Demonstrating the Photoelectric Effect (OCR AS Physics)

• 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