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Presentation On A-level Photoelectric

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Published in: Physics
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PPT on Photoelectric emission for A level Physics

Areesha A / Dubai

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  1. EFFECT
  2. Photoelectric Effect What is it : When metal surfaces are exposed to electromagnetic radiation with sufficient energy they absorb the photons of energy and emit electrons. This process is called the photoelectric effect. • How did it all start? • Henrich Hertz was the first to discover this phenomena in 1887 when he was investigating radio waves. • In 1901 Max Planck showed that energy is quantized, E=hf. • Albert Einstein explained the photoelectric effect in 1905. 2
  3. N/A
  4. The effect of light on a metal surface The photo-electric effect can be demonstrated by means of an ultraviolet lamp, a zinc plate, an electroscope and two ordinary light bulbs of 40 W and 200 W. ultraviolet lamp Zn plate 40 W white light lightbulb Zn plate 200 W white ultraviolet lamp light lightbulb Zn plate Zn plate ultraviolet lamp Zn plate The photo-electric effect is investigated
  5. Photoelectric effect 5 U-V photons Go to 9 Start with an electroscope that is charged negatively. Polished zinc The U-V light causes photoelectrons to be emitted. These are repelled by the surface and escape. Charge is lost by the electroscope so the leaf falls. Back to 6 Back to 3
  6. Why? Photoelectric effect 6 Why do ultraviolet photons liberate photoelectrons whilst visible light photons do not? Answer: none of the photons in white light has enough energy to release even one photoelectron The energy of a photon is given by: E = hf where h is Planck's constant and f is the frequency. Also E = because c = n (the wave equation) This means that the higher the frequency, the greater the energy. Visible light contains frequencies that are too IOW for photoelectric emission. Alternatively, the shorter the wavelength, the greater the energy of the photons. Visible wavelengths are too long. Back to 3
  7. E photon = hv = 6.22x105 m/s 700 nm 1.77 ev no electrons 550 nm 2.25 ev max = mis 400 nm 3.1 ev Potassium - 2.0 ev needed to eject electron Photoelectric effect 7
  8. Einstein's photoelectric equation During the photon-electron interaction, energy must be conserved. Einstein equated the energy of the incident photon with the energy required to release the electron, to produce the photoelectric equation where is the work function of the metal, and the maximum kinetic energy of the released electron. It is a maximum because some electrons may be closer to the nucleus, requiring more energy than the work function amount to be released, leaving less energy left over as kinetic energy. For the equation to work, all three quantities must be given in the same unit — when completing calculations, ensure that all values have been correctly converted 8
  9. Observations Ultraviolet light causes a negatively charged electroscope to discharge — the leaves of the electroscope collapse when UV light shines on it. White light does not release e- from the zinc plate even when irradiated with light of a much higher intensity or for a longer period. When the electroscope is positively charged nothing happens because it is much more difficult to remove e- from a positive object. 9
  10. • When a glass plate is placed between the ultraviolet source and the zinc plate, the electroscope stops discharging. CONCLUSION 1. 2. 3. 4. Photoelectrons are emitted for a specific metal if the frequency of radiation exceeds a certain limit (threshold frequency, fo). The rate of photoelectron emission for a single frequency radiation beam is proportional to the intensity of radiation i.e. the more intense the radiation of the same frequency the more photoelectrons are emitted. The emitted photoelectrons have kinetic energy ranging from zero to a maximum. Maximum kinetic energy depends on frequency. 10
  11. 5. 6. 7. The intensity of radiation has no effect on the kinetic energy of the emitted photoelectrons. Emission starts as soon as the surface is irradiated with effective radiation. Photoelectric current depends on intensity. 11
  12. Graph of KE of electron and frequency of incident light on metal slope = h frequency threshold frequency 12
  13. WHY IS THE PHOTOELECTRIC EFFECT SO IMPORTANT? It helped explain the particle nature of light. It is the basis of the quantum theory. It is used in photocells e.g. in solar calculators, alarms and automatic door openers 13
  14. The Dual Nature of Light What is light — a wave or a particle? The wave theory cannot explain all the known facts in connection with light. Diffraction and interference can only be explained by the wave theory. The quantum hypothesis offers an excellent explanation for the photo-electric effect but use the concept of frequency to calculate the energy of a photon. Light has both a wave- and particle nature. The wave nature predominates during the propagation of radiation, while the particle nature predominates during the interaction with matter. 14
  15. A plications of the photoelectric • T le photoelectric effect has many practical applications which include the photocell, photoconductive devices and solar cells A photocell A photocell is usually a vacuum tube with two electrodes. One is a photosensitive cathode which emits electrons when exposed to light and the other is an anode which is maintained at a positive voltage with respect to the cathode. Thus when light shines on the cathode, electrons are attracted to the anode and an electron current flows in the tube from cathode to anode. The current can be used to operate a relay, which might turn a motor on to open a door or ring a bell in an alarm system 15
  16. eresponsive to light, as described above, or sensitive to the removal of light as when a beam of light incident on the cathode is interrupted, causing the current to stop. Photocells are also useful as exposure meters for cameras in which case the current in the tube would be measured directly on a sensitive meter. The photocell is at the centre of the many applications of the photoelectric effect. It consists of a curved emitter and a rod as collector, so as not to inhibit light from reaching the emitter. 16
  17. The structure of a typical photocell is shown below: he flash of a camera uses the photoelectric effect emitter Photocell ultra violet Photoelectric Effect collector
  18. Photocells are used in garage door openers. n example is shown in the diagram below: Infrared ligtt beam enitter 18
  19. Spacecraft The photoelectric effect will cause spacecraft xposed to sunlight to develop a positive charge. his can get up to the tens of volts. This can be a major problem, as other parts of the spacecraft in shadow develop a negative charge (up to several kilovolts) from nearby plasma, and the imbalance can discharge through delicate electrical components. The static charge created by the photoelectric effect is self-limiting, though, because a more highly-charged object gives up its electrons less easily 19
  20. Closely related to the photoelectric effect is the hotoc ndu iv eff which is the increase in le tri I nd ivi of certain non metallic materials xposed to light. This effect n be quite large so that a very small current in a device suddenly becomes quite large when exposed to light. Thus photoconductive devices have many of the same uses as photocells. Solar-cells=usually made from specially prepared silicon, act like a battery when exposed to light. Individual solar cells produce voltages of about 0.6 volts but higher voltages and large currents can be obtained by appropriately connecting many solar cells together. 20
  21. EMISSION SPECTRA CONTINt'ous SPECTRUM BRIGHT 1 INF SPECTRA cont. spectra LAMP What caused spectra of atoms to contain discrete "lines" • it was apparent that only a small set of optical frequencies (wavelengths) could be emitted or absorbed by atoms Each atom has a distinct "fingerprint" • Light only comes off at very specific wavelengths • or frequencies • or energies Note that hydrogen (bottom), with only one electron and one proton, emits several wavelengths
  22. Electron Diffraction https://www.youtube.com/watch?v=CP dZZkE19GA https://www.youtube.com/watch?v=IYn U4T3jbgA 22
  23. Electron diffraction. AQAA Level Physics Unit 1 Particle Physics Lesson 13 Electron Diffraction and de Broglie 0 4:01 16:51
  24. Let's start with photon energy Light is quantized into packets called photons Photons have associated: frequency, v (nu) wavelength, = c) speed, c (always) energy: higher frequency photons higher energy more damaging spring 2008 24
  25. Quantum Wavelength Every particle or system of particles can be defined in quantum mechanical terms • and therefore have wave-like properties The quantum wavelength of an object is: (p is momentum) called the de Broglie wavelength typical macroscopic objects masses kg; velocities m/s -+ p- 1 kg•m/s • 10-34 meters (too small to matter in macro environment!!) typical "quantum" objects: • electron (10-30 kg) at thermal velocity (105 m/s) —Y 10-8 m spring is 100 times larger than an atom: very relevant to an 25

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