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From Ideas to Implementation 1. Increased understandings of cathode rays led to the development of television Students learn to: explain why the apparent inconsistent behaviour of cathode rays caused debate as to whether they were charged particles or electromagnetic waves Apparent observations fitted particle-wave theory; Wave – travel in straight lines (bent cathode ray tube) - create shadows (Maltese cross) - can pass through thin metal foils without inducing damage (Hertz) Particle – could be deflected by magnetic field and electric fields (J.J.Thompson) - had momentum (paddle wheel) - they were slower than the speed of light explain that cathode ray tubes allowed the manipulation of a stream of charged particles A cathode ray tube is an evacuated vacuum tube, with cathode rays (electrons – a stream of charged particles) being emitted from the cathode and traveling to the anode. These cathode rays could be manipulated by; - adding electrodes inside the CRT - using external magnets and electric fields - placing solid objects in path of traveling cathode rays identify that moving charged particles in a magnetic field experience a force Moving charged particles produce a magnetic field of their own, this field interacts with other magnetic fields causing the particles to experience a force. F = Bqvsin identify that charged plates produce an electric field Two parallel charged plates when separated by a distance d create a uniform electric field between the plates. Follows: E = V/d i.e. voltage supplied (volts) / distance of separation (metres) Answer is in Vm-1 OR E = F/q Which is the force experienced by a charged particle in an electric field, thus F = Eq describe quantitatively the force acting on a charge moving through a magnetic field F qvB sin The force is dependent on the charge on the particle, the particles speed, the magnetic field strength and the sin of the angle at which it is traveling relative to the magnetic field it is in. Physics HSC Course : Ideas to Implementation : page 1 discuss qualitatively the electric field strength due to a point charge, positive and negative charges and oppositely charged parallel plates Point charge – electric field strength at any point is = to force that a (+) 1 coulomb charge would be subjected to in that position. *Test charges are always positive. Positive and negative charges – strength is inversely proportional to distance, direction points radially away/towards particle (dependant whether neg or pos, positive lines point away from charge. Oppositely charged plates – proportional to voltage supplied to plates and inversely proportional to the distance between the plates. Direction is at right angles to direction of plates, and points away from positive plate. describe quantitatively the electric field due to oppositely charged parallel plates Particle E = F/q Plates E = V/d outline Thomson’s experiment to measure the charge/mass ratio of an electron Question; were cathode rays waves or particles? He showed that the rays were deflected towards positive plate when subjected to electric field by placing fluorescent screen at end of tube, thus the rays were negatively charged (a particle property). He measured the curvature of the rays movement when subjected to a magnetic field, and found radius of their motion. i.e. Bqv = mv2/r By equating a magnetic field with the electric field to make the rays flow through the tube undeflected he could equate the two known formulae; F = Eq and F = Bqv thus, E = Bv and could find velocity of the rays, which was slower than light (particle property). From this he could then apply; Bqv = mv2/r (sub. v = E/B) giving, q/m = E/B2r and thus could find the charge to mass ratio, proving that cathode rays were a negative particle smaller then the smallest known piece of matter at the time (atom). outline the role of: – electrodes in the electron gun – the deflection plates or coils – the fluorescent screen in the cathode ray tube of conventional TV displays and oscilloscopes - The electron gun produces a narrow beam of electrons. The electrodes in the gun accelerate the electrons. - The deflection plates or coils establish an electric field that controls the deflection of the electron beam from side to side and up and down. - The fluorescent screen is coated with a material that emits light when struck by electrons in the cathode ray. This allows the position of the beam to be seen where it strikes the screen. Physics HSC Course : Ideas to Implementation : page 2 Students: o perform an investigation and gather first-hand information to observe the occurrence of different striation patterns for different pressures in discharge tubes Using discharge tubes A common piece of apparatus used for this investigation is a set of glass discharge tubes at different pressures, arranged side-by-side on a board. The tubes have been sealed after having had varying amounts of air pumped out of them (the more air pumped out, the lower the air pressure). Each tube contains an electrode at each end to allow the application of a large voltage, which is provided by an induction coil. The high voltage causes an electrical discharge through the air in the tube, causing the air to glow. Different discharge patterns are formed at different pressures. Sample observations At 5% of atmospheric pressure, long, thin red-purple streamers appear between the two electrodes. At lower pressure, these streamers give way to a soft red glow. Upon further pressure reduction, the glow is broken into striations, bands of light and dark. The amount of dark space between the glowing bands increases with further reductions. At 0.01% of atmospheric pressure, the dark space extends throughout the tube. At this very low pressure, the glass near the anode glows a yellow-green colour. o o perform an investigation to demonstrate and identify properties of cathode rays using discharge tubes: – containing a maltese cross – containing electric plates – with a fluorescent display screen – containing a glass wheel analyse the information gathered to determine the sign of the charge on cathode rays Maltese cross – cathode rays travel in straight lines as they create a shadow at the end of the tube. Electric plates – cathode rays are attracted and move towards the positive plate, thus have a negative charge. Fluorescent screen – the cathode rays produce fluorescence. Glass paddle wheel – the cathode rays move the paddle wheel and thus have momentum, i.e. mass and velocity. Thus cathode rays were negatively charged particles. solve problem and analyse information using: F qvB sin F qE and V E d Physics HSC Course : Ideas to Implementation : page 3 2. The reconceptualisation of the model of light led to an understanding of the photoelectric effect and black body radiation 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 observed that the spark between the gap in the transmitter loop caused an electrical disturbance between the gaps in the detecting loop. Hertz observed that the gap in the detector could be made larger and still generate sparks when the radiation from the transmitting spark shone directly into the gap in the detecting loop. Hertz did not recognise that the UV component in the transmitter spark removed free electrons from the surface of the metal, thus allowing the discharge (spark) to occur across a wider gap. outline qualitatively Hertz’s experiments in measuring the speed of radio waves and how they relate to light waves - Induction coil w/ spark gap succeeded in generating and receiving radio waves (EMR). Focused using parabolic mirrors. - Hertz measured their speed (Hertz was able to calculate the velocity of the waves by reflecting the generated waves off a metal sheet and measuring the wavelength of the standing wave set up by interference – was found to be close to 3×108ms-1, which was predicted by Maxwell), and observed their interference, reflection, refraction and polarization. identify Planck’s hypothesis that radiation emitted and absorbed by the walls of a black body cavity is quantised Planck's explanation for the observations involved the radical idea that energy could only be radiated or absorbed in small discrete amounts, later called quanta, now identified as photons. The size of each quantum of energy is characteristic of the frequency of light emitted. This explained why the black body curve ‘peaked’, classical physics expected it to continue rising. identify Einstein’s contribution to quantum theory and its relation to black body radiation Einstein proved/explained Planck’s work; Stated that energy was given out and absorbed in packets of photons, a photon is the smallest amount of energy to be transferred at a particular frequency. The amount of energy held by the photon is proportional to its frequency, i.e. E = hf. The intensity of light is proportional to the number of photons present. Thus the shorter the wavelength, the higher the frequency and thus greatest amount of energy radiated from black body. Einstein also explained how waves and particles behaviours can coexist. Physics HSC Course : Ideas to Implementation : page 4 explain the particle model of light in terms of photons with particular energy and frequency Instead of seeing light as a wave it is seen as a stream of particles called photons. The energy of these photons is dependant on their frequency, i.e. E = hf. All energy is seen as quantized. Photons have zero rest mass and travel at 3×108ms-1 in a vacuum. identify the relationships between photon energy, frequency, speed of light and wavelength: E hf h (Planck’s constant) = 6.626×10-34Js E is in Joules or Ev (electron volts) and c f ALSO c = 3×108ms-1 E = hc/ Students: o perform an investigation to demonstrate the production and reception of radio waves - 9 volt battery short circuited with 20c coin with a radio on a station nearby. Static will be heard. o identify data sources gather, process and present information to summarise the use of the photoelectric effect in: - solar cells - photocells Solar cells (photovoltaic) convert sunlight into electrical energy with the use of silicon semiconductors. When sunlight falls on a junction between n-type and p-type semiconductor material, electrons are ejected from atoms. These electrons are collected to form a direct electric current (DC). Photocells are common in electric eyes, radiation detectors and light meters. Many utilise the photoelectric effect to detect the presence of light or radiation at particular wavelengths. E.g. Astronomers use a photoelectric photometer to measure/analyse frequencies or stars emissions. OR in an alarm circuit when an intruder cuts a beam of light falling onto a photocell. o solve problems and analyse information using: E hf and c f o process information to discuss Einstein and Planck’s differing views about whether science research is removed from social and political forces Einstein – Wanted peace and no war, believed research should be for beneficial reasons only, not for military purposes and to destroy things. Realised potential of Atomic bombs and helped initiated Manhattan project and moved from Germany to America as he believed current trends were becoming quite concerning. Planck – Pro-political and social uses of science, especially if help nation in war efforts and gaining control of nations/world, he believed in war and had strong nationalistic views. Physics HSC Course : Ideas to Implementation : page 5 3. Limitations of past technologies and increased research into the structure of the atom resulted in the invention of transistors Thermionic devices – easily damaged (using glass tubes which are heated), slow, inefficient, not easily transportable (large devices). Used heating (thermal) to emit particles (ions). identify that some electrons in solids are shared between atoms and move freely Atoms contain specific stationary states in which electrons can exist. Molecular bonding allows for electrons to move freely and be shared between atoms. Ionic bonding allows outright sharing and transfer of electrons. Solid state elements – conduct electricity when electrons in outer orbit are free to move. describe the difference between conductors, insulators and semiconductors in terms of band structures and relative electrical resistance Conductivity/electrical resistance – the easier electrons can move, the lower the electrical resistance. Conductors – Valence and conduction bands overlap. (low resistance – e- easily jump to conduct. band. Semiconductors – Contain small energy gap between valence and conduction band, with energy electrons can jump up (medium electrical resistance). Insulators – Contain large energy gap between valence and conduction band, electrons need large amount of energy to jump to conduction band. identify absences of electrons in a nearly full band as holes, and recognise that both electrons and holes help to carry current When an electron jumps to the conduction band, it creates a positive ‘hole’ in the lattice, other electrons attempt to fill this gap but they only move it around, this creates an electric current. The ‘hole’ acts as a positive charge carrier. Conduction/current – electrons in conduction band - positive holes in valence band compare qualitatively the relative number of free electrons that can drift from atom to atom in conductors, semiconductors and insulators Conductors have more free electrons that are free to drift from atom to atom than semiconductors and insulators. Certain conditions can influence easier conduction; - Temperature - Lighting conditions - Applying a potential difference Physics HSC Course : Ideas to Implementation : page 6 identify that the use of germanium in early transistors is related to lack of ability to produce other materials of suitable purity Germanium was used in transistors since contextually there was suitable industrial techniques to purify it, other substances which acted better (silicon) could not be used effectively with current technology and resources. Negatives; - Too good of a conductor when heated - Too much current/heat flows through circuits damaging them - Creates large resistances with large currents, thus large heat production (inefficient) Then; silicon was used since; more abundant, cheaper, work well at temperatures. describe how ‘doping’ a semiconductor can change its electrical properties A small amount of impurity (GV or GIII) is placed in an otherwise pure crystal to disrupt its structure and electrical properties. Either extra electrons added (GV) with negative charge (n-type) carriers or deficiency (GIII) with positive charge carriers ‘holes’ (p-type). identify differences in p and n-type semiconductors in terms of the relative number of negative charge carriers and positive holes N-type – extra electrons which act as negative charge carriers, this type is more efficient that p-type semiconductors. - Doped with GV element. P-type – deficiency of electrons creates ‘holes’ which act as positive charge carriers, current created with flow of holes throughout lattice. describe differences between solid state and thermionic devices and discuss why solid state devices replaced thermionic devices Thermionic devices – use thermionic emission to emit electrons (heat cathode), this is performed in a vacuum environment. There is two or more electrodes which act as rectifiers or amplifiers and the value of these machines lies in the task of these electrodes. Solid state devices – uses semiconductors, no heat required, small, transportable, work fast and reliable, use less power (more efficient), cheaper, simpler. Used in integrated circuits as transistors etc. Students: o perform an investigation to model the behaviour of semiconductors, including the creation of a hole or positive charge on the atom that has lost the electron and the movement of electrons and holes in opposite directions when an electric field is applied across the semiconductor EGG carton experiment w/ ball bearings (electrons) with one hole without e- representing a positive ‘hole’ charge carrier. When a voltage is applied e- flow from the n-type through the p-type semiconductor towards the positive terminal, this is caused by attraction. Physics HSC Course : Ideas to Implementation : page 7 o plan, choose equipment or resources for, and perform a first-hand investigation to predict and verify the effect on a generated electric current when: the distance between the coil and magnet is varied the strength of the magnet is varied the relative motion between the coil and magnet is varied - As distance increases, current decreases. - Increase in magnet results in increase of current. - The faster the motion (more flux cut) the larger the current. o identify data sources, gather, process, analyse information and use available evidence to assess the impact of the invention of transistors on society with particular reference to their use in microchips and microprocessors Transistors impact – increase leisure time, decrease in social interaction, greater access to communicational resources, cheaper entertainment, increase in obesity, increased unemployment, smaller electrical devices (practicality). Microchips and Microprocessors – allows multiple circuits on one crystal of silicon, miniaturised and made faster the storage, processing and transfer of information, are easily incorporated into devices and many manual tasks can be performed by the use of these devices. 4. Investigations into the electrical properties of particular metals at different temperatures led to the identification of superconductivity and the exploration of possible applications outline the methods used by the Braggs to determine crystal structure Braggs used X-ray diffraction to determine the internal structure of 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). By firing X-rays into a crystal, with a receiving photographic plate above it, the rebounded X-rays which have ‘hit’ particles in the crystal are presented on the screen above. The angles these create allowed Braggs to determine distance between particles in crystals since he knew the wavelength for X-rays. This was direct evidence for the periodic atomic structure of crystals. The application of this technique has been crucial in determining the structure of important biological substances, such as DNA, and in the development of the transistor and microchip. identify that metals possess a crystal lattice structure The atoms in a crystal are in a regular repeating pattern called the crystal lattice. A crystal lattice is defined by a repeated three-dimensional unit. When a pure metal starts to form from a cooling molten state, the atoms arrange themselves in an ordered geometrical pattern that is repeated over and over again producing a crystalline structure. describe conduction in metals as a free movement of electrons unimpeded by the lattice Metal ions are surrounded by a ‘sea of electrons’ which are able to move freely throughout the lattice. These electrons are shared by all the ions in the lattice. Physics HSC Course : Ideas to Implementation : page 8 identify that resistance in metals is increased by the presence of impurities and scattering of electrons by lattice vibrations Impurities – disturb structure and integrity of lattice by distorting it’s shape, this impedes the free movement of electrons, increasing resistance. Lattice Vibrations – as temperature increases, lattice vibrations also increase. These vibrations cause electrons to ‘collide’ with more particles in the lattice, increasing resistance since the electrons are deflected from their normal linear progress through the crystal lattice. describe the occurrence in superconductors below their critical temperature of a population of electron pairs unaffected by electrical resistance When a substance is cooled below its critical temperature, the substance exhibits properties of zero electrical resistance. Electrons are the charge carriers in a metal. At room temperatures, the metallic bonds (the lattice) holding the conductor together vibrates and interferes with electron movement through the conductor. Type 1 – Pure metals, at temperatures close to 0K – 23K. Type 2 – Alloys, ceramics and metal oxides become superconductive at higher temps that type1’s –up to 120K. Electrons in superconductive substances are ‘assisted’ rather than impeded as they move through the lattice, this is due to BCS cooper pairs where electrons in pairs travel through the lattice. discuss the BCS theory 1957 - The BCS theory (after is proponents 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). At temperatures below the critical temperature for particular metals (or metal alloys), the movement of electrons is enhanced by lattice vibrations (phonons) 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. No lattice vibrations to impede electron flow either. THIS IS EXPLANATION FOR TYPE 1 SUPERCONDUCTORS discuss the advantages of using superconductors and identify limitations to their use Advantages – No energy losses (saves power + environment), cost and power efficient, allow very fast transport (no resistance) – uses Meisner effect. Limitations – Type 1 – critical temp is too low to get to easily and cannot be used in conjunction with A.C. power supply. Type 2 – Critical temp too low, Can’t work with A.C. and are too brittle. Students: o process information to identify some of the metals, metal alloys and compounds that have been identified as exhibiting the property of superconductivity and their critical temperatures Substance Zinc Mercury YBCO (YBa2Cu3O7) Type 1 1 2 Physics HSC Course : Ideas to Implementation : page 9 Tc (K) 0.88 4.15 92.00 o perform an investigation to demonstrate magnetic levitation o analyse information to explain why a magnet is able to hover above a superconducting material that has reached the temperature at which it is superconducting A superconductor will not allow a magnetic field to penetrate its interior. Mangnets remain ‘stuck’ above the superconductor at a set distance. The varied created poles of the superconductors magnetic field repel all fields, thus pushing magnet away but also attracting it in place. o gather and process information to describe how superconductors and the effects of magnetic fields have been applied to develop a maglev train The maglev train is a magnetically levitating train which is achieved through the use of superconductors. It uses the Meisner effect to allow the train to hover on the ‘tracks’, the train is accelerated by creating ‘poles’ in the ‘tracks’ and rails which push and pull the train along the path whilst holding it in place. The 1st one was produced in Japan in 1972, these trains are faster, more efficient (no friction) and more environmentally friendly than normal train systems. o 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 Applications – Computers – improve speed, connection, capacity and performance, enable them to run at speeds over 120GHz (30 times faster than current), use less power and create more hard drive space. Motors and Generators – these are currently inefficient w/ large energy losses, superconductors would reduce coal consumption, increase efficiency of devices (99% efficient) and use less power. Transmission of electricity and power grids – could improve transmission, storage and generation of electricity. There is the large drawback that they materials used have to be cooled to very low temperatures which would be a terribly difficult task. Other applications – Maglev transport, MRI scans, particle accelerators, SQUID’S, quantum computers and magnetometers. Physics HSC Course : Ideas to Implementation : page 10