Presentation on theme: "Caroline Chisholm College"— Presentation transcript:
1 Caroline Chisholm College PhysicsIdentify Planck’s hypothesis that radiation emitted and absorbed by the walls of a black body cavity is quantisedColours change as a body is heatedTo understand how the wavelength of the radiation varies with temperature, scientists use a ‘black body’A BLACK BODY ABSORBS ALL INCOMING RADIATION AND IS A ‘STANDARD’ FOR EXPERIMENTAL COMPARISONSe.g. an oven with a small hole drilled into the wall The walls absorb all incoming radiation and only emits (through the hole) radiation which is characteristic of the oven temperatureULTRAVIOLETCATASTROPHE!Classical wave theory suggested that as the walls get hotter and shorter wavelengths are emitted, radiation intensity should continue to increase without limitHowever, in experiment the radiation was found to have a definite peak for a given temperaturethere was always a lumpthat wasn't exceededBlack body simulationPlanck explained that the radiant energy was absorbed by atoms (until a peak was reached) andthen emitted in a ‘lump’, so energy was always a multiple of ‘lumps’ having energy E=hfKnowing c=fl and E=hf, the energy of each ‘lump’ or ‘quantum’ (a photon) could be calculated
2 Caroline Chisholm College Physics Identify the relationships between photon energy, frequency, speed of light and wavelength: E = hf and c=flCaroline Chisholm CollegePhysicsSolve problems and analyse information using: E = hf and c = flGiven the wavelength of light, you can work out the frequencyf=c/lOnce you know the frequency, you can work out the energy of each photonE = hfIf you know power of a light source in Watts (J/s),you can work out how many photons are released per second!
3 The Photoelectric Effect Caroline Chisholm CollegePhysicsThis occurs when electrons are released from a surfacewhich is exposed to electromagnetic radiationHertz first noticed the effect when he observed that stronger sparks were produced in the receiving loopwhen it was exposed to UV radiation from the transmitter. Materials which blocked UV reduced the sparkJ. J. Thomson set up a metal surface and a collector in a vacuum tube and detected current flowwhen light was directed onto the metal surface - photoelectrons were being liberatedLenard found that when he applied a negative voltage to the collector,for a given frequency of light there was adefinite voltage that would stop the electron flow (current)SO THE LIGHT FREQUENCY DETERMINED THE K.E. OF THE ELECTRONSIncreasing the frequency of light increased theKE of the electrons and hence the required stopping voltageIncreasing the light intensity did not change the KE of the electronsDoubling the light intensity doubled the currentINTERNET SIMULATION
4 Caroline Chisholm College Physics Explain the particle model of light in terms of photons with particular energy and frequencyIdentify data sources, gather, process and analyse information and use available evidence to assess Einstein’s contribution to quanta and their relation to black body radiationCaroline Chisholm CollegePhysicsClassical Physics predicted an absorption of light energy, regardless of frequency, until there was enough KE to liberate the electron. Experiment showed that different frequencies liberated electrons with different K.E’sIdentify Einstein’s contribution to quanta and its relation to black body radiationEinstein used Planck's particle theory to propose the following:LIGHTE=hfPHOTONSFrom Planck’s work with black bodies:MORE PHOTONSMORE LIGHT INTENSITYHIGH FREQ. LIGHTHIGH ENERGY PHOTONSThe energy required to release the electron is called the WORK FUNCTIONIf the photon has more energy than the work function,the electron will have KE equal to the excessThe frequency required to overcome theWork Function is the threshold frequencyPhoton Energy, E = hf = W + (K.E)electron
6 Caroline Chisholm College PhysicsProcess information to discuss Einstein and Planck’s differing views about whether science research is removed from social and political forcesPhysics FactJacaranda p.216
7 Caroline Chisholm College The following diagram shows light incident upon a cathode in a discharge tube.Caroline Chisholm CollegePhysics
8 Caroline Chisholm College Identify data sources gather, process and present information to summarise the use of the photoelectric effect in: – solar cells – photocellsCaroline Chisholm CollegePhysicsPhotocellsSUNLIGHT(LIGHT ENERGY)ELECTRICALENERGYUSING THE PHOTOELECTRIC EFFECTA type of photocell where light is focused onto the junction ofp-type silicon (not exposed to light) and n-type silicon (exposed).The photoelectric effect causes photoelectrons to be released which arecollected by a fine metal grid on the n-type silicon surface.Electrons (electricity) flow from n-type to p-type silicon via a circuit.Solar cellsBreathalysersIn a photoconductive cell, the photoelectric effect causes additionalelectrons to be released which increases conductivity (decreasesresistance) i.e. a material may conduct electricity in light but not in dark.In a breathalyser a light source is shone through condensed alcoholvapour - more alcohol means less light is transmitted to the detectorso the current (and hence the reading) changes