HSC II.txt

  1. EDS Maglev Train
    Trains that employ the electrodynamic suspension (EDS) system, super-cooled superconducting magnets are placed on the train's undercarriage while electromagnetic coils are placed along the track (guideway). When the train gets close to the coils a current is induced allowing the train to levitate 1-10 cm above the guideway. Does not require as much computer monitoring while travelling as EMS (electromagnetic suspension system) - but requirement for very low temperatures, makes it not very practical.
  2. EMS Maglev Train
    • Conventional electromagnets mounted under train on structures wrapping around the guideway - provide lift and create a frictionless travelling surface - lifting force produced by electromagnets of like polarity in both train and guideway thus repulsion lifts train.
    • EMS is unstable - varying distances between magnets and guideway - needs to be monitored closely using computers to correct instability.
  3. What is a superconducting magnet and why is it used on Maglev trains?
    An electromagnet made from coils of superconducting material. In its superconducting state the wire can conduct much larger currents and hence intense magnetic fields. They are cheaper to operate as no energy is dissipated as heat. Superconducting magnets also conduct electricity even after the power supply is switched off, hence energy can be stored if needed.
  4. Describe how the Maglev train is propelled?
    Once the train is levitated, power is supplied to the propulsion coils within the guideway walls to create a unique system of magnetic fields that constantly change polarity so that the train is continually attracted fromt he front but repelled from behind. Using this technique, speeds of over 500 km/h have been achieved!!!
  5. Use of photoelectric effect in solar cell
    • Solar cell converts sun's light energy into electrical energy.
    • Semi-conductor diode - p-type layer in contact with n-type layer.
    • At p-n junction, electrons from n-type flow into holes of p-type, creating an electron deficiency in n-type and surplus electron in p-type so that n-type is now positively charged, p-type negatively-charged.
    • Hence depletion zone is created between the two layers, and an electric field is set up preventing further flow of electrons.
    • Photon strikes electron-filled hole in the depletion zone.
    • Electron is liberated.
    • Electron is attracted towards the n-type due to the positive electric field.
    • This creates a current from which we can derive energy and do work by lighting a light bulb for example.
    • Electrons around the outercircuit from n-type to p-type.
    • ffdffdImage Upload 1
    • n-type layer is exposed to light and p-type layer is not.The photoelectric effect is observed as photons transfer energy equivalent to E = hf to the electron, providing the energy to be liberated and provide a current.
  6. Use of photoelectric effect in photocell
    • most common type of photocell - PHOTORESISTOR
    • produce a current due to the photoelectric effect
    • electrons escape from cathode and travel to the anode
    • made out of a semiconductor of high resistance - either intrinsic (silicon - needs quite a bit of light to conduct) or extrinsic with the help of a dopant (don't need as much light - dopant lowers amount of energy for it to conduct)
    • magnitude of current proportional to the intensity of light
    • resistance changes depending on how much light is falling onto it.
    • when it absorbs enough photons that the electrons can then conduct, resistance is significantly reduced.
    • USES: displays in clocks may incorporate a photoresistor so that the face is always readable i.e. it lights up in the dark. Street lights also use photoresistors so that they automatically turn on when light becomes low
  7. What is a cathode ray tube (CRT)?
    • Vacuum tube - evacuated gas tube
    • 2 Electrodes (cathode and anode) at either ends - conductive metal having contacts outside tube
    • High voltage passed through tube when electrodes connected to a circuit, electrons jump from cathode to anode - crossing the tube.
    • Does not occur in noromal air - high density air molecules prevents electrons going far distances
  8. How is light emitted in a CRT? What is the shape and colour of the light pattern dependent on in a CRT?
    • Electrons collide with particlces inside the tube
    • Particles absorb and then release the energy of the electron during their collision - causing the glass to fluoresce
    • Appearance of light i.e. shape and colour dependant on chemical composition of the gas inside the tube and the pressure of the gas.
  9. How do cathode ray tubes allow the manipulation of a stream of charged particles?
    • Stream of charged particles = cathode rays = negatively charged electrons
    • CRTs are a source of a steady stream of charged particles.
    • DIRECT MANIPULATION: Objects - small thin metals (Maltese cross, paddlewheel) can be placed insidethe tube
    • REMOTE MANIPULATION: Fields (electric & magnetic) created by charged plates or field coils could permeate the glass tube - Remote manipulation
  10. Cathode Rays: Charged Particles or Electromagnetic Waves??
    • As a WAVE they: travelled in a straight line to produce shadows when obstructed by objects and they could pass through thin metal foils without damaging them.
    • As a PARTICLE they: did not radiate out like a wave, left the surface of the cathode at 90 degrees; were deflected by magnetic fields; could turn a paddlewheel that lay in the path of the ray indicating they had transferred momentum; travelled far slower than light

    CROOKES insisted it was a particle while HERTZ maintained it was a wave.

    Debate resolved when THOMPSON used an electric field to deflect the rays. Before that the older vacuum pumps not strong enough to remove enough air to make the deflection visible; also electric fields used before weren't strong enough.

    Thompson provided strong evidence - impossible to deflect electromagnetic waves with an electric field.

    Hence, cathode rays were particle streams.
  11. PRAC: identify properties of cathode rays using Maltese cross, electric plates, fluorescent screen, glass wheel
    • Maltese cross tube - anode mounted on base of tubeImage Upload 2
    • shadow was formed - indicated cathode rays travelled in straight lines.
    • cathode rays blocked quite easily.
    • shadow had very sharp edge - diffraction not occuring
    • could be particles, not waves.
    • Electric plates - cathode rays deflected
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    • bending of the cathode ray beam indicated beam was electrically charged - deflected towards positive plate - cathode rays were negatively charged. (beam also deflected with a magnetic field using bar magnet)
    • Fluorescent screen - lit up as it was struck by the cathode rays - carried enough energy to produce the reaction in the screen necessary to produce light - property used in TVs and computer monitors.
    • Glass paddlewheel - mounted inside tube on runners so it was able to move. cathode rays struck wheel caused it to rotate and roll along the tube. movement was away from cathode, indicating rays emitted from cathode. Conservation of momentum - cathode rays moved the wheel by colliding with it indicated they had mass and therefore particles.Image Upload 4
  12. PRAC: Observing different striation patterns for different pressures of gas in discharge tubes
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    • Striation patterns - light and dark areas inside discharge tube
    • Whether light is released is dependent on whether the energy of the electrons is sufficient and also the pressure of the gas inside the tube.
    • In our experiment, the pressure of the gas changed the striation patterns. (actual nature of striation pattern dependent on gas used e.g. normal air, hydrogen etc)
    • 4 discharge tubes and each with different air pressures (5%, 2%, 0.5% and 0.1%) - percentage of standard atmospheric pressure.

    • 5% air - purple/pink streamers passing through from cathode to anode continuously
    • 2% air - alternating light and dark bands PERPENDICULAR to the length of the tube.
    • 0.5% air - dark gaps widened between bands - fewer lines - purple/pink glow concentrated around anode
    • 0.01% - NO STRIATIONS - glass around anode glowed yellow/green

    The dark bands appearing as gas pressure decreases due to electrons having insufficient energy to excite the gas molecules in the tube - loss energy from previous collisions and had to accelerate and speed up again.

  13. Moving charged particlces in a magnetic field experience....
    A FORCE (e.g. electrons in a wire, the Van Allen belts)

    NB A stationary charged particle does not experience a force.
  14. Explaining patterns found in discharge tubes. (Extra content)
    • Before trying to explain the patterns formed in the discharge tubes we will look at what happens when an atom absorbs energy. The electrons in the atom originally orbit in the ground state, i.e. the state of minimum energy. They can absorb energy and lose an electron. Chemists will recognise this as ionisation energy. They can also absorb a lesser, but fixed, amount of energy that will make the electron orbit at a higher energy level. The electron at this level is unstable and will drop back to its ground state, emitting a photon of light as it does so. The photon has a definite frequency as governed by Planck’s equation; E = hf and we see it as light of a specific colour.
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    • In the following explanations we will refer to cathode ray electrons and atom electrons. They are exactly the same sort of electrons, identical in every respect, but the distinction will be made to describe their origin.

    • A cathode ray electron can hit a gaseous atom. The atom’s electron will gain energy and proceed
    • to a higher energy level where it will be unstable. It will then revert to its original energy level and give off a quantum of energy of a specific wavelength as it does so. The colour of the light emitted is characteristic of the gas in the tube.

    • Why is the negative glow blue?
    • When atoms are ionised some of the positive ions recombine with electrons. Two attracting bodies have potential energy and this is converted into other forms of energy as the bodies come together. As the ion and the electron come together a lot of energy is released so it is released as light of a short (high energy) wavelength i.e. blue. So the light of the negative glow is due to the energy released when positive ions and electrons combine and emit blue light in the process.

    The potential drop across the discharge tube is uneven with most of it occuring close to the cathode. Electrons are accelerated rapidly and attain a high velocity as they move through Crookes’ dark space. The average distance that the cathode rays travel before colliding with atoms depends on the amount of matter and so the pressure in the tube. Crookes’ dark space becomes longer as the concentration of matter decreases when the pressure is reduced.

    • As the cathode rays move through Crookes’ dark space they gain sufficient energy to ionise the
    • gas in the tube and produce the blue negative glow. They are slowed down by collisions in this region and don’t have much energy as they enter Faraday’s dark space.

    They accelerate through the Faraday's dark space until they have sufficient energy to excite atoms rather than ionise them. The atom’s electrons attain a higher energy level and then return to the unexcited state, emitting a photon of light as they do so. This is seen as a specific colour that is characteristic of the gas in the tube. The cathode rays lose energy in their collision with an atom and are accelerated until they again have sufficient energy to excite an atom. Thus the positive column can consist of a number of bright regions where the cathode rays have excited the atoms, separated by a number of dark regions. In this way the positive column can appear striped or striated.
  15. Charged plates
    • plates with a potential difference between them
    • electric field runs between the plates
    • field lines run from positive plate to negative plate, are parallel, and the field strength is equal at all points between the plates
    • field does not exist outside space betweent he plates.
    • direction of the field is the direction that a positive charge would move if placed in the field.
    • F = qEImage Upload 7
  16. Field strength between parallel plates
    • depends on only two things - potential difference between the plates and the distance between them
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  17. Thompson's experiment to measure charge/mass ratio of an electron
    • Modified cathode ray tube
    • thermionic cathode - heated by a separate heating circuit to release more electrons.
    • anode - small hole through centre to produce a thin stream of electrons - travelling into space rather than between a potential difference
    • charged plates - above and below the beam - deflect the electrons
    • Hermholtz coil - mounted to produce a magnetic field to deflect the electrons.
    • fluorescent screen - at the end of the tube - indicating how electrons were deflected, if at all.
    • With only magnetic field acting, a measurement of the resultant deflection from the straight line path of electrons to teh fluorescent screen was made, allowing the radius of the beam in the magnetic field to be determined.
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    • Thomson then applied a variable voltage on the metal plates to produce an electric field. The voltage was adjusted until the electric field produced a force on the electrons, equal and opposite to the force created by the magnetic field. This resulted in no deflection of the beam of electrons (as for no fields)
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  18. Electrodes in the electron gun, deflection plates or coils, and the fluorescent screen in the CRT of a TV and oscilloscope
    Cathode ray tube has the:

    ELECTRON GUN producing narrow beam of electrons. 2 filaments: the cathode and two open-cylinder anodes. Cathode releases electrons by thermionic emission, while anode accelerates and focuses the electron beam.

    DEFLECTION SYSTEM - 2 sets of parallel plates connected to a potential difference - produces electric field between plates. The 'Y' plates - vertical deflection; 'X' plates - horizontal deflection. Deflection is necessary in order to form an image on the fluorescent screen as the electron beam must sweep over the screen rapidly. A TV uses magnetic fields for stronger deflection due to the large screen.

    FLUORESCENT SCREEN - coated with a material e.g. ZnS. that fluoresces (emits light) when struck by electrons, making the electron beam visible.

    • Differences between TV & CRO:
    • In a TV deflection by magnetic field; CRO electric fields
    • TV has a grid between cathode and anode, allowing the intensity of electrons hitting the screen to be controlled. CRO has none.
    • The horizontal sweep (time base) of a TV is fixed, while the time base of the CRO can be controlled.
    • Colour TV - 3 independent electron guns for red, green, blue while CRO has just 1 electron gun.
  19. What must happen in order for the photoelectric effect to occur (keywords: photon, threshold frequency, work function, photoelectron, intensity)
    • The FREQUENCY of the individual photon must be greater than the threshold frequency of the metal (i.e. photon required to have sufficient energy E = hf) to allow the photon to provide enough to overcome the work function binding the electron to an atom in the material, given byImage Upload 12
    • where fo = threshold frequency.
    • There is one photoelectron produced per photon absorbed. Unless the photon energy is high enough it will make no difference how intense the beam of light is; the metal will still not release any electron to register a current. In order for photoelectrons to be released, the energy of the photon striking the metal surface must be greater than the work function of the metal.
  20. What is the velocity of the photoelectrons ejected dependent on in the photoelectric effect? What is the quantity of electrons ejected dependent on in the photoelectric effect?
    • Velocity of photoelectron- frequency of incident radiation
    • Quantity of photoelectrons - intensity of incident light
  21. Hertz's observation of the effect of a radio wave on a receiver and the photoelectric effect he produced but failed to investigate. (know procedure of Hertz's experiment)
    • 1. Production of EMR: Current was fed into the primary loop from the induction coil and oscillated back and forth. This oscillation of charges (accelerating electrons) in primary loop generated EMR (radio waves in this case), which was emitted from the gap. There was also a spark generated between the electrodes due to charges being conducted through the air.
    • 2. Transmission of the EMR: The EMR (radio wave) travelled to the receiving coil
    • 3. Reception of the EMR: The EMR (radio wave) reached the receiving coil - causing electrons in the receiving coil to oscillate - regenerating and inducing current in the receiver although much fainter than the primary loop's due to energy lost during transmission. oscillation of charges causes another spark across air gap

    conclusion: receiving loop was a detector of electromagnetic waves generated by the transmitter as it received the radio waves, and converted them into sparks in the receiver's gap.

    • Hertz also discovered the photoelectric effect. Illuminating the spark gap in the receiving loop with UV radiation from a mercury vapour lamp stronger sparks produced in the receiving loop (the UV light knocked electrons from the surface of the loop making it easier to jump the spark gap - Hertz did not realise this). When glass was used as a shield between the 2 loops, it blocked the UV and intensity reduced in the detector loop's spark. However quartz as a shield - no effect on intensity - allowed UV to fall on detector from the transmitter. Aso when conducted in a dark box to see spark more clearly, spark greatly diminished in intensity. HERTZ OBSERVED the photoelectric effect but FAILED TO INVESTIGATE any further.
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  22. Hertz's experiment in measuring the speed of radio waves and how they relate to light waves
    • Hertz connected two loops together with a wire - interference between the AC wave in the wire and wave caused by EMR transmission. He used standing waves set up with a carefully placed polished reflector to determine the wavelength of the radio waves. He already knew the frequency from the oscillations of his electric current and thus using the universal wave equation
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    • He calculated the speed of the radio wave to be the same as light.

    In relation to light waves: Hertz showed that they were reflected off a zinc plate, refracted by pitched or asphalt blocks, diffracted around obstructions, polarised (when rotating the receiving coil, sparks were stronger at certain angles compared to others)

    Hertz concluded and verified Maxwell's earlier prediction - an entire spectrum of EMR exists which all travel at the same speed, the speed of light.
  23. What is a black body? How does it emit energy?
    • A black body is a theoretically ideal object that absorbs and radiates energy at all electromagnetic wavelengths.
    • When a black body is HEATED to some temperature in a vacuum, it beings to emit radiation perfectly - black body radiation covering entire range of EMR spectrum, with intensity varying with wavelength.
  24. Planck's hypothesis - radiation emitted and absorbed by the walls of a black body cavitiyi is quantised.
    • Classical physics predicted a black body emits radiation infinitely as wavelength decreases. This could not account for the experimental radiation curves obtained, demonstrating an intensity peak at a certain wavelength depending on the black body's temperature. Also an exponential curve would violate CONSERVATION OF ENERGY since total energy (area under graph) would be infinite. This prediction (Rayleigh-Jeans Law) of radiation being emitted with infinite intensity, made by classical theory was known as the ULTRAVIOLET CATASTROPHEImage Upload 15
    • Planck hypothesised quantised radiation replacing the continuous radiation waves initially predicted - explained the experimental data - radiation only occured in small packets of energy called 'quanta'. Each quanta is produced by an ATOMIC OSSCILATION of an atom in the black body wall. Energy of these quanta or photons are related to their frequencies: E = hf.

    Why did Planck put forward this idea? Simply to mathematically derive the black body radiation curve.

    Developed a totally NEW BRANCH OF PHYSICS - quantum theory.
  25. Einstein's contribution to quantum theory & its relation to black body radiation
    Used Planck's formula (E=hf) to create a more detailed quantum theory of light with light packets, called photons, and also created an explanation for the PHOTOELECTRIC EFFECT.

    Stated photons were smallest units of light possible with energy E = hf. He also defined light by relating intensity with quantity and frequency with energy. Light intensity depends on number of photons i.e. more photons, greater light intensity. Photon with a lot of energy meant they had a high frequency.

    • Einstein then explained photoelectric effect in terms of WORK FUNCTION and THRESHOLD FREQUENCY - also providing an explanation for photoelectron kinetic energy as Image Upload 16
    • - kinetic energy of emitted photoelectron is equal to the light photon's energy minus the energy required to remove an electron from an atom in a metal i.e. the work function.
    • - i.e. any extra energy that a photon has over the work function PROVIDES THE KINETIC ENERGY OF THE PHOTOELECTRON EMITTED
    • NOTE: if hf < φ electrons cannot escape, no photo electric effect, intensity is useless
    • hf = φ electrons can escape, photoelectric effect happens. This hf or E value corresponds to the threshold frequency, fo, no extra energy.
    • hf > φ electron escapes with energy left over which is given to the photoelectron.
    • Image Upload 17
    • Graph of energy of photoelectrons being emitted plus threshold frequency of five different metals.
    • N.B. Gradient of all lines equal to Planck's constant.
    • IN RELATION TO BLACK BODY RRADIATION - Einstein's theories stemmed from Planck's work on black bodies.

    Einstein brought quantum theory further into the mainstream where other scientists took interest in building onto it.
  26. ASSESSING Einstein's contribution to quantum theory
    • significant contribution
    • took Planck's theories on black body radiation and applied it to a separate situation to come up with quantisation of the energy of light
    • expanded on the work of Planck and turned quantum theory into a set of ideas with concrete principles and modelling
    • Einstein took it seriously, Planck believed he had come up with a mathematical trick.
    • Einstein validated quantum theory in using it to explain photoelectric effect with its experimental observations
    • opened the door for further research based on quantum ideas
  27. PRAC: production and reception of radio waves
    • induction coil DC power supply
    • strong spark between electrodes
    • electromagnetic radiation - radio waves produced due to rapidly discharing spark
    • AM radio low frequency region used as the receiver to detect radio waves from the induction coil
    • loud static and cracking sounds produced when induction coil was turned on and most pronounced at lowest frequencies
  28. Einstein and Planck's differing views about whether science research is removed from social and political forces
    • Planck played a major role as a basis for Einstein's ideas but their views on the role of science and scientific research were very different.
    • Einstein PACIFIST - 1 of 4 scientists sign petition for world peace at start WW1.
    • Planck PATRIOT - 1 of nearly 100 distinguished scientists signed manifesto produced by German government condoning invasion of Belgium

    Einstein viewed SCIENTIFIC RESEARCH SHOULD BE REMOVED FROM SOCIAL AND POLITICAL FORCES - devoted to pursuit of knowledge and understanding.

    Planck argued the purpose of science was to support social and political agenda - saw rapid progress of German's scientific research as benefit to all Germans and as a clear indication of their superior abilities.

    Relationship between Planck and Einstein put under increasing tension as Hitler and Nazi party won control. Einstein's work on relativity opposed by scientists involved with Nazi party. As WW2 began, Planck strongly supported Nazi and worked for the war effort and ask Einstein to resign from the Prussian acadmy because of his views on the Nazi gov and opposition to activities leading up to the war. As war progressed, Planck stayed in Germany, with his research directed to the war effort - faithful to Nazi doctrine. Despite their respect for each other, Einstein could not forgive Planck for his contributions to the Nazi war effort. Planck did realise that scientific research should be removed from social and political forces but this was only an ideal situation.

    War and Nazi atrocities forced Einstein to act politically, encouraging Roosevelt in getting the funds for the scientific research of the development of a nuclear weapon or atomic bomb before the Nazi regime. Hence by this time he realised science was inevitably linked political issues.
  29. Intrinsic semiconductors
    pure semiconductor crystals consisting of one element. Band gaps smaller than insulator but bigger than conductors - initially insulators but when heated (or other energy inputs) moderately become conductive
  30. Extrinsic semiconductors
    Semiconductor crystals with deliberate impurities added - a dopant - group 3 or group 5 element. Contain an extra energy level inside the forbidden energy gap for electrons to exist, reducing the energy required to get an electron into the conduction band.
  31. Transistors: Germanium vs silicon
    • Germanium first group 4 element that could sufficiently be purifised as a semiconductor in early transistors.
    • used because of purification - for predictable properties, semiconductor crystal need be very pure. However germanium became too conductive with just moderate heating so can burn out i.e. germanium chip performance dependent on temperature.
    • Silicon is the superior material, MORE ABUNDANT and therefore CHEAPER, easier to dope, and superior thermal properties however in the 1940s techniques used to purify germanium crystals did not work on silicon crystals. Hence silicon crystals could not be manufactured pure enough to make reliable chips.
    • Germanium used in early transistors until suitable purity silicon was developed.
  32. Braggs experiment to determine crystal structure
    • Studied diffraction patterns of x-rays with wavelength similar to separation of the planes of the crystal lattice to investigate its structure.
    • directed a beam of X-rays at metal salts.
    • examined the ways the X-rays reflected off various layers of the atom, interfere with each other, produce an interference pattern
    • analysed the distances between the maxima in the interference pattern of x-rays
    • They came up with Braggs' Law - diffraction angle was related to the distsance between planes of the lattice and wavelength of the x-rays. This allowed them to determine lattice structure, where atoms formed an ordered lattice surrounded by a sea of delocalized electrons.
    • Image Upload 18

    allowed for an understanding of conduction through metallic resistors to be developed, led to production of doped silicon crystals and microprocessors improtant for computers. Also significant in determining structure of biological substances e.g. DNA.
  33. Identify that some electrons in solids are shared between atoms and move freely
    • IONIC & COVALENT substances - electrons bound in strong bonds within the structure and thus influenced little by electric fields.
    • METALS - valence electrons loosely bound and are mobile forming a sea of localised electrons around the positive ions - easily influenced by electric fields i.e. conduct.
  34. What is the ratio of the impurities to the atoms in a doped semiconductor?
    1/ 200,000 atoms!
  35. Differences between solid state and thermionic devices. Why did solid state devices replace thermionic devices?
    VALVE - a thermonic device in which two or more electrodes are enclosed in a glass vacuum. A filament, heated by an electric current, liberates electrons by thermionic emission i.e. electrons are 'boiled off' the filament to be available to create a current. The filament where electrons are released is the cathode. These electrons are accelerated by a high potential difference towards the anode.

    A SOLID STATE DEVICE - utilises the junction between n-type and p-type semiconductors - doping of silicon to produce electrons and holes to move through lattice and carry current.

    A TRANSISTOR - is a solid-state device - consists of a thin film of n-semiconductor sandwiched between two thin layers of p-semiconductor (or vice versa), forming a triode. Electrons pass from the top (emitter) layer through the central (base) to the bottom (collector). A voltage is applied to the base, strongly influences the current flowing from the emitter to the collector, thus amplifyingn the current.

    • REASONS why solid state devices largely replaced thermionic devices in modeorn appliances (disadvantages of thermionic devices):
    • --- THERMIONIC bulky & heavy though portable; required large power source
    • --- THERMIONIC inefficient - waste heat energy produced as energy wasted simply to heat the filament in order for electrons to be boiled off
    • --- THERMIONIC expensive to run - constant need to replace them - large voltages and time to heat up
    • --- THERMIONIC fragile - vacuum glass tube.

    • ADVANTAGES of SOLID-STATE devices:
    • --- Smaller, leading to integrated circuits (single sized chip can hold THOUSANDS of transistor circuits - whereas thermonic devices required individual vacuum tubes), and miniaturisation of electronic devices and equipment.
    • --- Required far less energy to operate, ENERGY EFFICIENT due to lower voltages
    • --- Time taken for signals to travel through circuit were decreasing and thus great advances in computing speeds.
    • --- cheaper to produce - mass production
    • --- highly portable - do not require high voltages, were light and more durable
    • --- more reliable; no fragile parts; i.e. longer lasting
  36. How does p-type semiconductors conduct electricity?
    • Addition of a Group 3 element e.g. boron - lattice structure will be missing an electron in the valence band and so a positive hole is created.
    • When a p-type semiconductor experiences an electric field, an electron falls into the hole, protomting the hole into the conduction band. The positive holes can then drift through the lattice of the semiconductor as conventional current.
  37. What is the function of a thermionic device and solid-state transistor?
    • Amplification of a signal e.g. in hearing aids
    • Electrical switching in computing devices.
  38. Invention of the transistor in replacing thermionic devices.
    Radio & electronics - need for increase in voltages of signals to drive loudspeakers and other electrical devices. Vacuum tubes achieved this but had many downsides including losing its vacuum slowly.

    Demand for new radar and radio technology during WW2 including amplifiers - sparked renewed interest in SS devices - smaller, more energy efficient, reliable alternative to builky thermionic components. Thermionic shortcomings lead to development of superior technology to overcome problems.

    In 1947 Bardeen and Brattain of Bell Laboratories observed when gold electrical contacts, fractions of millimetres apart, were applied to a crystal of germanium - OUPUT VOLTAGE greater than INPUT. Shockley became interested in this observation and in a few months developed a transistor. It consists of a thin film of one type of semiconductor sandwiched between 2 thin layers of the other type, forming an npn or pnp triode. Small changes in the input voltage into the base or middle semiconductor, resulted in great variations in a current passing through emitter and collector, the semiconductor at the ends.

    • Image Upload 19
    • At the time, methods for producing pure germanium crystals were used for early transistor production. Germanium however was not very abundant yet other superior elements such as silicon could not be sufficiently purified at the time. Silicon is more abundant, retains its semiconducting properties at high temperatures, and forms an electrically insulating oxide film. Development of better methods in purification led to the development of INTEGRATED CIRCUITS.

    The transistor is SMALLER, CHEAPER, more ENERGY EFFICIENT, and more DURABLE than thermionic components. It quickly replaced the vacuum tube in many electronic devices.
  39. Impact of the invention of transistors - use in microchips and microprocessors
    • Immense, significant impact
    • Improvements to quality of life
    • Earliest solid-state transistors consisted of discrete components such as the diode. Each component separate item; small, used less power, more durable than thermionic devices; cheaper to construct; energy efficient; energy not lost as heat.

    After invention of integrated circuits, miniaturization of electronic circuits came about; thousands of transistors and electronic circuits built on a tiny single chip.

    Amplifiers could be built on single chip; connected to a circuit board i.e. less connections and wires - less heat produced than thermionic; signals travelled much faster i.e. faster transfer, storage and processing of info.

    Invention of transistors led to development of microprocessors - capable of controlling very complex processes - wide range of appliances; resulted in development in medical diagnosis and treatment, entertainment, industrial design and communications. Also increased recreational time for society; improvements in education and communications technology.

    HOWEVER!! socio-economic division - widening gap between rich and poor - those who can afford technological upgrades and those who can't; less face-to-face contact due to developments in automated customer service (unemployment) and digital communications.

    OVERALL transistors have GREATLY benefited society; improvements in technology helping medical industry, entertainment and communications fields. They have become an integral part of modern life.
  40. BCS theory (keywords: phonon, cooper pairs, lattice distortion, sound energy)
    • When in the superconducting state, a superconducting material lattice vibrations drop down to negligible levels.
    • When an electron travels through the lattice, it induces vibrations or phonons as the positive ions are attracts to the passing electron. The phonons, packets of sound energy which are exchanged, enabling the two electrons to overcome their electrostatic repulsion, in binding together and forming a Cooper pair, flowing unimpeded through the lattice as they interact less with the lattice - very little resistance.
  41. Types of superconductors.
    • Type 1: Metal and metal alloys (critical temp below 30K) e.g. aluminium, mercury; more malleable (beaten into thin sheets) or ductile (extruded into thin wires); tough; easily produced. However have very low Tc and hence hard to reach these low temps and maintain them; usually required liquid helium which is much more expensive than common coolants such as liquid nitrogen (BP -196 C, 77K - not low enough to these metal/metal alloy superconductors.
    • Type 2: Oxides and ceramics new ones constantly being developed. E.g. of Type 2 - YBa2Cu3O7 having critical temp of 90 K. Can use liquid nitrogen to reach Tc as well to maintain it. Disadvantages: do not have some adv. of Type 1 i.e. brittle, fragile, generally less workable, less ductile, chemically less stable tend to decompose in extreme conditions; difficult to produce that's why they were discovered later.
  42. Why does a magnet hover above a superconducting material which is below critical temperature?
    • Superconducting materials below critical temperature are diamagnetic - do not allow magnetic fields to pass through them.
    • If a magnet is placed above the superconductor, induced currents flow in the superconductor to produce a magnetic field that cancels the applied magnetic field inside the superconductor - the superconductor excludes the magnetic field.
    • The upward magnetic force created by the superconductor equals the weight force of the magnet, causing the magnet to 'levitate' above the superconductor. This is called the Meissner Effect.
  43. PRAC: Demonstrating magnetic levitation
    • Ceramic superconducting disk in a Petri dish and small magnetic cube
    • Teacher poured liquid nitrogen onto superconducting disk and into the dish, so that temperature is below critical temp, becoming superconductive.
    • Used insulated plastic tongs to place magnet just above disk, magnet floated
    • nudging the magnet with tongs caused it to rotate
    • Eventually, magnet fell as disk warmed up, losing its superconductivity
    • SECOND TRIAL left magnet on disk then poured liquid nitrogen. As disk cooled, magnet floated upwards off the disk - showed Meissner effect is due to exclusion of magnetic fields from superconductors.
    • Magnet rose upwards by itself - levitation of magnet due to exclusion of the magnetic field not due to eddy currents because if the magnet moved up, induced eddy currents would drag the magnet down.
  44. Application of superconductivity - computers, motors and generators, electricity transmission through power grids
    • Computers
    • using a superconductor - little, if any, waste heat produced, resulting in a processor (CPU) that can operate at much faster speeds. In 1962, Brian Josephson invented a superconductor that functions that same as a transistor switch but a lot faster! He called it the 'Josephson Junction' - CPUs incorporating these would be so much faster than current CPUs. You could also replace transistors with superconducting quantum switches (SQUID - superconducting quantum interference device), also allowing processors to operate faster.

    • If superconductors were used to make electromagnets, to provide necessary magnetic fields, less energy used to create and maintain magnetic fields, also giving out very strong magnetic fields with lighter electromagnets. Hence, efficient generators.
    • Also transmission of electricitiy using superconducting wires would result efficient transmission with eliminated energy loss through heat (Ploss = I2R and 'r' is negligible). This would reduce cost of power. A superconducting electricitiy grid was successfully triald in America 4 years ago - currently used in certain areas of the New York grid.

    • A long superconducting loop functions as a storage device. Current introduced into loop continues to flow around the loop indefinitely, as there is negligible resistance. Hence energy can be retrieved when required. A Power storage device using superconductors allow power stations to run continuously at peak efficiency, regardless of fluctations in demand.

    2 major obstacles impede the use of superconductors in nearly all applications - EXTREME COLD TEMPERATURES inconvenient and in computers, difficult and unwiedly. also TYPE-2 SUPERCONDUCTORS only realistic option for real-world applications as they only require liquid nitrogen cooling, and are ceramic compounds so are not ductile or malleable.
  45. Safety precaution when using a discharge tube and induction coil.
    • high velocity electrons produced represent ionising radiation and their collisions, especially if directly to the metal anode, result in X-rays which are potentially hazardous.
    • Safe working distance of 1-2m needs to be maintained when operating the coil.
Card Set
HSC II.txt
Ideas to Implementation topic