Physics 9.3 – From Ideas to Implementation1. Increased understandings of cathode rays led to the development of television.1.
Explain that cathode ray tubes allowed the manipulation of a stream of charged particles.A cathode ray tube is a glass tube with an electrode attached to each end, and by use of a vacuum pump, most of the air evacuated to create a low atmosphere system. When a high voltage is passed through the electrodes, a current is produced and mysterious pale green lights are seen on the glass ??“ thus we know ???cathode rays??™ are passing from the cathode to the anode.Light caused by cathode rays striking air particle in the tube that becomes excited and releases a photon of visible light, which forms in strips or striations depending on the pressure. The gas type changes the colour of the striations.
A cathode ray tube allows the manipulation of the cathode ray because:The pressure and voltage can be varied, and objects can be placed around the tube (electric and magnetic plates) or inside (Maltese cross, paddle wheel.)In everyday life, cathode ray tubes are used in neon signs. A small amount of gas is in the tube, which, when excited by a high potential, glows with a certain colour (neon = red)PRAC 1: Perform an investigation and gather first-hand information to observe the occurrence of different striation patterns for different pressures in discharge tubes.Apparatus: ??? A set of glass discharge tubes at different pressures are arranged on a board??? Each tube contains an electrode at each end to allow the application of a large voltage, provided by the induction coil. 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??? Under normal conditions of temperature and pressure, air is an insulator, but at reduced pressure, air and all other gases conduct electricity.
Observations:??? At 5% atmospheric pressure, long thin red-purple streamers appear between the two electrodes??? At lower pressure, a soft red glow appears.??? At even lower pressure, the glow is broken into striations, bands of light and dark; the amount of dark space between the glowing bands increases with further reductions in pressure.??? At 0.01% pressure, Crookes??™ dark space extends throughout the whole tube; the glass near the anode glows a yellow-green colour.[pic]Explanation:In discharge tubes, as you lower the pressure, you are removing particles that electrons would have collided with.
Electrons can accelerate to faster speeds before colliding with gas particles, meaning they have more energy to transfer when they collide, resulting in different striation patterns.Cathode rays collide with gas, metal electrodes, glass or phosphorescent materials placed in their path, and as they strike excited atoms, electromagnetic radiation is emitted as heat, light, UV and X-rays. The tube walls glow as energy is released by electrons when they collide with the walls.Elastic collisions occur in dark spaces, and kinetic energy is conserved.
Therefore, no packets of energy are released. The electrons haven??™t accelerated enough to get energy to give.Inelastic collisions occur in light spaces – Kinetic energy is changed into other forms of energy (light, heat etc.) Therefore packets of energy are released or transferred.
Electrons in valence shells go up, and then drop down a shell. They absorb energy to jump up, then emit it as they go back down, and this is seen as coloured gas.So, in light spaces, electrons give away an energy packet; and in dark spaces, they gain an energy packet. Depending on the gas and amount of energy supplied, there will be different coloured gases.2. Explain why the apparent inconsistent behaviour of cathode rays caused debate as to whether they were charged particles or electromagnetic waves.
In the 19th century, it was observed that the anode glowed with a pale green light ??“ fluorescence originating at the cathode. They were thus named cathode rays by Eugene Goldstein. This observation was due to the air particles being ionised and attracted to the respective electrodes, where they deposit their charge.Various properties of cathode rays were discovered, and these caused a debate between German researchers (electromagnetic wave theory) and English researchers (particle theory.)Electromagnetic Wave theory ??? The rays are identical regardless of the material used ??? Travel in straight lines ??? Causes glass to fluoresce ??? Can cause chemical reactions ??“ eg: photographic plates ??? Can penetrate thin metal foils ??? Could heat materials they struckParticle theory ??? Can be deflected by magnetic fields ??? Can be deflected by electric fields ??? Rays carry energy and momentum and can do work ??? Ray are attracted to positive charges ??? Rays deposit a negative charge on impact ??? Travelled much slower than lightThe debate was finally settled by J.J Thompson in 1887 who showed that the rays could be deflected by an electric field, and thus were a negatively charged particle ??“ electrons.Hertz had ???showed??™ that cathode rays weren??™t deflected by electric fields (he was incorrect, the deflections were just so small he couldn??™t see them) and concluded they weren??™t charged particles.PRAC 2: 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 wheelAnalyse the information gathered to determine the sign of the charge on cathode rays.
Maltese cross: a Maltese cross was placed in the path of the cathode rays, a shadow of the cross was observed indicating cathode rays travel in straight lines like light and are absorbed or reflected by solid objects.Electric plates: rays were deflected towards the positive plate, showing the rays have a negative charge.Fluorescent display: Shows cathode rays can cause fluorescence and therefore have energyThe screen can also be used to trace the path of rays being manipulated by other means.Glass paddle wheel: when struck by the cathode rays, the wheel slowly rotated showing that the rays can do work and have mass/momentum, and that the ray was produced from the cathode.i.
e. the cathode ray has a negative charge3. Identify that charged plates produce an electric field.
Charge plates produce a uniform electric field (except near the edges), thus the field runs from positive to negative and deflects a negative particle (cathode ray) toward the positive plate.Strength of the field is given by E = V/d4. Describe quantitatively the electric field due to oppositely charged parallel plates.The field due to oppositely charge plates is determined by:[pic] where E = electric field in Vm-1, V = voltage in volts, d = distance in metresForce due to a chargeStrength of a field is defined as the force a 1 coulomb charge would experience at that point.F = Eq Force in newtons, Field in Vm-1, and q is charge in coulombs5. Identify that moving charged particles in a magnetic field experience a force.An electric field exists in any region where a charged object experiences a force. Charged plates exert a force on charged objects brought close to them because their movement fives rise to a second magnetic field.
It is the two magnetic fields interacting which produces forces that change the motion of the charge.E = F/q ( electric field strength in N/CV=w/q ( potential difference – the work done moving a charge between 2 points in an electric field, per unit charge.6. Discuss qualitatively the electric field strength due to a point charge, positive and negative charges and oppositely charged parallel plates.Positive point charge Negative point chargeStrength of electric field diminishes with distance from the charge, E (7. Describe quantitatively the force acting on a charge moving through a magnetic field [pic]For a charged particle moving parallel to the magnetic field, ( = 0o and F=0For a charged particle moving perpendicular to the magnetic field, ( = 90o and F= maximumElectrons follow a circular path, because the magnetic field is always acting perpendicularly to the electron??™s velocity.So, FB = FCqvB =r = (r = radius of the curvature of the particle)PRAC 3: Solve problem and analyse information using:[pic]Find the force of an electron of charge -4×10-18 C enters a field of 0.8T at 120m/s.
[pic] = 120 x 0.8 x 4×10-18 = 3.84 x10-16N clockwiseFind the electric field strength if 10 000V is applied across 1mm. = 10 000 / 0.001 = 10 000 000 Vm-1Find the force if a 2C charge is placed in this field = 2 x 10 000 000 = 20 000 000 NC-18. Outline Thomson??™s experiment to measure the charge/mass ratio of an electron.Joseph Thomson incorporated electric charged plates and a magnetic field into the cathode ray tube. He verified Crookes??™ hypothesis that cathode rays could be deflected by electric fields.
In his experiment he attempted to measure the charge to mass ratio of the cathode ray particle ??“ the electron. Using a cathode ray tube, he deflected cathode rats using an electric field, and straightened their path using an electromagnetic field, (cancelling each other out) equating their forces and finding the velocity of the cathode ray particles.He then deflected them using only the magnetic field and traced their path on a fluorescent screen, measured the radius of their circular path and the calculated the charge/mass ratio.His results were combined to find the charge to mass ratio:FB = FE Fc = FBqvB = qE mv2/r = qvBv = E/B q/m = v/rBThis discovery was important because the charge to mass ratio was found to be the same for all cathode rays regardless of the medium: 1.
76 x 10-11Ckg-1This was also found to be 1800 times greater than for hydrogen ions , and showed that the atom was divisible and cathode rays were subatomic particles.He also assumed they were the particles responsible for electricity.9. Outline the role of:??“ electrodes in the electron gun??“ the deflection plates or coils??“ the fluorescent screenin the cathode ray tube of conventional T.V. displays and oscilloscopes.
Electron gun:Consists of a filament, a cathode and two open cylindrical anodesElectrons are emitted from the cathode by heating it to high temperatures, causing the electrons to boil off the filament (thermionic emission.) These are then accelerated and focused by the anodes. The flow of electrons is controlled by the grid (a ring shaped electrode), often negative to reduce the number of electrons and therefore the brightness.Deflection plates or coils:Two pairs of parallel plates are used to deflect the electron flow to the required x and y coordinates by means of varying the potential difference between the plates.
Often an alternating current is used to quickly vary the deflection making it seem like a lineCoils do the same job as the plates except they use magnetic fields as opposed to electric fields and current will determine the amount of deflection.Y plates control vertical deflection: X plates control horizontal deflection:Fluorescent screenThe inside of the glass is coated in a fluorescent material such as zinc sulphide. When the screen is hit by an electron beam, the coating will fluoresce and light is emitted at that point, thus providing accuracy for the location of the particles2. The reconceptualisation of the model of light led to an understanding of the photoelectric effect and black body radiation.
1. 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 used an induction coil used to generate a high frequency spark. He observed that discharges in one coil produced discharges in a second coil about 1.
5m away. He proposed that an invisible radiation produced by the discharges in his first coil travelled through the air to the second coil and produced the second discharges as a result.This showed that these ???long radio waves??™ could be reflected, refracted, interfered with, polarised and diffracted ??“ just like light.Using the wave equation, and by measuring the wave length, he predicted the speed: 3*108ms-1, proving these waves were like light.Hertz also found that when ultraviolet light was shone onto the spark gap, it became polarised – the spark became brighter and jumped more readily. He didn??™t investigate this but later it was found that UV light caused electrons to be ejected from the surface of the metal.
Hertz realised that light and electricity were connected in some way – the photoelectric effect.PRAC 1: Perform an investigation to demonstrate the production and reception of radio waves.An induction (transmitter) coil was used to generate sparks. An AM radio was placed 3m away but off station, to act as a receiver.A buzz was heard from the radio in response to each spark.An accelerating charge is needed at the transmitter which generates radio waves which travel out at the speed of light.
A charge experiences a force from the radio wave which sets it moving back and forth in the receiver.2. Outline qualitatively Hertz??™s experiments in measuring the speed of radio waves and how they relate to light waves.In 1887 Hertz studied the radiation associated with electrical discharges. He calculated the wavelength of his radiation by studying the interference pattens produced when one ray, reflected off a metal plate to a detector was superimposed on a ray that travelled directly to the detector. Knowing the frequency of the oscillator producing the sparks, he used V = f? to find the rays speed. He found this to be equal to the speed of light, proving them to be part of the electromagnetic spectrum.
Spark generator Detector Reflecting Plate3. Identify Planck??™s hypothesis that radiation emitted and absorbed by the walls of a black body cavity is quantised.Around the 1860??™s many scientists started observing the light given out by hot.
Incandescent metals such as tungsten. The observed that hot materials, no matter their composition, emitted radiations of varying wavelengths that depended not on the substance, but on the temperature of the substance. Plank proposed a model to explain black body radiation at all temperatures.
(Radiation radiated from a hot body that does not reflect light – absorbs all EMR.) Classical physics predicated that as the wavelength emitted decreased, the intensity of the emission would increase indefinitely because the more wavelengths, the more oscillations and therefore greater emission of waves. However, experimental data confirmed this was untrue ??“ ultraviolet catastrophe.This difficulty was overcome in 1900 when Max Planck proposed that an atom could be only be given energy in a specific whole number ??“ a multiple of a minimum/quanta of energy. This meant that an atom could only absorb energy in a set amount (not limitlessly ??“ and therefore unlimited intensity), and thus decreasing the wavelength would not mean infinite oscillations, but just more atoms oscillating and therefore a limited level of intensity.The electromagnetic energy associated with the oscillation of the atoms was quantised.
It could only have energy values consistent with the equation E = hf.An atom could absorb or release only integral number of quanta of energy. Quanta were absorbed or emitted only when an atom changed from one quantised energy level to a different one. If the atom did not change quanta levels, it could neither absorb nor emit energy..4. Identify Einstein??™s contribution to quantum theory and its relation to black body radiation.Einstein was the first to give quantum theory a real basis for acceptance in is 1905 paper on the photoelectric effect.
He incorporated Planck??™s idea of quantised energy and suggested light consisted of photons with a quantum of energy given by E = hf. He used Planck??™s idea of quanta as developed from black body radiation experiments to explain the photoelectric effect, which classical wave theory had been unable to do. Einstein??™s suggestions, and the equation he produced to explain the photoelectric effect, were to prove a stunning success in explaining the observed results in experiments on the photoelectric effect. Einstein??™s explanation had shown that the emission of a photoelectron was related to the frequency of the photons rather than the intensity of the light (as predicted by classical physics) this clearly supported Plank??™s idea of quantised energy and created a change in scientific thinking, giving quantum theory much more credibility in the scientific world.The results if the photoelectric effect cast grave doubt on the wave model of light that had been previously so successful in explaining the properties of light including interference and diffraction. The genius of Albert Einstein was required to give an adequate explanation of the phenomenon. The basis of his explanation lay in Planck??™s quantum idea, but Einstein expanded this concept.Adopting a particle model in conjunction with Planck??™s hypothesis, Einstein proposed that: 1.
The energy of light is not evenly spread out over the wavefront, but concentrated in ???bundles??™ or ???packets??™ of energy ??“ photons. 2. Each photon has energy given by Planck??™s relationship E=hf 3. A photon could give up all or none of its energy to one electron, but it could not give only part of it. 4. The maximum kinetic energy of the emitted electron was equal to the initial photon energy minus the work done in overcoming the attractive forces near the surface Ekmax = hf ??“ O = qVFrom this, Einstein predicted that different surfaces would have similar graphs or gradients of h.
PRAC 2: Identify data sources, gather, process and analyse information and use available evidence to assess Einstein??™s contribution to quantum theory and its relation to black body radiation.Einstein successfully showed how ???light??™ was made up of a stream of particles with a quantum of energy E=hf now called a photon. However, some credit must go to Planck as Einstein used Planck??™s idea of a quantum of energy and his equation.
Einstein also explained black body radiation as photons of light being absorbed by atoms of the black body if they had the correct amount of energy and also then emitted a photon of light of a particular frequency. However, Planck had originally made the first explanation of black body radiation by saying the ???atomic oscillators??™ were responsible for absorbing and emitting a quanta of energy.Einstein made the most significant discovery that light was a stream of particles called photons which had a quantum of energy. He solved the debate about the nature of light, and so gained the Nobel Prize for the photoelectric effect which showed the dual nature of light and explained black body radiation.
PRAC 5: Process information to discuss Einstein and Planck??™s differing views about whether science research is removed from social and political forces.Governments can demand the right to decide what their search grants will be spent on. So political agenda can drive University research and lead to universities taking on research for the military. Should scientific research be dedicated to human needs and universities have independence, to put at the disposal of politicians, industry or military which could have devastating consequences if they were headed by dictators of militarists.
Planck was a nationalist German. In the fervour of the first few weeks of WWI Planck signed his name to the ???manifesto of the 93 German intellectuals to the civilised world.??? This was a formal declaration that the leaders of the German Art and Sciences supported acts of the German army. He then supported research to support the war effort.Albert Einstein was a German-Jew. In contrast he was not loyal to any government.
Einstein signed a rival document ???manifesto to the European??? which was an anti-war manifesto. Einstein saw clearly the role of science as something for the good of man that was not to be manipulated for the good of the state.Planck and Einstein had their differences however they were in fact friends. Einstein had made use of Planck??™s quanta to explain the photoelectric effect. The emerging Nazi party openly pursued a policy of anti-Semitism. Nazi physicists and their followers violently denounced Einstein??™s theory of relativity as Jewish-communist physics.
Planck tried to convince Hitler forced emigration of the Jews was bad for German science and that Jews could be good Germans. However Hitler disregarded him. After the war Einstein fought hard to have the rights of German scientists returned.The war and the Nazi atrocities did prompt Einstein to act politically, despite his best instincts, to encourage Roosevelt to direct necessary fund for the scientific research to develop a nuclear weapon before the Nazis. Einstein is noted for his total opposition to all war and went on to warn of the dangers involved in the creation of a nuclear weapon and the devastating energy that would be released.
He regretted the consequences from this research, clearly showing his ongoing belief that scientific research should be free from social and political forces.5. Explain the particle model of light in terms of photons with particular energy and frequency.6. Identify the relationships between photon energy, frequency, speed of light and wavelength:[pic]Einstein generalised Planck??™s ideas to all electromagnetic radiation. The worry with this was that it implied that light consisted of particles ??“ photons.
Each photon carried energy directly proportional to its frequency and given by Planck??™s equation E = hf. This did not sit well with classical wave theory, and did not fit in with the wave properties exhibited by light (diffraction, interference.) this model started some scientists thinking that there must be some truth in quantum theory and so took it more seriously and looked for other applications, which further strengthened the validity of the theory.Photon Explanation:1.
Intensity is measured by the number of photons per unit area. As intensity is increased, number of photons is increased and thus photocurrent.2. A photon transfers all its energy to the electron. Provided this energy is greater than the work function, the electron will leave instantaneously. If hf < O then the electron will not be emitted.
3. Frequency ??“ since the energy of the photon is dependant on frequency, if the frequency is increased then the ejected electron will have a greater kinetic energy. If frequency is not high enough, the electron will not be emitted.4.
Threshold frequency is the minimum frequency required for emission; where hf = OPRAC 3: Identify data sources, gather, process and present information to summarise the use of the photoelectric effect in photocells.Photocells are based on cathode ray tube technology and use the photoelectric effect directly to produce an 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 relatively large size (compared to solar cells) of photocells and their sensitivity to shock led them to be replaced by more robust solar cells using semi-conductors made from silicon.Solar cells also use the photoelectric effect to convert the energy from sunlight into electrical energy. However, rather than relying on the sensitive and breakable vacuum tubes in earlier photocells, solar cells use semi-conductor technology. Free electrons in the n-type drift across to the p-type material (due to charges.) this creates a potential difference ??“ an electric field ??“ between the semi-conductors, preventing more electrons flowing into the p-type semiconductor. When (U.V.) sunlight falls on a junction between n-type and p-type semiconductor material, electrons in the n-type material are ejected around the circuit from atoms due to the photoelectric effect.
Electrons in the junction move up to fill the holes in the n-type material, and electrons from the p-type then move up to fill these holes, and the process repeats to form a direct electric current.Solar cells are often assembled into modules designed to produce a certain current at the design voltage. The current produced depends directly on the amount of light striking the module. Typically, simple solar cells convert around ten to fifteen percent of the incoming solar energy into useful electrical energy. This efficiency can be increased by using two or more materials sensitive to light of different frequencies in tandem, in order to utilize more of the Suns spectrum, or by using focusing devices to concentrate the sunlight and converting >37% into energy.
The 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.[pic]———————–[pic][pic][pic][pic][pic]