Photoelectric effect


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Photoelectric effect

  1. 1. Quantum Phenomena<br />Photoelectric Effect<br />Work Function j<br />Photoelectric Equation<br />Wednesday, 09 September 2009<br />
  2. 2. The Photoelectric Effect<br />When u.v. light shines upon a zinc plate it causes the plate to emit electrons. We call these PHOTOELECTRONS, because they have been emitted using photons!<br />What happens if the electroscope is negatively charged?<br />u.v. light<br />The photoelectrons are emitted by the zinc plate and the electroscope slowly discharges, i.e. the gold leaves fall back down.<br />Zinc plate<br />
  3. 3. The Photoelectric Effect<br />What happens if the intensity (i.e. amplitude) of the u.v. radiation is increased?<br />The number of photoelectrons emitted per second increases, so the electrode discharges more quickly.<br />The number of photoelectrons per second is proportional to the intensity of the radiation (for any given metal). <br />u.v. light<br />Zinc plate<br />
  4. 4. The Photoelectric Effect<br />What happens if the electroscope is positively charged?<br />No loss of charge takes place. The free electrons in the zinc plate need much more energy to leave the zinc plate because of its positive charge.<br />On what does the maximum Ek of the photoelectrons depend?<br />On the frequency of the radiation and on the metal.<br />u.v. light<br />Zinc plate<br />
  5. 5. The Photoelectric Effect<br />Why couldn’t we use visible light to cause photoelectric effect in zinc?<br />Visible light is below the threshold frequency of zinc, therefore no photoelectrons can be emitted.<br />Could we increase the intensity of the light to cause photoelectricity?<br />No, because having more light of the same frequency doesn’t change the energy of the light photons, which is hf, i.e. dependent on the frequency.<br />Give a definition of threshold frequency.<br />The threshold frequency for a given metal is given as the minimum frequency of electromagnetic radiation for which photoelectric emission occurs.<br />
  6. 6. Photon energy<br />To escape the potential well of the metal, an electron must do a certain amount of work. So, a certain amount of energy must be transferred to the electron by the incident radiation. This energy is similar to the escape velocity from a planet! <br />The energy of a photon is:<br />What happens if f < threshold frequency?<br />If the energy of the photons is smaller than the work the electron needs to do to escape the potential well, the electron will be pulled back.<br />
  7. 7. The work function<br />The work function is defined as the minimum work needed to remove an electron from the surface of the metal. So, the maximumEk of the emitted photoelectron will be equal to the energy of the incident photon (hf) minus the work function (j), because the electron has to do that work in order to escape the potential from the metal. In other words, the photoelectron outside the metal will not have the same energy of the incident photon, because it has used some of that energy up to escape the metal surface.<br />
  8. 8. The work function<br />What do you think the work function depend on?<br />It depends on the metal.<br />Can you explain now why we need different frequency radiation to achieve photoelectricity in different metals?<br />Different metals have different work functions. Therefore, for a metal with lower work function a radiation with low frequency will suffice to give enough energy to the electrons to escape, but for metals with a higher work function we need higher frequency radiation to win the work function.<br />
  9. 9. Threshold frequency<br />Check again the definition of threshold frequency and work out an equation for it.<br />For electrons to escape their max K.E. has to be greater than zero. So, if we put max K.E. = 0, the previous equation becomes:<br />i.e.<br />
  10. 10. Threshold frequency<br />But, because the threshold frequency is the minimum frequency needed to produce photoelectric emission for a given metal:<br />