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Caroline Chisholm College

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1 Caroline Chisholm College
Physics Identify Planck’s hypothesis that radiation emitted and absorbed by the walls of a black body cavity is quantised Colours change as a body is heated To 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 COMPARISONS e.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 temperature ULTRAVIOLET CATASTROPHE! Classical wave theory suggested that as the walls get hotter and shorter wavelengths are emitted, radiation intensity should continue to increase without limit However, in experiment the radiation was found to have a definite peak for a given temperature there was always a lump that wasn't exceeded Black body simulation Planck explained that the radiant energy was absorbed by atoms (until a peak was reached) and then emitted in a ‘lump’, so energy was always a multiple of ‘lumps’ having energy E=hf Knowing c=fl and E=hf, the energy of each ‘lump’ or ‘quantum’ (a photon) could be calculated

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Identify the relationships between photon energy, frequency, speed of light and wavelength: E = hf and c=fl Caroline Chisholm College Physics Solve problems and analyse information using: E = hf and c = fl Given the wavelength of light, you can work out the frequency f=c/l Once you know the frequency, you can work out the energy of each photon E = hf If 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 College Physics This occurs when electrons are released from a surface which is exposed to electromagnetic radiation Hertz first noticed the effect when he observed that stronger sparks were produced in the receiving loop when it was exposed to UV radiation from the transmitter. Materials which blocked UV reduced the spark J. J. Thomson set up a metal surface and a collector in a vacuum tube and detected current flow when light was directed onto the metal surface - photoelectrons were being liberated Lenard found that when he applied a negative voltage to the collector, for a given frequency of light there was a definite voltage that would stop the electron flow (current) SO THE LIGHT FREQUENCY DETERMINED THE K.E. OF THE ELECTRONS Increasing the frequency of light increased the KE of the electrons and hence the required stopping voltage Increasing the light intensity did not change the KE of the electrons Doubling the light intensity doubled the current INTERNET SIMULATION

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Explain the particle model of light in terms of photons with particular energy and frequency Identify data sources, gather, process and analyse information and use available evidence to assess Einstein’s contribution to quanta and their relation to black body radiation Caroline Chisholm College Physics Classical 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’s Identify Einstein’s contribution to quanta and its relation to black body radiation Einstein used Planck's particle theory to propose the following: LIGHT E=hf PHOTONS From Planck’s work with black bodies: MORE PHOTONS MORE LIGHT INTENSITY HIGH FREQ. LIGHT HIGH ENERGY PHOTONS The energy required to release the electron is called the WORK FUNCTION If the photon has more energy than the work function, the electron will have KE equal to the excess The frequency required to overcome the Work Function is the threshold frequency Photon Energy, E = hf = W + (K.E)electron

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Physics Process information to discuss Einstein and Planck’s differing views about whether science research is removed from social and political forces Physics Fact Jacaranda p.216

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The following diagram shows light incident upon a cathode in a discharge tube. Caroline Chisholm College Physics

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Identify data sources gather, process and present information to summarise the use of the photoelectric effect in: – solar cells – photocells Caroline Chisholm College Physics Photocells SUNLIGHT (LIGHT ENERGY) ELECTRICAL ENERGY USING THE PHOTOELECTRIC EFFECT A type of photocell where light is focused onto the junction of p-type silicon (not exposed to light) and n-type silicon (exposed). The photoelectric effect causes photoelectrons to be released which are collected 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 cells Breathalysers In a photoconductive cell, the photoelectric effect causes additional electrons to be released which increases conductivity (decreases resistance) i.e. a material may conduct electricity in light but not in dark. In a breathalyser a light source is shone through condensed alcohol vapour - more alcohol means less light is transmitted to the detector so the current (and hence the reading) changes

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