3. Ideas to implementation
Identify that moving charged particles in a magnetic field experience a force
=qvbsinθ Moving charged particles in a magnetic field experience a force E= F/q Electric field strength = force experience per unit charge E= v/d Electric field strength = voltage experiences per unit distance (between horizontal plates)
Explain that cathode ray tubes allowed the manipulation of a stream of charged particles
A cathode ray tube is a highly evacuated glass tube - negligible obstruction from gas particles Two electrodes - cathode (negative) and anode (positive), High voltage applied across electrodes Structures built in or around the tube allow the cathode rays to be manipulated Further electrodes create an electric field that changes the path of the cathode rays Magnetic fields applied from the outside (through the glass) deflect the cathode rays Solid objects can placed inside the tube block the path of the rays
Identify that the use of germanium in early transistors is related to lack of ability to produce other materials of suitable purity
A transistor is a semiconducting device used to amplify and/or switch current Ge was used in transistors < 1960s because the technology needed to purify it to a ↑ grade was available Other materials such as Si could not be obtained at a sufficiently pure level at this time Germanium Disadvantages Conductivity changes with temperature, higher temperatures, good a conductor - excessive/dangerous currents Germanium is rare and costly. Silicon eventually replaced germanium as the semi-conducting material of choice in transistors Silicon Advantages 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.
Discuss the advantages of using superconductors and identify limitations to their use
Advantages No resistance - no energy wasted as heat when a current flows through a superconductor Perfect diamagnetism (shields out magnetism) - Meissner Effect Extreme sensitivity to small magnetic fields - detection Can be used to generate large magnetic fields Particle accelerators that use superconducting electro-magnets are cheaper to run because they use less electricity to produce magnetic fields These properties are useful in: Power transmission: no resistance removes the need for transformers, allows longer distance, cheaper transmission to remote areas Magnetic levitation: can greatly increase speed by removing friction between moving parts Medical imaging; may be able to deflect extremely weak electrical signals in the brain, allowing disorders to be imaged. Computer chips allow miniaturisation and increased speed Disadvantages Cost Brittle Type 1 - temperatures below the critical temperature are hard to maintain Type 2 - critical temperatures are higher but ceramic materials → brittle + not easily shaped into wires
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
Aluminium 1.2 Lead 7.2 Ni-Al-Ge 21 Tin-Niobium 18 TBaCuO 90 HgBaCaCuO 133
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 (Meissner effect)
An earth magnet will hover above a superconductor due to the Meissner effect Below the critical temperature → superconductors do not allow magnetic fields to penetrate their interior Superconducting material → produces eddy currents that generate an equal and opposite magnetic field to the magnetic field supplied by the rare earth magnet (Lenz law) Magnetic fields cancel each other out Repulsion occurs → Earth magnet hovers above the superconducting material
Perform an investigation to demonstrate the production and reception of radio waves
An induction coil was used to create a rapidly oscillating B-field which caused a rapid sparking across a gap between spherical electrodes in a conducting circuit. When circuit was on signal could be heard in hand help radios
Discuss the BCS theory
BCS theory explains superconductivity in terms of electron pairs (Cooper pairs) and packs of sound energy which are due to lattice vibrations (phonons) Phonons are packets of sound energy present in a solid because the lattice vibrates Lattice vibrations are minimal in superconductors (due to their low temperature) Despite this → a moving e- can cause the lattice to distort by attracting surrounding +ve cations As a result → increased +ve charge density forms near the e-, a second electron is attracted to the excess of positive charge, forming a cooper pair The transient relationship of cooper pairs results in continuous interacts of electrons forming a population Certain amounts of energy are required to break a cooper pair, however not enough energy is present to divid the population there copper pairs move through lattice structures with zero resistance. Implications Resistance is effectively zero - very narrow wires can carry very large currents Temperatures far below critical temperature can carry increased currents Current produces a magnetic field around the conductor Strength of the magnetic field eventually causes the loss of the superconducting state → current limit
Describe the difference between conductors, insulators and semiconductors in terms of band structures and relative electrical resistance
Band structure - Electrons in the valence band orbit the nucleus in the outermost electron shell - Electrons in the conduction band are delocalised (they don't belong to any particular atom) - free moving nature allows the material to conduct electricity - Energy gap = amount of energy needed by an electron to a) overcome electrostatic attraction to nucleus and b) move from the valence band to the conduction band Electrical resistance - Conductors: overlapping valence and conduction bands -free moving sea of electrons, with no extra excitement = good electrical conductivity = low resistance - Insulators: large energy gap between valance and conduction bands due to ↑ energy required to move from one to the other - few delocalised electrons = poor electrical conductivity = high resistance - Semiconductors: intermediate energy gap - intermediate number of delocalised electrons = medium conductivity = medium resistance
Identify Planck's hypothesis that radiation emitted and absorbed by the walls of a black body cavity is quantised
Black body = object that emits and absorbs 100% of the radiation in discrete amounts called quanta E=hf No radiation is reflected - the object appears perfectly black, as temp increases when radiation is absorbed emission wavelength changes Black body is in equilibrium when the amount of incoming radiation = amount radiated original beliefs - As wavelength emitted by BB decrease, radiation energy increase - Therefore, as BB gets hotter graph of energy/intensity would rise infinitely as wavelength approaches zero. - Violated the principle of the conservation of energy - Not supported by the experimental data planks hypothesis - Radiation emitted and absorbed by the walls of a black body is in discreet "packets" or quanta of energy - The amount of energy per quanta is characteristic of the frequency of light emitted - The number of quanta radiated depends on the intensity of radiation absorbed by the black body - This hypothesis matched experimental results
Explain why the apparent inconsistent behaviour of cathode rays caused debate as to whether they were charged particles or electromagnetic waves.
Cathode ray/discharge tubes are sealed glass tubes from which most air has been evacuated. A beam of electrons travels from cathode to anode and can be deflected by electrical or magnetic fields Hertz (wave) - cathode rays not deflected by electric fields - Result incorrect, deflections too small to observe (inaccurate equipment), ∴ ray isn't charged = wave - Hertz rays penetrated thin metal foil causing no holes Wrong → charged particles pass through spaces between the crystal lattice (limited knowledge of atomic structure) ∴ Ray did not have mass = waves particle Crooks - Crookes used a 'paddle wheel' cathode ray tube - When cathode rays struck the wheel it spun - therefore Ray could exert momentum, ∴ had mass = particles - Crookes got deflection in electric field experiment ∴ ray particles Debate resolved by JJ Thompson: Showed that negative charges were left by cathode rays when they hit metal plate, suggesting cathode rays were negatively charged particles Gas becomes ionised when ray hit remaining gas particles, neutralising the plates, leaving no charge. So when beam was fired - no deflection was detected. When vacuum was evacuated to a higher standard a deflection was detected.
dentify that charged plates produce an electric field
Charged plates produce an electric field The electric field strength determines the direction of and force experienced by a positive particle The nature of this field is dependent on the charge of the plates
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
Computers - Superconducting film used as connecting conductors - MRI scans requiring large magnetic fields - Higher data transmission speed and further miniaturisation - Field strength of 4T easily generated - Once current level achieved 'persistent current mode' occurs - current cycles without energy loss Motors and Generators - Superconducting materials for generating electricity - Superconducting materials for power storage - Superconductors can produce ↑ magnetic fields - no need for an iron core and generator smaller in size - ↓ fossil fuels needed to produce electricity = reduce greenhouse gases + pollutants - Electricity stored in SMES (superconducting magnetic energy storage) - DC flows around a large ring structure without energy loss until it is required - Power generation machinery operate at peak efficiency - Minimises the need for new power stations - Possibility of solar-power stations - electricity harnessed during day → stored → used at night Transmission of electricity - Use of superconductors in transmission wires - Experimental electricity transmission line - Reduces energy loss due to resistance - HTS (High Temp SC) material → wound around a hollow core with a liquid helium coolant - Large current densities through thinner wires - More energy efficient - Cheaper - DC → changing direction → heating → energy loss
Compare qualitatively the relative number of free electrons that can drift from atom to atom in conductors, semiconductors and insulators
Conductors = lots of free electrons Semiconductors = intermediate number of free electrons (depends on conditions → heat and dopants) Insulators = none
Describe how 'doping' a semiconductor can change its electrical properties
Doping = adding impurities → changes the number of e- / holes → improves electrical conductivity Semiconductor elements are group IV e.g. silicon and germanium which have 4 valence electrons Adding atoms of group III elements (boron, aluminium, gallium) will create a hole in the lattice Adding atoms of group V elements (phosphorus, arsenic, antimony) supplies an additional free electron, can be easily promoted in conduction band
Describe quantitatively the electric field due to oppositely charged parallel plates
E=V/d=F/q F=qV/d Force on a charged particle
Identify and assess Einstein's contribution to quantum theory and its relation to black body radiation
Einstein explained the photoelectric effect Stopping voltage: the voltage at which the energy of an electron falls to zero Cut-off frequency: the frequency at which photon energy is not sufficient to produce electrons Work function: the minimum energy needed to liberate an electron from a metal surface Used Planck's idea of quanta, named energy packets as photons - light considered a particle and wave: Trends observed Energy carried by a photon is proportional to its frequency Higher frequency = more energetic photons w/ greater Ek, Same number of electrons released Intensity changes the amount of photons Higher intensity = More electrons are released, same Ek No emission of e- occurs below the cut-off frequency fo No current flows at voltages below Vo (stopping voltage) Assessment: Einstein's theory explains what classical physics could not, lending significant support to quantum theory. The wave and particle properties of light described by Einstein also led to de Broqlie's hypothesis of the wave-particle duality of matter. Therefore, Einstein's predictions were an important step towards the development of Quantum theory.
Process information to discuss Einstein's and Planck's differing views about whether science research is removed from social and political forces
Einstein's beliefs - Jewish - fled Germany to America - Science research should be divorced from political agendas Planck's beliefs - Nationalist - stayed in Germany and directed the Kaiser Wilhelm Institute helping nazi effort for country benefit Einstein's instrumental role in the development of the atomic bomb - Wrote a letter to President Roosevelt, Manhattan - Feared that Germany would get there first Planck's efforts aimed at slowing the development of the bomb in Germany
Discuss qualitatively the electric field strength due to a point charge, positive and negative charges and oppositely charged parallel plates
Electric field extends radially Strength decreases as the distance from the point charge increases For opposite charges the electric field is in the direction of the negative charge and field strength decreases as the distance from the two charges increases Oppositely charged parallel plates - Electric field is uniform between the plates - Field strength decreases - As the distance between the plates increases - As the potential difference between the plates decreases - Field lines curve at each end of the plates
Outline the role of electrodes in the electron gun and the deflection plates or coils the fluorescent screen in the cathode ray tube of conventional TV displays and oscilloscopes
Electrodes - Cathode and anode allow for a potential difference - Cathode rays are emitted from the cathode - Electrons are attracted to anode, they arrive at it with very high speed, the majority of electrons keep travelling towards the screen Deflection plates/coils - Used to direct cathode rays to certain parts of the screen - Electrons are charged - their path can be altered by the presence of a magnetic or electric field - In oscilloscopes o Two sets of plates are used - one set alters the x coordinate and the other the y coordinate o By varying the voltage in the plates, the position of the cathode ray on the screen can be altered - In televisions o A coil produces a magnetic field - it exerts a force on charged particles such as electrons o 3 electron guns in RGB produce rays Fluorescent screen - The electron collides with an atom on the screen - The energy of the electron excites the atom, causing a photon to be emitted - Oscilloscopes have a phosphorus coating, phosphorus atoms emit green light when bombarded with cathode rays
Describe conduction in metals as a free movement of electrons unimpeded by the lattice
Electrons are shared by a rigid lattice of cations through which they flow unimpeded Metals = good conductors of electricity → lots of negatively charged particles (e-) that are free to move
extrinsic vs intrinsic semi conductors
Extrinsic semiconductor: if conduction is dominated by donor or acceptor impurities Intrinsic semiconductor: if conduction properties occur naturally without doping.
Forward and reverse biasing
Forward Biased External voltage is applied (positive at p side and negative at n side) Electrons entering the n type neutralise the positively charged ions in the depletion zone, while the holes flowing into the p type region neutralise the negatively charged ions in the depletion zone This decreases the potential difference across the p-n junction (narrows the depletion zone) Reverse Biased External voltage is applied (negative at p and positive at n) - electric current is negligible 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.
Outline Thomson's experiment to measure the charge/mass ratio of an electron
High voltages from an induction coil are supplied to cathode and anode, creating cathode ray which travel at high speeds in the low pressure glass tube and strike the glass at the other end Charged plates produce an electric field, electromagnets produce a magnetic field Since the fields are oriented at right angles to each other, the force on the electron due to the magnetic field will be in the opposite direction of the force due to the electric field Experiment stages - Magnetic and electric fields are varied until their opposing forces cancelled and no deflection occurs, cathode rays undeflected, equate magnetic and electric force equations to calculate velocity of e- - Apply the same strength magnetic field (without electric field) - determine radius of circular path travelled by e-, equate magnetic and centripetal force equations to calculate mass. magnetic field and electric field are equal when no deflection occurs v= e/b electric field off - deflection of magnetic field so force magnetic = force centripetal derive Thomson's value for charge/mass ratio for cathode ray particles was extremely large suggesting the existence of a subatomic particle with either a very small mass or aery large charge Thomson concluded that the size of the particle must be very small as the charge would have been so large, it would not have been contained stably inside an atom. The direction of deflection suggested that this particle was negatively charged and this led to discovery of the electron. Thomson's experiment put to rest the wave-particle debate on cathode rays.
Identify that resistance in metals is increased by the presence of impurities and scattering of electrons by lattice vibrations
Impurities Interrupt the regular structure Large atoms protrude further into gap between cations → ↓ space for e- and ↑chance of collision Lattice vibrations (temperature) ↑ temperature → cations in the lattice to vibrate more e- more likely to be impeded and scattered in different directions
different striation patterns for different pressures in discharge tubes
Induction coil acts as set up transformer, delivering a high voltage across the set of discharge tubes at different pressures. High pressure Streamers Constant stream Cathode glows with faraday's dark spaces, then striations All black with green at anode
Gather and process information to describe how superconductors and the effects of magnetic fields have been applied to develop a maglev train + draw
Levitating the train Superconductors (cooled to below their critical temperature) on the train Repelled from permanent magnets on the track Levitation eliminates friction with the track → faster speeds possible Forward motion Propelled forward by permanent magnets on the track and electromagnets on the train Track = permanent magnets that alternate in polarity Electromagnets on the train → change polarity in phase w/ permanent magnets on track (AC electricity) Electromagnet → repels adjacent track magnet but attracts the magnet in front Repulsion pushes and attraction pulls the train forward Initial forward momentum is provided by the standard wheel propulsion used in normal trains EDS - electro dynamic suspension Limitation Have to keep cold
Explain the particle model of light in terms of photons with particular energy and frequency and Identify the relationships between photon energy, frequency, speed of light and wavelength: E hf and cf
Light travels in photons, where the photo's energy is proportional to the frequency of light as E=hf A photon can either give all or none of its energy to an electron (discrete) A photon travels at 3 × 108 ms-1 and has a particular frequency and wavelength c=fλ
investigation to demonstrate and identify properties of cathode rays
Maltease cross: - Beam accelerated past anode - Travels in a straight line - doesn't pass through solid (shadow) - Anode accelerates cathode ray Electric plates: - Cathode ray beam was deflected by electric plates when potential difference was applied - Beam was deflected towards positive plate therefore, was negatively charged Fluro display screen: - Fluorescent screen in the path of the cathode ray was lit up - Suggested ray carried enough energy to produce reaction in the screen necessary to produce light. glass wheel: - Wheel mounted inside tube and allowed to spin - When ray struck wheel it rotated - Demonstrates how cathode ray has mass due to conservation of momentum
Identify that some electrons in solids are shared between atoms and move freely
Metals = rigid crystal lattices with a sea of delocalised electrons are free to move Unbound moving charges = ability to conduct electricity
Identify that metals possess a crystal lattice structure
Metals possess a crystal lattice structure Different metals have different lattice arrangements Cations are in fixed positions - surrounded by a 'cloud' of free-moving electrons
Identify differences in p and n-type semiconductors in terms of the relative number of negative charge carriers and positive holes
N-type = excess number of electrons If silicon is doped with a group V atom (five valence electrons = one more than silicon), an extra electron (not involved in bonding) is supplied to the material This semiconductor is an n-type semiconductor because it has excess negative charges Doesn't take much energy to promote electrons into conduction band as donor level is increased P-type = electron deficiency If silicon is doped with a group III atom (three valence electrons = one less than silicon), a 'hole' is created in the silicon crystal lattice structure This semiconductor is a p-type semiconductors because it has a deficiency of negative charges
Identify data sources, gather, process and present information to summarise the use of the photoelectric effect in photocells
Photocell = device that uses the photoelectric effect Solar/photovoltaic cell Sunlight → electrical energy A solar cell has an n-type semiconductor in contact with a p-type semiconductor At the p-n junction → excess e- in the n-layer fill the positive holes in the p-layer = depletion Photons (sunlight) supply e- in the n-type layer with sufficient energy to migrate across the depletion layer → causing a potential difference across the two layers e- (from p to n) + voltage = current Phototube/photocell Metal plate is excited by light source emitting electrons as photoelectrons when struck by photons above a certain threshold, Electricity flows from cathode to anode, passes through an external circuit as current Application: If the light beam is interrupted the lack of photocurrent can trigger an alarm e.g. motion detectors.
Outline qualitatively Hertz's experiments in measuring the speed of radio waves and how they relate to light waves
Production of EMR radiation Induction coil used to produce forms of EMR other than light Rapidly oscillating the electric field caused rapid sparking across a gap in the conducting circuit Spark induced in the detection loop - EMR generated by the transmitter were detected In order to determine velocity of wave, wavelength are required as v=fλ (frequency was known) → proved these waves are EM waves speed = c To find wavelength, a wave was reflected creating a standing wave, same frequency, wavelength and amplitude but different directions, creating an inference pattern of max and min receiver readings according to position of receiver (max: antinode vs min: node) The distance between successive maxima and minima gave wavelength
Describe Hertz's observation of the effect of a radio wave on a receiver and the photoelectric effect he produced but failed to investigate
Rapidly oscillating B-field in induction coil caused rapid sparking across electrodes in circuit This resulted in sparks at the receiver (metal loop with spark gap) o Sparking caused by AC voltage creates electric and magnetic field (together EM waves i.e. radio waves) o Theses waves travelled to receiver and energies its electrons on the conducting surface, causing them to jump the gap Hertz also demonstrates the waves created by the induction coil circuit have other properties the same as EM waves o Reflection - reflected off zinc plate and received o Refraction - refracted through prism and received o Polarisation - rotated receivers plane relative to transmitter, receivers intensity max parallel, min perpendicular o Speed - measured to be C , see next dot These observations strongly supported Maxwell's predictions of EM radiation: self-propagating, transverse waves, alternating electric and magnetic fields (perpendicular). Photoelectric effect: o Quanta of energy from EM radiation give electrons sufficient energy to escape the metal surface o Generates a sparks across the gap o Hertz observed saw spark emitted from a receiver when EM waves were present o Spark induced in the detecting loop - caused by propagating EMR He observed the photoelectric effect but he failed to investigate it further
Describe the occurrence in superconductors below their critical temperature of a population of electron pairs unaffected by electrical resistance
Resistance to e- movement falls to zero at the critical temperature Two types of superconductors (response to B feild) As external field is increased, internal field is 0, however at critical magnetic field the internal magnetic field suddenly transitions to normal state - can't use for applications, which use high B-field. Can withstand high amounts of external magnetic field, without high critical magnetic field point, seen in graph, useful to make electromagnets.
Gather, process and present secondary information to 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
Shortcomings WWII, the increased need for more efficient and sensitive radar and radio communications sparked research into transistors. Thermionic devices used were fragile, bulky, inefficient, noisy, unreliable and insensitive However, Ge lost its semi conducting properties at high temperatures and is rare and expensive so it became necessary to develop more reliable semiconductors. increased knowledge Experimentation of Germanium - methods of Ge purification, doping and the new understanding of electron flow through semi-conductors allowed the creation of transistors, which utilised pnp/npn junctions. More reliable and efficient amplification of weak electrical signals. i.e. radio waves in long distance communications. 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.
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
Table tennis balls are placed and manipulated within an egg carton Demonstrate metal conductor: lower tray (valence band and crystal lattice), upper tray (conduction band, free moving), electric field applied by raising one end pushing free electrons to one end of tray (demonstrating application of potential difference Demonstrate pure semi conductor: vibrating package to demonstrate heat required to move electrons (ping pong balls) N-type: add extra ping-pong balls (excess electrons), P-type: take away Ping-Pong balls leaving holes. Limitation of model, Failed to demonstrate: Increase in conductivity from pure to doped silicon Concentration of dopant element to silicon accurately Size of band gaps/donor acceptor levels
Identify data sources, gather, process and present information to summarise the effect of light on semiconductors in solar cells
The effect of light on semiconductors in solar cells is the production of electricity n-type is exposed to light while the p-type is not At the junction, electrons flow from n top, 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 and 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 Can do work when joined to an external circuit
Describe quantitatively the force acting on a charge moving through a magnetic field F qvBsin(theta)
The force is a vector quantity so the direction can be determined using the R-hand palm rule. Force is perpendicular and forward velocity changes position thus moving particle in circle. The speed of the charged object remains constant even though its direction changes. Therefore following a helical path
Why are x rays diffracted
The x-rays is scattered off atoms of different layers → travel a different distance → thus out of phase, therefore diffraction pattern occurs Constructive interpherance: difference of path wave travles is multiple of wavelength, 0,λ,2λ.... Distructive interpherenace: difference of path wave travles isnt multiple
Describe differences between solid state and thermionic devices
Thermionic diode A negative filament is heated sufficiently to allow thermionic emission of electrons from the surface, these electrons move towards positively charged electrode at end of vacuum. Metal filament inside the tube → current flows to generates heat → provides e- with sufficient energy to escape the metal surface Potential difference applied → escaping e- accelerate through the tube to the +ve terminal at other end Solid state devices: Made from a solid semiconductor material, p-n junction acts as diode allowing current to flow in a single direction Electrons at p-n junction diffuse into holes to form a depletion layer (depleted of free charged particles), This movement of charge creates a net potential difference across the junction, setting up an E-field, which prevents continued charge movement. It is possible to reduce/increase height of this barrier, by the application of an external voltage If a voltage is applied to a p-n junction, it will act as a diode, allowing current to flow from p to n
why solid state devices replaced thermionic devices
Thermionic diode Fragile valves - breakage and extreme care Bulky - portable appliances impractical Metal coating cathode - boiled off over time, reacts with gas Time to warm up - reduced efficiency solid state devices Less fragile Much faster current flow with rapid switching Significantly less heat produced Quiet
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 are key technology used in: Transistors - photovoltaic cells (environment), hearing aids (health) Microchips - portable technology (convenience), prosthetic devices (health) Microprocessors - computers (Internet), robots (manual labour) ADs Increased ability to transfer and store information globally Increased ability to communicate Automated technology in work force, faster production/lower costs Greater ability to analyse and assess data, using information to improve society e.g. climate, transport, economy, military disads: More instances of copyright violation/invasion of privacy Reduced personal contact and physical activity Automation of industry has reduced the need for unskilled labour, causing loss of jobs.
Outline the methods used by the Braggs to determine crystal structure
When X-rays enter a crystal they are scattered (absorbed and reemmitted) in all directions - treat as reflection, not reflection Crystals have planes, which xrays are scattered from, to produce a diffraction pattern the size between the slits must be close to the wavelength of light falling on them Knowledge of wave length can help determine the slit seperation Braggs examined patterns produced by x-rays when they hit atoms of a crystal and were scattered onto a photographic screen (via the photoelectric effect) X-rays → produced striking a metal anode with high energy cathode rays Parallel beams of x-rays → short wavelength to match gaps between the layers of atoms Collimator - produced coherent beam of light Scattered x-rays were observed - the diffraction pattern depends on Wavelength of the x-rays Distance between the planes of atoms in the crystal Angle of incidence of the x-rays on the crystal planes
Identify absences of electrons in a nearly full band as holes, and recognise that both electrons and holes help to carry current
e- (conduction band) and holes (valence band) are moving charges → therefore they contribute to the overall electrical conductivity of the material Holes - The absence of an electron in a semiconductor lattice = hole - Hole produced when an electron moves to the conduction band, or when a dopant atom is added - Electron of a neighbouring atom fills the hole → attracted to the more positive neighbouring atom - Electrons continue slot into these generated holes = movement of +ve charge (current) Electrons - Delocalised electrons are free to move in the conduction band - Movement of -ve charge (current)
Einstines equations
kmax = hf-w hf = v0 E = hc/wvavelength w = hf0/e
Bragg equations
n(wavelength) = 2dsintheta n(wavelength) = y times d/L
