure of the gas determines the density of atoms and thus the nature of collisions.
- Therefore different discharge effects can be observed at different pressures.
- Excitation: Electrons moved to higher orbit
- Ionisation: Electrons removed from atom
- Observe the discharge pattern
- Repeat for tubes of different pressures.
220.127.116.11.2 Perform an investigation to demonstrate and identify properties of cathode rays using discharge tubes
- Place a metal barrier in the form of the maltese cross in the centre of the tube
- A shadow is cast on the end on the tube
- Suggesting that the rays travelled in straight lines and originated at the negative terminal.
- Cathode ray tube was equipped with two horizontal plates which could be attached to a battery
- Uniform electric field established between the two plates and the beam was deflected
- Beam bent towards the positive electrode consistent with the rays being particles carrying a negative charge.
- Fluorescent display screen
- When a magnet was brought close to the tube the rays are deflected
- Happened at right angles to the direction that the beam was travelling and that of the magnetic field.
- Place a light paddle wheel in the cathode ray tube
- Observe the wheel spinning and move away from the cathode
- Suggest that the ray has momentum (however later proved to happen as one side of the sail becomes hotter)
18.104.22.168.3 Solve problems and analyse information using:
F = Eq
E = v/d
W = qV = ½ mv2
To work out the velocity of a charge between plates qV = ½ mv2
22.214.171.124.1 Describe Hertz’s observation of the effect of a radio wave on a receiver and the photoelectric effect he produced but failed to investigate
Hertz noted that sparks could be produced across the gap for distances of several meters.
He noticed that the spark jumped more readily when it was eliminated by UV light. He called it the photoelectric effect but failed to investigate it.
126.96.36.199.2 Outline qualitatively Hertz’s experiments in measuring the speed of radio waves and how they relate to light waves
He used the Lloyd's mirror experiment where:
- Waves of known frequency were reflected from a metal plate and allowed to interfere with the direct beam from the transmitter.
- This resulted in a diffraction pattern (from constructive and destructive interference) which showed maximum and minimum intensity at right angles to the plate. The separation of maxima and minima is proportional to the wavelength
- Using V = f lambda he was able to calculate their velocity
- Velocity was very close to the speed of light measured previously and that predicted by Maxwell.
188.8.131.52.3 Identify Planck’s hypothesis that radiation emitted and absorbed by the walls of a black body is quantised
A black body is any object that will absorb 100% of radiation that hits it and emits radiation at all wavelengths and therefore appears black.
This spectrum shows the amount of each colour being emitted at each temperature.
The classical wave theory predicted that as the wavelength of emission became shorter, the radiation intensity would increase approaching infinite amounts of radiation. This cannot be true as it violates the law of conservation of energy and was described as “the ultraviolet catastrophe”.
The experimental data (that the radiation intensity curve corresponding to a given temperature has a given peak and then declines) could not be explained.
Planck suggested that the energy radiated and absorbed by a black body was not continuous but rather quantised - it should be treated as packets of energy (photons). The energy could be described by E = hf
184.108.40.206.4 Identify Einstein’s contribution to quantum theory and its relation to black body radiation.
220.127.116.11.2 Assess Einstein’s contribution to quantum theory and its relation to black body radiation
- Albert Einstein incorporated the idea of ‘quantised energy’ in a 1905 appear
- He suggested that all electromagnetic radiation was emitted or absorbed in discrete units of energy called photons
- Individual photons were directly proportional to the frequency of light (E = hf)
- Einstein produced an equation for the photoelectric effect
- Ek is the kinetic energy of the emitted photoelectrons
- Ephoton is the energy of the quantised photon
- Phi is the work function or energy holding the electron to the atom
- Einstein’s theories were able to be confirmed by shining light onto a metal cathode to emit photoelectrons and in turn produce a current.
- Thus confirming Planck’s ideas
- Whilst Planck came up with the concept of quanta to solve the UV catastrophe he could not explain why they worked.
- Einstein produced an equation
18.104.22.168.5 Explain the particle model of light in terms of photons with particular energy and frequency
- The particle model of light is that light energy travels in photons; and it is that energy in these photons that activate electrons in a metal. When a photon strikes the metal, all (or none) of the energy will be passed on to the electron and then be emitted as a spark.
- Photons have a particular energy and particular frequency
- This particle theory was used by Einstein and explains how the intensity of light depends on the rate of photons.
22.214.171.124.6 Identify the relationships between photon energy, frequency, speed of light and wavelength
126.96.36.199.4 Solve problems using
188.8.131.52.1 Perform an investigation to demonstrate the production and reception of radio waves
- Have one student hold a walkie talkie powered on in the corner of the classroom
- Have the other student stand in the other corner of the classroom with a powered on walkie talkie
- Push the button on one walkie talkie and whisper something into the microphone of the walkie talkie whilst pressing the transmit button
- Record observations from the audio produced by the other walkie talkie
184.108.40.206.3 Summarise the use of photoelectric effect in photocells
A photocell is a device that uses the photoelectric effect (the interaction between a photon and an electron in a metal to produce an electron no longer bound by the metal) to operate.
These devices include Phototubes and photomultipliers. They could additionally be photoresistors, which are used to detect light.
Photocells operate by the photoelectric effect, which dictates that electromagnetic radiation (above a certain cut-off frequency - principally in the ultraviolet range for most materials) falling onto the surface of a material causes the emission of photoelectrons and a current to flow in the photocell.
Photoresistors and light detectors rely upon the change in resistance caused by the emission of electrons from materials when exposed to electromagnetic radiation.
220.127.116.11.5 Process information to discuss Einstein and Planck’s differing views about whether science research is removed from social and political forces
Political Views of Planck and Einstein
- Both Einstein and Planck lived in Germany at the same time; they were both friends who both worked in the same area of physics. Einstein was a pacifist and opposed the German war effort, while Plank was a staunch patriot who strongly supported the war.
- Einstein held the view that science research was not removed from social and political forces. He believed that political and social forces directed the course of science and use its discoveries. This led him to leave Germany before WW2 and become active in movements for world peace and cooperation. He wrote a letter to the American president at the time warning of the dangers of atomic research in Nazi Germany. This resulted in the Manhattan Project. He was strongly opposed to the building and use of atomic and nuclear weapons and remained a believer in the fact that scientific research should be free from social and political forces.
- Planck during the war directed all his research to the war effort and was faithful to the aims of the Nazi doctrine even despite his son being assassinated and home town raided. His work was not removed from social and political forces.
18.104.22.168.1 Identify that some electrons in solids are shared between atoms and move freely
Some electrons in solids are shared between atoms and move freely
- In metals, a lattice of positive ions is surrounded by a sea of delocalised valence electrons which are free to move between atoms.
- These electrons drift randomly with essentially no net movement
- When an electric field is applied, they drift against it and are able to conduct electricity
- In insulators, atoms are held in a lattice structure by strong covalently bonded valence electron pairs.
- These electrons are not free to move and so cannot conduct electricity
22.214.171.124.2 Describe the difference between conductors, insulators and semiconductors in terms of band structures and relative electrical resistance
No energy gap
As temp increased, vibration of ions increases, hindering the straight movement of electrons
More electrically conductive than insulators BUT less electrically conductive than conductors
Not electrically conductive - high resistance
Large energy gap
Valence band: the highest energy band in which electrons exist in an unexcited state
Conduction band: band where excited electrons can move freely and conduct electricity.
As can be seen only when the electrons in a semiconductor's valence band have enough energy to move into the conduction band will it conduct electricity. This energy is provided by heat.
126.96.36.199.3 Identify absences of electrons in a nearly full band as holes, and recognise that both electrons and holes help to carry current
The net absence of an electron in a full valence band is known as a positive hole.Holes act as positive charge carriers and move in the direction of conventional current.
Positive current flow relies on the movement of holes from one atom to another. i.e. the hole is filled by a neighbouring electron which simultaneously leaves behind a new positive hole.
Thus electron current is much faster as they are able to flow through the conduction band, whereas positive current flow is slower (relies on “leapfrogging”).
188.8.131.52.4 Compare qualitatively the relative number of free electrons that can drift from atom to atom in conductors, semiconductors and insulators
Conductors have many electrons in the conduction band able to act as free charge carriers
Semiconductors have a few free electrons
Insulators have no free electrons
184.108.40.206.5 Identify that the use of germanium in early transistors is related to lack of ability to produce other materials of suitable purity
Germanium was the first element to be used in early transistors as it was the only element which could be purified to a sufficiently high level.
- At higher temperatures, it becomes too good a conductor – allows excessive and dangerous currents
- Germanium is rare and costly.
- Silicon eventually replaced germanium as the semiconducting material of choice in transistors
- It is the second most abundant element on earth – widely available and cheap to obtain.
- It retained its semiconducting properties at relatively high temperatures
- It forms oxide wafers which can be doped with impurities.
- Processing techniques were developed to produce very pure crystals.
220.127.116.11.6 Describe how ‘doping’ a semiconductors can change its electrical properties
Doping: Addition of a group III or group V elements to group IV semiconductors to change its electrical properties
Addition of a group V dopant: N-type
- Group V atoms have one more valence electron than group IV atoms.
- When added, 4 of its 5 electrons form covalent bonds with adjacent atoms but one remains unbounded.
- This electron can be easily promoted to the conduction band and act as a free charge carrier, increasing negative charge carrier density.
- This increases conductivity and reduces resistance.
Addition of a group III dopant: P-type
Group III atoms have one less electron in valence shell than group IV.
- When added into a semiconducting lattice, the 3 valence electrons form covalent bonds with surrounding atoms.
- The absence of a 4th electron leaves a positive hole in the lattice which is able to act as a positive charge carrier in the valence band.
- This increases positive charge carrier density, increasing conductivity.
18.104.22.168.7 Identify differences in p and n type semiconductors in terms of the relative number of negative charge carriers and positive holes.
- Majority of charge carriers are negative electrons due to addition of dopant. (extrinsic)
- Minority of charge carriers are positive holes due to natural excitement of an electron into the conduction band. (intrinsic)
- Majority of charge carriers are positive holes due to doping. (extrinsic)
- Minority of charge carriers are negative electrons.
22.214.171.124.8 Describe differences between solid state and thermionic devices and discuss why solid state devices replaced thermionic devices
- A negative filament is heated sufficiently to allow thermionic emission of electrons from the surface.
- These electrons move towards a positively charged electrode at other end of the vacuum.
- In diodes, this is the process by which current is rectified (AC to DC)
- If the filament is negative and the electrode is positive, e- will flow.
- If supplied current is reversed, then e- will not travel to the electrode and no current will flow.
Solid State Devices
- Solid state devices use semiconductors and properties of p-n junctions to direct the flow of
- The junction between a p-type semiconductor and an n-type semiconductor acts as a diode, allowing current to only flow in a single direction.
- Operation of a p-n junction
- Electrons close to the p-n junction tend to diffuse from the n-type across the interface and combine with positive holes in the p-type semiconductor.
- This movement of charge creates a net potential difference across the junction, setting up an E-field that goes from nàp .
- This E-field acts as a potential stopper against further movement of charge across the junction and is called the “depletion zone”
- If a voltage is applied to a p-n junction, it will act as a diode, allowing current to flow from p to n
The application of current can be either “forward” or “reverse” biased:
- This involves positive current being applied p → n and hence negative current from n → p.
- The flow of electrons into the n-type repels electrons already present in the n-type semiconductor.
- They are forced towards the depletion zone where they neutralise the positive holes at the junction.
- Similarly, the positive holes in the p-type will move towards the depletion zones and neutralise negative charges at the junction.
- This decreases the potential difference across the p-n junction. (narrows the depletion zone)
- Eventually the movement of charge across the junction will no longer be opposed by the potential across the depletion zone, allowing the flow of conventional current from p-type to n-type.
- If voltage is applied so that positive current is directed from n→ p and the negative current from p→ n, the influx of positive charge to the n-type will attract electrons in the semiconductor away from the p-n junction.
- This increases the potential difference across the junction (widens the depletion zone)
- It is very difficult for charge carriers to attain sufficient energy to overcome the strong electric field and cross the depletion zone so no current will flow
Thermionic vs solid state pros and cons:
Used in communication systems like wireless radio to act as an amplifier
Glass is fragile
Warming up time
Huge rooms to hold a small computer
126.96.36.199.1 Perform an investigation to model the behaviour of semiconductors
- 10 people are seated in ten chairs to represent electrons in the valence band of the atoms of a semiconductor at 0 K
- 2 people stand at either end holding up + and - signs representing the applied voltage
- Play music, representing energy
- Some people stand up, representing electrons excited by the energy moving into the conduction band and becoming free to move within the lattice
- Standing people move towards the + representing electron flow
- Seated people move one chair towards the + if there is an empty chair next to them, so empty chair space moves towards -, representing the movement of holes
- There is nothing in this model representing the size of the band gap, and therefore the behaviour of different semiconducting materials cannot be modelled
- There is nothing representing the strength of the applied voltage, so the behaviour of semiconductors under different voltages cannot be modelled
188.8.131.52.2 Discuss how shortcomings in available communication technology lead to an increased knowledge of the properties of materials with particular reference to the invention of the transistor
During WWII, an increased need for more efficient and sensitive radar and radio communications sparked research into transistors.
- Thermionic devices used at the time were fragile, bulky, inefficient, noisy, unreliable and insensitive
- Researchers tried to improve amplification technology
Germanium – Experimentation yielded transistors using Germanium crystals.
- This enhanced our scientific understanding of the semiconducting properties of Ge and its potential for use in transistor devices.
- When electrical contacts were applied to a crystal of Germanium, the output power was larger than the input
- Transistors are made up of two p-n junctions, in the form of an n-p- n or p-n- p transistor. When a small positive voltage is placed between the base and the emitter, a large current is produced between the collector and the emitter.
- Development in methods of Ge purification, doping and the new understanding of electron flow through semiconductors allowed the creation of transistors.
- This allowed more reliable and efficient amplification of weak electrical signals. i.e. radio waves in long distance comms.
- Advances in knowledge about Ge crystal production allowed them to be used for more sensitive transistors capable of handling weaker higher f. signals.
- However, Ge lost its semi conducting properties at high temperatures and is rare and expensive so it became necessary to develop more reliable semiconductors.
Silicon – this led onto research which enhances our understanding of methods of silicon production, eventually allowing the creation of Si transistors which overcame shortcomings of Ge use.
- Si could also be made into thin oxide wafers allowing the miniaturisation of transistors and increasing the portability of communication technology.
184.108.40.206.3 Assess the impact of the invention of transistors on society with particular reference to their use in microchips and microprocessors
The invention of the transistor paved the way for the development of miniature electronic circuits - the most profound being integrated circuits (IC). Devices could now be made that required less space and less power and had far more reliability than existing devices. With the introduction of the transistor, electronic systems have continued to become smaller, more sophisticated and cheaper. It led to the so called computer revolution which has changed our lives greatly. Today, IC’s are used in a huge variety of microprocessor based equipment, ranging from mobile phones and calculators to ATM’s. The first microprocessors appeared in 1971 and have been reducing in size ever since.
Fast computers and tiny electronics have connected the world in an incredible amount of ways. For example, communication technologies have increased in efficiency and reliability, and are so advanced that through GPS, they are able to pinpoint any location on Earth to within a few metres. This has had a profound impact on missile and war technology, satellite technology and communication technology. It is now possible to speak and communicate with others all the way around the world, wirelessly and cheaply.
Computers no longer require a full cooling system and a storeroom full of computer circuits. They can now process huge volumes of information which has reduced repetitive manual labour. Transistors are particularly useful in memory chips. They control the flow of charges to a tiny capacitor. Other transistors act as amplifiers when information is retrieved.
In terms of environmental impact, there are positives and negatives. It has led to the development of solar cells in an attempt to battle scarce resources and reduce our dependence on fossil fuels. There is less pollution because transistors require less energy to run. Furthermore, there is no EMR produced from transistors. The main ingredient of semiconductors, silicon, is also an extremely abundant material.
On the other hand, while transistors are small in size, they have contributed to waste disposal problems. The process of producing transistors uses more toxic chemical and creates pollution, especially on non-degradable plastics which disturb the ecosystem when they are thrown away.
Essentially impacts of the transistor have occurred in communication, technology, daily lives, and of course the environment. These impacts have on the whole been positive to society as they have greatly improved living standards and encouraged rapid growth in many areas of science.
220.127.116.11.4 Summarise the effect of light on semiconductors in solar cells.
Solar cells convert the sun’s light energy into electrical (chemical) energy using p-n junctions
- For a solar cell to work, the n-type is exposed to light while the p-type is not
- At the junction, electrons flow from n → p, setting up a voltage gradient across the depletion zone, which eventually prevents the further flow of electrons.
- When a photon strikes an electron in the depletion zone, it is liberated and pushed towards the n-type layer by the electric field.
- This creates a voltage difference between n&p type layers.
- As a result, the excess e- in the n type flows into electrical contacts created by a fine metal grid and through an external circuit to reach the p-type layer to replenish the overall e- balance
18.104.22.168.1 Outline the methods used by the Braggs to determine crystal structure
- Sir William and Lawrence Bragg studied crystals using X-rays. They examined the patterns produced by the X-rays after the rays passed through the crystal and hit a photographic screen. The patterns were used to determine the internal structure of the crystals.
- X-rays were produced by allowing high energy cathode rays to strike a metal anode. These rays were directed at a crystal of a metal salt. (The first tried were sodium chloride, NaCl, and zinc sulfide, ZnS).
- The individual atoms in the crystal scattered the x rays
- A photographic plate was placed in the path of the X-rays exiting the crystal. The X-rays hitting the photographic plate produced a pattern of bright spots.
- This interference was analysed
- Calculation of the angles between the bright spots forming the pattern on the photographic plate allowed the Braggs to determine the internal structure of the crystal.
- The Braggs’ work was direct evidence for the periodic atomic structure of crystals postulated for several centuries.
- A mathematical expression, Bragg’s Law, developed for explaining these patterns of X-rays, allowed the future study of material structure using other types of electromagnetic (e-m) beams.
- Wavelength (lambda) is known
- N is the number of the bright spot (1 is the first bright spot)
- d is the distance between atoms
- Theta is the angle between the metal surface and the x ray beam.
- n= 2dsin
22.214.171.124.2 Identify that metals possess a crystal lattice structure.
Metals possess a crystal lattice structure. Metals are three dimensional lattices of ions in a sea of delocalised electrons (which come from the valence shell) which hold the ions in an array.
126.96.36.199.3 Describe conduction in metals as a free movement of electrons unimpeded by the lattice. (Include a comparison of conduction in superconducting materials above and below their critical temperature)
The sea of electrons in the lattice (made up of valence electrons) are loosely called a ‘sea’ due to the fact that they are loosely bound to the metal ion and so can drift between ions. As they can move, they can transfer charge and thus conduct electricity.
In a superconductor that is below critical temperature, the electrons pass almost unobstructed through the lattice (explained through the BCS theory for Type 1 superconductors). The lack of collision means that very little energy is lost. Above their critical temperature, the superconductors behave ‘normally’, that is like a metal or a compound or an alloy depending on what substance they are.
188.8.131.52.4 Describe the effect, on the resistance of a metal, of scattering of electrons by lattice vibrations and by the presence of impurities in the lattice. Include a description of the link between lattice vibrations and temperature.
Essentially, resistance measures to what degree a substance opposes the passage of an electric current.
As the delocalised electrons drift around the lattice, any impurities and imperfections in the lattice slows down their flow. This leads to an increased resistance.
An increase in temperature leads to the atoms that make up the lattice shaking. The atoms that make up the lattice structure of conductors start to vibrate more vigorously (over greater distances). As a result of this, mobile electrons in the conductor may collide more often with phonons (packets of energy). Hence an increase in electrical resistance.
184.108.40.206.5 Describe the occurrence in superconductors below their critical temperature of a population of electron pairs unaffected by electrical resistance
- At room temperatures, the metallic bonds (the lattice) holding the conductor together vibrates and interferes with electron movement through the conductor. Along with impurities and imperfections in the lattice itself, these three factors are responsible for resistance effects (energy loss and restricted current flow) in a conductor.
- At temperatures below the critical temperature, lattice effects impeding electron movement changes dramatically from impeding to assisting electron flow. That assistance comes about by an effect that pairs electrons and assists them to move freely through the conductor. The theory is called the BCS theory (see next point)
220.127.116.11.6 Discuss the BCS theory with reference to Cooper Pairs and Phonons - use diagrams.
The BCS theory (named after US physicists John Bardeen, Leon Cooper and John Schrieffer) explains superconductivity in terms of electron pairs and packets of sound waves related to lattice vibrations (called phonons). It only explains type I superconductors.
Formation of Cooper Pairs
At temperatures below the critical temperature for particular superconductors, the movement of electrons is enhanced by lattice vibrations (phonons - the lattice vibration equivalent of a photon (the quantum of light), i.e. the quantum of vibrational mechanical energy.) which cause electric field effects resulting in electron pairing (by overcoming what would normally be strong repulsive forces between like charges) and an assisted passage through the lattice with negligible energy loss.
Why they stay together
When an electron part of a cooper pair passes close to an ion in the lattice, the attraction between the negative electron and the positive ion causes a vibration to pass from ion to ion until the other electron of the pair absorbs the vibration. The net result is that the first electron has emitted a phonon and the second one has absorbed it. This is what keeps the cooper pair together.
Why is the negligible energy loss.
The reason for the negligible energy loss can be thought of a line of skaters who are linked. If one experiences a bump, they group won’t slow down as he is supported by the rest. Because resistance is effectively zero, very narrow wires can carry very large currents. The lower the temperature, below the critical temperature, the higher that current can be. That current produces a magnetic field around the conductor. The strength of the magnetic field will reach a point where it will cause the loss of the superconducting state thus putting an effective limit on the current that can flow in any particular superconductor.
18.104.22.168.7 Discuss advantages of using superconductors and identify limitations to their use
- Superconductors carry large currents with no energy loss and can generate very strong magnetic fields.
- Particle accelerators use this so they are cheaper to run and use less electricity
- Superconductors have a low heat buildup as they have a low resistance so not a lot of energy is wasted.
- Faster transmission of electrical signals
- Most known superconductors need to be very cold to pass their critical temperature. The cost of maintaining these temperatures is high.
- With higher temperature manmade superconductors:
- The materials, of which they are made, are often brittle, are hard to manufacture and they are difficult to make into wire.
- Some superconductors are chemically unstable in certain environments
- Some synthetic superconductors are expensive to manufacture
22.214.171.124.1 Process information to identify some of the materials that have been identified as exhibiting superconductivity
Material Critical Temperature Tc (K) Metal, alloy or compound
126.96.36.199.2 Perform an investigation to demonstrate magnetic levitation
188.8.131.52.3 Explain why a magnet is able to hover above a superconducting material that has reached the temperature at which it is superconducting (see Meissner Effect)
When a superconductor is placed in a weak external magnetic field, and cooled below its Critical Temperature, the magnetic field is ejected (the superconductor becomes a perfect dimagnet). This is because the induced currents generate a magnetic field inside the superconductor that just balances the field that would have otherwise penetrated the material.
184.108.40.206.4 Gather and process information to describe how superconductors and the effects of magnetic fields have been applied to develop a maglev train
- Propulsion and Levitation is achieved by the attractive and repulsive forces of magnetism.
- There are superconducting magnets on the carriage.
- These electromagnets are made from a superconductor and the temperature is kept under the critical temperature.
- The superconductor allows huge currents and hence huge magnetic fields to be achieved by these electromagnets on the train.
- The movement of these superconducting magnets as the train moves along the track induces a current in levitation coils in the track.
- These coils will then become electromagnets themselves, repelling the train upwards. This counteracts the downward force of gravity or weight force and thus the carriage will levitate.
- Other coils in the track are responsible for propelling the carriage along.
- Superconductors are essential to produce the huge magnetic fields required for levitating a train
220.127.116.11.5 Process information to discuss possible applications of superconductivity and the effects of those applications on computers, generators and motors and transmission of electricity through power grids.
Reducing the size of silicon chips is limited to heat build-up from the resistance in connections to the chip. Present connections in the chip are also much slower in conducting electrical signals than superconductors. Using superconductors will allow more densely packed chips and much higher processing which can result in smaller and faster computers.
Generators & Motors
Motors & Generators using superconductor magnets would not require an iron core. They would also become smaller & lighter. This could result in lower energy input from fossil fuels which results in them being cheaper to operate (less power used) for consumers & better for the environment (reduction in greenhouse gases if the energy was generated using fossil fuels.)
Transmission of electricity through power grids
Traditionally electricity is transmitted at high voltages and through thick copper wires to minimise energy ‘loss’ in the form of heat from resistance. By using superconductors to transmit power there would not be minimal resistance and hence lower voltages, larger currents and thinner wires could be used. This would reduce cost to the power companies, increasing their profit or creating savings for consumers. This could also reduce greenhouse gas emission as not as much power is needed to be generated in the first place.