Explain why cathode ray tubes were an innovation.
Cathode ray tubes allow the manipulation of charged particles. Discovery of vacuum cathode ray tube enabled scientists to produce a continuous beam of cathode rays rather than short discharge sparks. This meant they could observe them for longer and take measurements that were not possible with discharges across small spark gaps.
Explain why the apparent behaviour of cathode rays caused debate over their nature.
- Deflected by B-field = charged and therefore particles. Waves can't carry a charge.
- Scientists in the 1800s didn't obseve deflection in electric field which refuted the charged particle hypothesis.
- They were unaware that rays were deflected by E fields since they couldn't produce a strong enough potential difference (E field) due to lack of technology.
Describe what you showed about cathode rays with the Maltese cross.
- Cathode rays are blocked (or absorbed) by a metal barrier (shadow formed on back of tube)
- Rays travel in straight lines (shadow edges were sharp)
- Rays cause fluorescence (glass behind cross glowed when hit by the rays)
Describe what you showed about cathode rays in the screen display tube (fluorescent screen).
- Cathode rays are charged (deflected by B-field brought near the tube in direction of RHpush rule with thumb pointing opposite direction to travel of electrons.
- Therefore consist of particles (waves can't be charged)
- Rays travel in straight lines (narrow beam formed)
- Rays cause fluorescence (chemical on screen glows where ray passes through it)
Describe what you showed about CRs in paddle wheel tube and the logical progression that led you to this conclusion.
- Rays have momentum (cause paddle wheel to move)
- Rays must have mass (they have momentum)
- Rays are particles (they have mass)
Describe what you showed about CRs in the tube with electric plates and why.
- Rays travel in straight lines (narrow beam formed)
- Cathode rays are negatively charged (deflected by electric fields towards positive)
- Cathode rays consist of particles (they are charged, waves cannot be charged)
- NB: The last two weren't observed in the 1800s.
Describe different striatoin patterns observed in CRTs at different pressures.
- Pressure --> Striations
- Gas --> Colour
- Highest pressure (most gas) = wavy coloured lines.
- Higher pressure = striations appear with dark spaces and holes
- Lower pressure = dark spaces increase and regions of colour are reduced
- Lowset pressure = no colour in tube, glass of tube fluoresces
Explain interraction between moving charged particles and B-field.
- Moving charges in magnetic field experience a force (like deflection in CR).
- Charges' movement gives rise to second B-field.
- Interraction of two fields produces forces that change the motion of the charge.
Describe the electric field around point charges.
- Definition => E field = region where an electric charge expreiences a force.
- Field lines point in direction small positive test charge would go. - Away from pos, towards neg.
- Strength = force = E =
- Uniform = equally spaced and parallel. Closer lines are, stronger the field.
Describe the field between charged plates
- Directed from +ve to -ve. i.e. from high voltage to zero or lower voltage.
- Uniform field.
- Strength of field --> E=V/d
Describe the use of F=qE
- Force on charge in field is charge times the field strength.
- Field strength = E = V/d.
Describe what F=BqvSin
means and units used.
- Force is equal to product of magnetic field strength, charge, velocity of charge multiplied by the angle between direction of B-field (field lines) and direction of movement.
- F = Newtons, B= teslas, q = Coloumbs, theta = degrees and v=meters/second.
- If parallel, sintheta equal to zero and therefore, no EM force will be acting on it.
Outline Thompson's experiments in 1897.
- 1: CRT to deflect cathode rays using E-field and straightened the path using B-field. Equated F = Bqv = qE to find v of cathode rays.
- 2: Deflected them using B-field only and from F = mv2/r = Bqv
- By measuring r, he could calculate q/m.
- Showed q/m was 1800 times greater than hydrogen.
- Calculated v at much less than c, therefore rays are not EMR.
- Assumed charge on rays was similar to hydrogen and calculated mass. Assumed they were particles responsible for electricity.
- Named the particles electrons.
Outline the roles of electrodes in electron gun, deflection plates/coils and fluorescent screen in CRT.
- Electrically charged plates or EM fields deflect the cathode ray beam to the required spot on the screen and to scan the beam across and up and down the screen.
- Screen covered with fluorescent substance so it fluoresces when electrons hit it.
- Glass casing contains vacuum for beam to be produced and travel.
- Electron gun = cathode that emits electrons when heated and a negatively charged cylinder to focus the emitted beam.
- Cylindrical, +ve charged anodes accelerate CRs to required speeds to cause fluorescence.
Describe the implications of Hertz's 1887 experiment to our increased understanding of radio waves.
- Hertz studied radiation associated with electrical discharges.
- Calculated wavelength of radiation by studying interference patterns produced when a ray, reflected off parabolic mirror, was superimposed on a ray that travelled directly to the detector.
- Knowing frequency of oscillator producing the sparks, used v=f to find ray's speed.
- Found it to be equal to c, proving them part of the EM spectrum.
Account for use of parabolic mirrors in Hertz' experiment
The mirrors collected more waves for the receiving coil as well as focusing them to increase the spark's intensity.
Describe Hertz' observation of the effect of a radio wave on a receiver and the photoelectric effect he produced but didn't investigate.
- He observed that the discharge in one coil produced a discharge in another coil 1.5m away.
- Proposed that invisible radiation produced by discharges in first coil travelled through the air to the second coil which produced a discharge as a result.
- Shining UV light on the second coil, caused the spark to jump the gap more easily. He realised from this that light and electricity must be connected - called it the photoelectric effect. Didn't investigate it in detail though.
Describe experiment used to show the production and reception of radio waves.
A handheld radio was tuned to AM. Spark generator set up using an induction coil. It was set to run and and the interference on the radio observed as crackling.
Identify Planck's hypothesis about black body radiation and the 3 revolutionary ideas that this spawned.
- Hot materials no matter their composition emitted radiations of vaying wavelengths that depended not on the substance, but on the temperature of the substance.
- Planck proposed a model to explain BBR at all temps. Atoms inside the cavity oscillated back and forth and emitted radiation in a similar way to a radio antenna.
- Three Revolutionary ideas in explanation of emitted radiation pattern were: 1. EM energy associated with osciallation of atoms was quantised. Could only have energy values of multiples of f -- i.e. E=hf. 2. An atom could absorb or release only integral numbers of quanta of energy. 3. Quanta of energy were absorbed or emitted only when an atom changed from one quantised energy level to a different quantised level. If the atom did not change quanta levels, it could neither absorb not emit energy.
- NB -- Modern physics recognises that radiations emitted from hot objects are due to electrons falling from a higher energy level to a lower one and that the quanta are the amounts of energy associated with these electron transfers.
Identify Einstein's contribution to quantum theory and relation to black body radiation.
- 1905 - Einstein applied Planck's idea of quanta as developed from black body radiation experiments, to explain the photoelectric effect.
- Extended Planck's quanta idea to light assuming light existed as quantised photons that carried specific quantities of energy.
- Explained various observations associated with the photoelectric effect and in doing so gave quantam theory much more credibility.
Assess Einstein's contribution to quantam theory and its reception.
- Planck's hypothesis wasn't supported by scientific community.
- Too different from classical physics,
- Was only theoretical and there were no practical applications to convince them.
- Einstein provided practical proof with his use of quantam theory to explain photoelectric effect.
- This caused scientists to consider the possibility of quantam theory being true and from then on tried to apply it to other applications (e.g. Bohr's model of the atom), further strengthening the theory's validity and acceptance.
Explain the particle model of light.
- Einstein generalised Planck's idea to all EMR. This implied that all light consisted of particles - photons.
- Each photon carried energy directly proportional to its frequency given by E=hf.
- This didn't sit well with classical wave theory and didn't fit in with wave properties of light (diffraction, interference).
Identify relationship between photon energy, frequency, speed of light and wavelength. Name the equations.
- therefore E=hc/
- E can be expressed in joules or eV where 1eV=1.6x10-19 J
- Wavelength can be in nanometers which must be used in equation as 1x10-9 m.
Summarise the use of the photoelectric effect in photocells and assess their use.
- Photocells are based on cathode ray tube technology. Photoelectric effect directly produces electric current in the circuit attached to them. Incident photons cause the emission of electrons from the cathode and these flow through the vacuum in the tube to the anode and then into the external circuit.
- The large size of photocells and their sensitivity to shock led hem to be replaced by more robust solar cells that use semiconductors.
Summarise use of photoelectric effect in solar cells.
- Solar cells use the photoelectric effect to convert energy from sunlight into electrical energy. Instead of sensitive + breakable vacuum tubes in photocells, solar cells use semiconductor technology.
- Photocurrent in both depends on the intensity of light (of appropriate frequency) hitting the solar cell. As the source of light is the Sun, the effectiveness of the cell is limited being affected by weather and season.
Discuss Einstein's and Planck's views on science.
- Early 20th C = friends.
- During WWI = strained friendship - Einstein = pacifist, Planck = opportunist
Both believed science should be free of social & political influence but were subject to heavy political influences which directed their research & science activity
- Differing views:
- - Planck was a patriot of native Germany, strong supporter of German caused & aimed to work with the system, believed scientific research was not removed from
- social & political forces
- strong pacifist, escaped Germany & took up Swiss citizenship, not loyal to any Government & believed science should not be manipulated for the good of
- the state, regretted research on A-bomb
- Planck – conservative patriotic German, often conflicted in his responses
- - Believed science was an activity to uplift all people, felt
- responsibility to create a better world
- - His leadership role as a scientists made him a public figure in the political arena
- - Advocated the war & political intolerance did not inflict harm on international relations between scientists
- - Unlike Einstein didn’t see moral imperative opposing Hitler – compromised & worked within system
- - Able to continue career in Berlin despite hostility of anti-Semitic groups towards the ‘decadent Jewish science’ of relativity & the quantum theory
- - However Planck tried to protect his Jewish friends & students, acted behind the scene to preserve the freedom of the action of science, struggled to keep his institute free from political pressure
- - Planck’s institute was funded by Kaiser & leading industrialists & he was often subject to pressure to accommodate views he found personally offensive
- Einstein – Jewish, with rise of
- Hitler & anti-Semitism movement emigrated to US during 1930’s
- - Einstein believed the act of science should be independent of national interests
- - However he did agitate for an end to nuclear weapons tests in his later life
- - Einstein remained a pacifist (some say he compromised his ideals with his support of the development of the atom bomb)
- - At start of WWI 93 leading German intellectuals, including Planck signed a manifesto defending Germany’s war conduct, Einstein & three others signed an anti-war counter manifesto
- - Initially favoured construction of A-bomb to ensure Hitler did not do so first
- - After the war he lobbied for nuclear disarmament & world government
- As a Swiss citizen, Einstein could feel free to spend his time on theoretical physics, but he kept looking for ways to reconcile the opposing sides.
Describe atomic bonding in semiconductors and account for semiconductivity.
- Atoms of semiconductors have four valence electrons. Bond to other atoms covalently, completing stable octet by forming covalent network crystal structure. Under normal conditions there are no free electrons to carry charge and conduct electricity.
- However some materials are photoconductors and thermionic emitters of electrons. Their valence electrons also break away from their bonding positions and move when a potential difference is applied.
- This accounts for their semiconductivity.
Describe in terms of band structures and relative electrical resistance, the differences in conductors, insulators and semiconductors.
- Band theory and electrical resistance = model to explain diferences in conductivity. It communicates the energies causing substances to conduct in terms of energy gap diagrams.
- Metals - valence elctrons are free and delocalised. Travel through the metal when potential difference is applied. Already in the conduction band. In band theory model = valence band overlaps with conduction band.
- Intrinsic semiconductors - valence band is full but there is only a small energy gap between it and the conduction band - <2eV
- Doping a semiconductor lowers the band gap.
- In insulators, the valence band is also full but eergy gap is larger - ~10eV
Describe holes in conduction bands.
- Absences of electrons in nearly full bands are called holes. Electrons and holes help to carry current. Impact of photons above the 0 or threshold frequency on the surface of the semiconductor has two consequences:
- 1. Free electrons are emitted from the surface. These form electric current.
- 2. Positive holes are left in the valence shell. Holes efffectively act as positive charges.
- Neighbouring electrons jump into positive hols, eaving their positions as more positive holes.
- Electrons move through the semiconductor towards the positive potential, and positive holes move in the opposite direction towards negative potential.
Compare number of free elctrons in conductors, semiconductors and insulators.
- Every atom in conductor contributes valence electrons for conduction --> 1022 per cm3 of conductor.
- Semiconductors have about 1015 per cm3.
- Doped n-type semiconductors have 100 times more electrons than pure semiconductors.
- P-type semiconductors, while having fewer valence electrons, have improved conductivity due to positive holes.
- No free electrons to form current when voltage is applied across the ends of an insulator.
Describe an experiment you performed to model the behaviour of semiconductors.
- Thought experiment - rows of chairs in a doctor's surgery. Seats are positive holes but only become active if an electron (patient) leaves the seat.
- Patients and chairs represent atoms in semiconductors. First patient is called to see the doctor (potential difference).
- Empty seat now exists. Patient second in queue moves to this seat leaving his and so on. The empty seat thus moves in the opp. direction until it is at the back of the cue.
- Next new patient to arrive will sit in this seat - representing an electron moving into the positive hole from further down the circuit.
Justify the use of germanium in early transistors and why it isn't used anymore.
- Germanium was used to make electronic devices that used semiconductors because silicon couldn't be purified enough.
- Gemanium wasn't ideal, though, as it stopped conducting if it became too hot. This can happen when an elecrical current flows through a substance.
Describe how doping a semiconductor can change its electrical properties.
- Pure semiconductors have too little natural electrical conductivity to be useful in circuits. Their conductivity can be increased by adding impurities. - called doping
- Doping adds either a group III (p-type) or group V (n-type) with concentration of about 1 in 200 000.
- Extra electrons in an n-type semiconductor add to current flow while the positive holes in the p-type make it easier for the electrons to move through the semiconductor from hole to hole. Because electrons have greater mobility than positive holes, n-type conduct (slightly) better than p-type.
Identify differences in p and n-type semiconductors in terms of relative numbers of negative charge carriers and positive holes.
- If silicon is doped with arsenic (group V), four of the arsenic's five falence electrons bond into the silicon lattice but the fifth is free to move when a small amount of energy is added. --> n-type conductor
- If doped with boron (group III) a positive hole results for every doped atom. This is a position where the fourth electron in the silicon lattice would normally be. Known as p-type semiconductor.
Account for introduction of solid state over themionic devices when it happened.
- Thermionic devices use CRTs and require a vacuum, making them fragile.
- They emit electrons when heated.
- Valves were replaced by solid state state devices in 1950s and then integrated circuits in 1958.
- Problems with germanium overheating and then not conducting and the inability to produce pure siicon guaranteed valves a longer lifetime than might've been the case.
Discuss how shortcomings in communications technology led to an increased knowledge of the properties of materials with reference to the invention of transistors.
Hint: 9 problems
- Problems with valves:
- Valves are much larger. = Large appliances. Modern integrated circuits have millions of components in a chip <5mm2.
- Portable valve devices needed up to 12 D-sized batteries.
- Valves are less efficient due to wasted heat energy.
- Heat developed by valves evaporated the thermionic coating on the cathode giving valves a limited life.
- Valves are more expensive to produce and run (less efficient).
- Valves are less reliable - easy to break and have fragile components that crystallise with continual heating and cooling, breaking easily when bumped.
- Valves mounted on metal bases insulated by bakelite rings and connected by insulated wires. Insulation often cracks or degrades due to movement or heat.
- High grid voltages are required to run valve amplifiers whereas equivelant transistors can work with 0.6V for same output.
- Valves take time to warm up to operating temperature.
Assess impact of transistors on society with particular reference to their use in microchips and microprocessors.
- Enabled communication revolution - advent of computers and processors in things like cars, radios, mobiles, electronic banking, games music and movie production... and changes in lifestyle for people, ease of travel, growth in entertainment sophistication, medical technology, general knowledge base.
- Reduction of unskilled work availability = more people on gvt welfare, growth of computing based jobs and need for tertiary training. = i.e. transition to more educated and enterprising world.
- Reliance on computors - businesses unable to function in their absense - crippled by computer issues, lives put at risk e.g. elecronic flight systems on aircraft.
Summarise the effect of light on semicondctors in solar cells.
- Solar cells operate on the principle of photoelectric effect.
- Emission of electrons from the surface when light falling on it is above the threshold frequency.
- As intensity increases photocurrent increases to a maximum value.
- As the frequency of incident light increases, phtocurrent increases as electrons are emitted with increases in Ek.
- Below the threshold frequency no photoelectrons are emitted from the surface regardless of incident light intensity.
Outline Bragg's experiment.
- He sought to determine crystal structure. He proposed that an analysis of X-ray diffraction patterns would give clues as to the crystal structure of substances.
- Bounced X-rays off metal specimens and analysed the diffraction patterns formed by the reflected rays caught on photographic film.
Identify the structure of a metal.
Crystal lattice (proved by Bragg) with valence electrons overlapping due to the atoms' proximity.
Describe and account for conduction in metals on an atomic level.
In metals, a sea of delocalised valence electrons surround the lattice. Electrons don't have an atomic identity - don't belong to a single atom. These electrons give metals their high conductivity.
Describe conduction in metals and account for different metals' resistances.
- Kinetic theory explains that particles of matter are in constant motion. Free electrons in metals also move, bumping back and forth among positive ions but having no specific forward motion.
- The application of voltage (poetneial difference) causes electrons to drift towards the positive potential and a current flows. The larger the voltage, the larger the current.
- The movement of electrons is not direct. They still bump into nuclei in the lattice and even move backwards as a result of these collisions. There is general movement forwards - drift velocity of the electrons.
- Drift velocity in conductors is lowered by impurities as they distort the crystal structure and offer more resistance by changing electrical energy into wasted heat energy.
Describe the occurence in superconductors below their critical temp of a population of electron pairs unaffected by electrical resistance.
- While all conductors (except semiconductors) conduct better as the temperature lowers due to decreased atomic vibrations in the metallic crystal lattice, superconductors lose all electrical resistance below a particular critical temperature.
- BCS theory accounts for the superconductivity by saying it is due to the flow of electron pairs unaffected by the resistance of the substance through the superconducting material when it is below crit temp.
Identify some metals alloys and compounds identified as exhibiting superconducitvity and their Tc's.
- Mercury - 4.15K
- Lead - 7.196K
- Aluminium - 1.175K
- Niobium/titanium - 9.2K
- Niobium/tin - 18.3K
- YBa2Cu3O7 - aka 123 - 92K
Discuss the BSC theory.
- As electrons move through the metal lattice, positively charged atoms are attracted to them, causing a distortion of the lattice. The distortion (known as a virtual phonon) concentrates the positive charge creating a region of increased positivity to which the following electron is attracted before rebounding back into its original position.
- As a result the electrons travel through the superconductor in pairs known as cooper pairs. The electrons in cooper pairs are held together by forces known as phonons.
- Because of the eextremely low temperatures (and therefore few atomic vibrations in the lattice) electron pairs travel unumpeded as long as the subsance is below the critical temperature.
Evaluate use of superconductors.
- Advantages - zero resistance to electricity, increase in efficiency of any device using them (no energy loss due to heating effects), perfect transfer of energy. Allows use of technologies such as MRI which are potentially lifesaving and invaluable for noninvasive research.
- As such, there are flow on environmental and cost to consumer in certain applications.
- Disadvantages come from mining and processing of raw materials but mainly from setting up and maintaining the critical temperatures.
Explain why magnet is able to hover above superconducting material below its crit temperature.
- Pagnetic fields do not permeate superconductors.
- If magnet is brought near superconductor, induced currents are set up in the superconductor. By Lenz's Law, the interaction between the B-field and the induced currents produces a force that causes the magnet to float above the superconductor.
- If a magnet is placed on top of a superconducting material (i.e. the system is not moving so no induced currents can be formed), then a spontaneous eddy current is generated in the superconductor.
- The interraction between B field of magnet and that due to spontaneous current in the superconductor repels the external B-field from the superconductor, causing it to float.
- The idea of repulsion of the external B-field from the material = Meissner effect.
Demonstrate magnetic levitation.
- Ytrium oxide superconductor was placed in Petri dish and liquid nitrogen poured all over it.
- Small magnet was released from above the surface of the superconducting material and it floated.