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AS Daily A Level Physics question

2025-08-17 OCR A A Level Physics 4.5.2(b) 4.5.2(f)

A filament lamp is powered by a variable DC supply. The light from this lamp illuminates a metal cathode in a photocell, causing a photoelectric current. The potential difference across the lamp is significantly increased, causing it to glow more brightly. Assume the emitted photons always have sufficient energy to cause photoemission. Which statement correctly describes and explains the effect on the photoelectric current?

  1. A The photoelectric current remains the same because the work function of the metal is constant.
  2. B The photoelectric current increases because the photons have a higher average energy, giving the photoelectrons more kinetic energy.
  3. C The photoelectric current increases because the lamp filament is hotter and emits more photons per second. (correct)
  4. D The photoelectric current decreases because the lamp's resistance increases with temperature, reducing the power dissipated as light.

Answer

The correct answer is C.

Increasing the potential difference across a filament lamp increases the current through it, leading to a large increase in the power dissipated (P=VI). This makes the filament significantly hotter and causes it to glow more brightly. A brighter light source has a higher intensity, which means it emits more photons per second. In the photoelectric effect, the magnitude of the photoelectric current is directly proportional to the rate at which photoelectrons are emitted. This rate is, in turn, directly proportional to the rate at which photons arrive (the intensity of the light), provided the photon energy is above the work function. Therefore, a brighter lamp emits more photons per second, which releases more photoelectrons per second, resulting in a larger photoelectric current.

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