From Ideas to Implementation - Module 2
KE of photo electrons
(KE= hf - ϕ) .Excess energy gives the photoelectron it's kinetic energy. Where ϕ is the work function
charge to mass ratio value
1.76 x 1011 C kg-1
Formula derived from Braggs
2d sinθ = nλ
JJ Thompson
A British mathematician and physicist who was the first to identify the electron in 1897. He measured the charge to mass ratio (q/m) of cathode rays and showed that all cathode rays had the same value. See also q/m ratio. above.
Max Planck
A German scientist credited with discovering quantum theory when investigating black body radiation. He found he could only get agreement between experiment and theory by postulating that light came in photons or quanta or bundles of energy. This differed from the classical view of the time
Extrinstic Semi-conductor
A doped semiconductor
Cathode Ray Tube
A glass tube that contain very low pressure gas (vacuum), with 2 electrodes connected to a high voltage source either side of the tube.
Semiconductor
A material that conducts electricity at low temperatures, but not at high temperatures
Photon
A particle of electromagnetic radiation with no mass that carries a quantum of energy
n-type semiconductor
A semiconductor material has electrons as the majority carriers and holes as the minority carriers; doped with Group V atoms.
p-type semiconductor
A semiconductor material has holes as the majority carriers and electrons as the minority carriers. The semiconductor is doped with Group lll atoms. See also Hole
Meissner effect
A superconductor below its critical temperature responds to the presence of an external magnetic field by the flow of internal currents, which set up a magnetic field cancelling the external field. That is, the resultant magnetic field inside the superconductor becomes zero and the superconductor is said to have expelled the applied magnetic field. The external and internal magnetic fields are in opposite directions so that there is repulsion of the magnet producing the external field above the superconductor. The upward repulsion cancels the weight force and the magnet is suspended above the superconductor
Electron diffraction
A technique used to study matter by firing electrons at a sample and observing the resulting interference pattern
Filament
A thin wire with high electrical resistance. When current passes through it. it gets hot.
Maglev Train
A train uses magnetic levitation for propulsion.
Insulator Structure of bands
A very large energy gap betweenthe valence and conduction bands- this is known as the forbidden energy gap. In order to conductelectricity, electrons in an insulator must gain enough energy to jump from their normal valence band to the conduction band.
Maltese cross
An evacuated tube with a metal cross in it. Used to show that cathode rays travel in straight lines.
A Black Body
An idealized body which is a perfect absorber and emitter.
ceramic
An inorganic, nonmetallic solid material comprising metal, nonmetal or metalloid atoms primarily held in ionic and covalent bonds.
photo electron
An ordinary electron that happened to be knocked out of the metal by a photon.
Intrinsic Semi-conductor
An undoped semiconductor
F = qvBsinθ and F= mv²/r
Because a charged particle moving in a magnetic field undergoes uniform circular motion, the force acting on it must also be a centripetal force - hence the magnetic force equation can be equated to the equation of centripetal force.
Cathode ray oscilloscope
CRO; an electronic device used to view electrical signals as waveforms
Solar cells
Cells made of several layers of light-sensitive materials which convert sunlight directly into electrical energy
Solid State Device Properties
Cheap Small and light weight Durable and reliable Long lifespan No warm up time Very low power requirement
Limitations of superconductors
Currently require super low temps to operate High temp ceramic superconductors are difficult to produce Use alot of energy to run
Planks Constant
Denoted h, this fundamental constant, equal to the energy of a quantum of electromagnetic radiation divided by its frequency, with a value of 6.626 × 10−34 joules.
Electron Gun
Device that produces a narrow beam of electrons in a cathode ray tube by thermionic emission
Thermionic devices
Devices that use thermionic emission in their operation. For example. the filament of a cathode ray tube
Positive Point Charge
Diminishes with distance from the object The direction of field is defined as pointing rapidly away from a positive point charge.
Negative point Charge
Diminishes with distance from the object The direction of field is defined as pointing rapidly towards a negative point charge.
Einstein and Planks differing views on scientific research
Einstein and Planck initially held differing views as to the relationship between science and politics,but in the end they both came to realise the two were intrinsically linked.Einstein at first refused to support the war or use science to help governments fight the war, believingthat science was removed from social and political forces. However, in the end he came to therealisation that the two are in fact linked together, and he ended up helping with the Manhattanproject which almost certainly contributed to the ending of the war.Planck initially felt that science definitely had a role to play in terms of politics, but eventually heturned against the Nazi regime, criticising it, believing that science should be separate. However,he understood that there is an unavoidable link between science and politics. Even after Planckattempted to separate science from politics, research science for the military continued throughother scientists.In a way, both Planck and Einstein are representative of the wider debate in science that continueseven today as to the role the government's agenda should be in terms of scientific research, but they,like today's scientists, realised that science and politics can never be separated, even if that is theideal situation
The electric field is uniform, the field lines running from the positive plate to the negative plate perpendicularly.
Electric field between 2 plates
E= v/d
Electric field strength in two plates formula
E= hf
Energy frequency formula
Thermonic Device Properties
Expensive Bulky / heavy Fragile, easily broken short life span significantly long warm up time Large power requirements
Formulas found from thompsons q/m ratio experiment
F = qvB = mv^2/r F = qE = qvB
G=(4π² x L) / T²
Gravity Pendulum Formula
Semi-metal Structure of bands
Have band gaps smaller than for insulators but bigger than conductors- they lie inbetween, so are initially insulators but when heated moderately become conductive.
Hertz radio wave experiment
Hertz used an induction coil to produce sparks between the spherical electrodes of the transmitter.Hertz then showed that these new electromagnetic waves could be reflected from a metal mirror, and refracted as they passed through a prism made from pitch. 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. Substituting this wavelength and the known frequency of the wave generator into the general wave equation, v = ? x f, Hertz calculated the wave speed at 3 x 108 ms-1, very close to the values for the speed of light earlier estimated by Maxwell and measured by Fizeau.
Hertz Discovery
Hertz was able to conclude that the radiation he was dealing with was part of the electromagnetic spectrum by analysing its properties in comparison to light. He carried out experiments to show that •It could be reflected by metal plates •It could be refracted by pitch or asphalt blocks •It could be diffracted around obstructions •It could be polarised (when he rotated the receiving coil he found that the sparks were stronger-at certain angles compared to others)
Deflection plates or coils
In a cathode ray tube dispaly device the electron beam is steered by
Deflection plate/ coils
In a cathode ray tube display device the electron beam is steered by.
How a charged particle produces a force
It sets up its own magnetic field when moving. This magnetic field will therefore interact with an external magnetic field producing a force.
Plum pudding model
J.J Thomsons model of an atom, in which he thought electrons were randomly distributed within a positively charged cloud
Effect of sunlight on a solar cell
Light shining on the semiconductor material frees electrons from the valence band to the conduction band of the semiconductor material, thus producing electron-hole pairs in the semiconductor. A photon of light with energy greater than the band gap energy between the valence and conduction bands but less than the work function energy for the semiconductor strikes an electron and is absorbed. This gives the electron sufficient energy to jump the forbidden energy gap from the valence to the conduction band. Once the electron is free it is forced by the barrier potential to move to the n-type material, while the hole left behind is forced to move to the p-type material. If an external circuit is supplied, the electron will move out of the n-type material, around the external circuit through a load and back into the p-type material to recombine with the hole. Clearly, as many electrons undergo the same process, an electric current has been produced by the solar cell. The size of the current produced is proportional to the light intensity incident on the solar cell. Overall, light energy has been transformed into electrical energy.
F = qvB = mv^2/r
Magnetic and centripital force formula to find velocity (v)
F = qE = qvB
Magnetic and electric field formula to find velocity (v)
Superconductors
Materials that have zero resistance when their temperatures are low enough. Superconductors allow electrons to flow unimpeded. See also BCS theory of superconductivity.
Type 2 superconductor
Metal oxides (ceramics) with much higher critical temperatures, some above 100k.
Type 1 superconductor
Metals or metal alloys with critical temperatures of 20K or lower.
Threshold frequency
Minimum frequency needed to cause the photoelec effect
Electrons
Negatively charged subatomic particles, that has a charge of -1.602x10^-19 C and a mass of 9.1x10^31 kg
Advantages of superconductors
No waste heat Very strong magnetic fields Used as energy storage Make thinner wires, save materials Used in MRI machines & strong particle accelerators Used in magnetic levitation (maglev trains) Can be used as switches, would replace transistors Much smaller More efficient, cost effective
Electron-hole pairs
Occur at temperatures above 0 K when some electrons gain sufficient energy to escape from their bonds and exist as free electrons. in doing so they leave a hole behind. The electron and hole form an electron-hole pair.
BCS model of superconductivity
Quantum-mechanical effect where two electrons pair up and pass unimpeded by the lattice.
Energy bands
Represent where the electron energies of large number of electrons in matter are spread over bands. The highest occupied energy band is the valence band. Above the valence band is the conduction band. Between these bands is the forbidden energy gap.
Factors affecting conductivity
Resistance of the metal (steady, linear lattice) Temperature (vibrations in lattice, affect electron path) How close lattice layers are to each other
Quantized
Restricted to certain quantities - the opposite of continuous; goes from one level to the next with nothing in between (e.g., the energy levels in which an atom's electrons exist.)
Photo cell
Sensors that allow you to detect light.
Cathode rays
Streams of electrons that are produced when a high voltage is applied to electrodes in an evacuated tube.
SQUID
Superconducting quantum switches (superconducting quantum interference device), future transistor replacements.
Quantum
The amount of energy needed to move an electron from one energy level to another. The minimum amount of energy that can be gained or lost by an atom
Diffraction
The bending of a wave as it moves around an obstacle or passes through a narrow opening
Communication technology impact from the transistor
The biggest problem with communication technology in the early days of the radio was amplification-the received signal was extremely weak and could not produce a loud sound without being amplified.This meant researchers were always trying to improve amplification technology to address the short-comings with valves such as their high failure rate, high power consumption, their weight and theirwarm-up time. When they first determined some of the properties of semiconductors this need forbetter amplifiers fuelled heavy research into the properties of semiconductors and the ways in whichthey could be used as amplifiers in the form of transistors. So the shortcomings in available com-munications technology led to the rapid development of the transistor which would have otherwisetaken many years longer
q/m ratio (charge to mass)
The charge to mass ratio for charged particles. Thomson measured this ratio for cathode rays and in doing so discovered the electron. See also Thomson.
Metal/ conductor Structure of bands
The conduction and valence bands overlap-this means that electrons in their normal valence positions can, without gaining any energy, be in theconduction band and move freely between atoms.
Electron sea model
The electron sea model describes the circumstances by which in a metal. positive ions in the lattice are surrounded by a moving 'sea' of electrons.
Photoelectric effect
The emission or absorption of electrons from a metal caused by light striking the metal
Electric field
The field around charged particles that exerts a force on other charged particles.
Electric current
The flow of electric charge
F = qvBsinθ
The force acting on a charge moving towards a magnetic field formula
Work Function
The minimum energy required by a single electron to escape the metal surface
The particle model of light
The particle model of light considers light to be transmitted by small particles. These particles have mass that depends on their energy, with more energetic photons having greater mass (although their rest mass is 0). To increase the energy of a photon, the frequency, not the amplitude of the light is increased. To increase the amplitude the number of photons is increased. Photon energies can only occur in multiples of Planck's constant.
The UV catastrophe
The prediction of late 19th century/early 20th century classical physics that an ideal black body at thermal equilibrium will emit radiation in all frequency ranges, emitting more energy as the frequency increases. When calculating the total amount of radiated energy, shows that it is infinite, contradicting the principles of conservation of energy.
Doping
The process of adding impurities to a crystal to alter electrical properties
Evidence Supporting cathode rays were Particles
The rays left the cathode at right angles to the surface Influenced/ detected by magnetic fields They were not affected by electric fields (later proved wrong by J.J Thompson) Could transfer momentum (paddle wheel) Travelled much slower than light
right hand grip rule
The rule used to determine the direction of the magnetic field around an electric current.
fluorescent screen
The screen of a cathode ray tube whose surface is coated with a material that fluoresces. that is. emits light when stnlck with electrons. Used to form an image of an electrical signal. See also Cathode ray. above.
Forbidden Energy Gap
The separation between valence band and conduction band
x-ray diffraction
The use of X-rays to determine the internal structure of crystals. X-rays are scattered by the crystal and the pattern of reflections is determined by the position of the atoms of the crystal.
Impact of microchips and microprocessors
They have enabled the building of small, efficient computers that nowhave widespread applications throughout society as well as in scientific research. It has allowed theautomation of repetitive tasks which has led to higher quality of life, at the expense of jobs and a risein unemployment. However, in terms of communication it has had a tremendous benefit enablingthe internet which has drastically changed society for the better. So overall transistors have had anextremely positive impact on society
Evidence Supporting cathode rays were waves
They travelled in straight lines (Maltese cross) They made shadows when an object was placed in its path (Maltese cross) They could pass through thin metal foils without damaging them Caused glass to fluorescent (Cathode ray tube)
Einstein's contribution to quantum theory
This contribution was twofold. Firstly, he used Planck's formula to create a more detailed quantum theory of light (with light packets called "photons"), and secondly he created an explanation for the photoelectric effect. In terms of defining light, he set up a concrete explanation for the particle theory, explaining intensity and frequency in terms of energy of and quantity of photons. He also stated that photons were the smallest units of light possible. In terms of its relation to black body radiation, Einstein's theories came about directly because of the work undertaken by Planck regarding black bodies. Einstein's work led him to explain the photoelectric effect in terms of work function and threshold frequency, also providing an explanation for photoelectron kinetic energy that matched Lenard's puzzling results. Further, he brought quantum theory further into the mainstream where other scientists continued to build on it.
The significance of the ratio of q/m obtained by Thompson
Thompson found the ratio to be 1800 times greater than that of a hydrogen ion. From this , because neither charge nor mass could be found he could conclude that; -The charge on cathode rays was 1800 times larger than the charge on a hydrogen ion, or -The mass of cathode ray particles was 1800 times less than that of a hydrogen ion From these results, Thompson assumed that the charge on them was similar in size to that on a hydrogen ion and calculated their mass to be 9.11 x 10^-31 kg. He also assumed they were the particles predicted by George Stoney in 1894 to be part of an atom responsible for electricity.
Thompsons Experiment to measure q/m ratio using E= v/d
Thomson's experiment involved two stages: 1- varying the magnetic field and electric fields until their opposing forces cancelled, leaving the cathode rays undeflected. By equating the magnetic and electric force equations, Thomson was able to determine the velocity of the cathode-ray particles. 2-applying the same strength magnetic field (alone) and determining the radius of the circle path travelled by the charged particles in the magnetic field Thomson combined the results and obtained the magnitude of the charge-to-mass ratio for the charged particles that constituted cathode rays.
Oppositely charged parallel plates
Uniform in strength and direction The direction of field is defined as at right angles to the plates and away from the positive plate.
Method used by Braggs to determine crystal structure
Used an x-ray tube as their x-ray source, and the x-rays travelled through a hole in a shieldwhich acted as a collimator to produce a tightly focussed beam of x-rays. The waves then reflectedthrough a crystal target which acted as a diffraction grating, and then the x-rays travelled to a sensorto analyse the interference pattern. From this they could calculate lattice separation distance, whichwas of great importance to science and understanding crystal structures.
v = fλ ∴ C = fλ
Velocity formula of electromagnetic waves.
Electromagnetic Waves
Waves produced by charges oscillating in conductors
Cooper pair
Where two electrons pair up and travel unimpeded through the crystal lattice of a superconductor. .
Heinrich Hertz
a German physicist who demonstrated the existence of electromagnetic waves after James Clerk Maxwell had predicted them. He also discovered the photoelectric effect but failed to investigate it further.
Planck's constant
a constant that relates energy and frequency for a photon: E - Itf. Equal to 6.626 x 10 3' J.s.
Silicon
a group IV element used extensively in semiconductor devices. See also Doping.
Germanium
a group N element. Originally used in semiconductor devices but now superseded by silicon as the preferred choice. See also Doping.
superconductor
a material that has almost zero resistance when it is cooled to low temperatures
phonon
a single quantum of vibrational or elastic energy
Quantum physics
along with relativity, is the foundation of modern physics. in 1900 Max Planck proposed that light came in bundles or quanta of energy. The energy was related to frequency by: E - hf. See aLso Planck; Planck's constant
Discharge tube
an evacuated glass tube containing electrodes and used to investigate the effect of passing electricity throughout gases at low pressures
Diffraction Grating
an optical component with a periodic structure which splits and defracts light into several beams traveling in different directions
Photocells
cells in which the electrons initiating an electric current are produced by the photoelectric effect.
Electron Gun
devices that produce a narrow beam of electrons in a cathode ray tube by thermionic emission
Paddle wheels
discharge tubes used to show that cathode rays carry energy and momentum.
Extrinsic conduction
electrical conduction in doped semiconductors.
Solid-state devices
electronic devices that use semiconductors rather than valves in their operation. Solid—state devices have all but replaced themtionic devices.
How Hertz measure the speed of his radio waves
he deflected some of the waves from a surface, making them travel a larger distance to a detector, and analyzed the interference patterns these waves produced with waves that traveled directly to the detector.
Conduction Band
is the lowest range of vacant electronic states
Bragg diffraction
n The diffraction of X-rays from crystals. British physicist Sir William Henry Bragg (1862-1942) and his son William Lawrence Bragg (1890-l97l) applied X~ray diffraction to the study of crystals; called X~ray crystallography.
unimpeded
not blocked; unobstructed
Positive Holes
represent the absence of an electron in an energy level. Formed when a Group [V element such as silicon is doped with a Group lll element.
Louis de Broglle
the French physicist who proposed that matter has wave characteristics It (wave particle duality). Derived the equation λ = h/mv where λ is wavelength. h is Planck's constant. m is mass and v is velocity.
Valance Band
the highest range of electron energies in which electrons are normally present at absolute zero temperature
Critical frequency/ Threshold frequency
the minimum frequency below which light will not cause the emission of electrons from a material. See also Photoelectric effect.
Drift velocity
the net velocity of electrons moving through a conductor
Striations
the patterns formed in a gas at low pressure as an electrical discharge is passed through it. See also Discharge tube.
Conduction
the process of charge moving through a medium. Conduction in metals results from the drift of a large number of electrons through the lattice