OCR AS Physics

Revision Notes

4.10.2 Demonstrating the Photoelectric Effect

Demonstrating the Photoelectric Effect

  • The photoelectric effect can be observed on a gold leaf electroscope
  • A plate of metal, usually zinc, is attached to a gold leaf, which initially has a negative charge, causing it to be repelled by a central negatively charged rod
    • This causes negative charge, or electrons, to build up on the zinc plate
  • UV light is shone onto the metal plate, leading to the emission of photoelectrons
  • This causes the extra electrons on the central rod and gold leaf to be removed, so, the gold leaf begins to fall back towards the central rod
    • This is because they become less negatively charged, and hence repel less

Observations of the Gold Leaf Experiment

  • Placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly
  • Using a higher frequency light source does not change how quickly the gold leaf falls
  • Using a filament light source causes no change in the gold leaf’s position
  • Using a positively charged plate causes no change in the gold leaf’s position
  • Emission of photoelectrons happens as soon as the radiation is incident on the surface of the metal

Typical set-up of the gold leaf electroscope experiment

Explaining the Observations

  • Observation:

Placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly

  • Explanation:
    • Placing the UV source closer to the plate increases the intensity incident on the surface of the metal
    • Increasing the intensity, or brightness, of the incident radiation increases the number of photoelectrons emitted per second
    • Therefore, the gold leaf loses negative charge more rapidly

 

  • Observation:

Using a higher frequency light source does not change how quickly the gold leaf falls

  • Explanation:
    • The maximum kinetic energy of the emitted electrons increases with the frequency of the incident radiation
    • In the case of the photoelectric effect, energy and frequency are independent of the intensity of the radiation
    • So, the intensity of the incident radiation affects how quickly the gold leaf falls, not the frequency
  • Observation:

Using a filament light source causes no change in the gold leaf’s position

  • Explanation:
    • If the incident frequency is below a certain threshold frequency, no electrons are emitted, no matter the intensity of the radiation
    • A filament light source has a frequency below the threshold frequency of the metal, so, no photoelectrons are released

 

  • Observation:

Using a positively charged plate causes no change in the gold leaf’s position

  • Explanation:
    • If the plate is positively charged, that means there is an excess of positive charge on the surface of the metal plate
    • Electrons are negatively charged, so they will not be emitted unless they are on the surface of the metal
    • Any electrons emitted will be attracted back by positive charges on the surface of the metal

 

  • Observation:

Emission of photoelectrons happens as soon as the radiation is incident on the surface of the metal

  • Explanation:
    • A single photon interacts with a single electron
    • If the energy of the photon is equal to the work function of the metal, photoelectrons will be released instantaneously
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