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AS Daily A Level Physics question

2026-05-09 OCR A Quantum (Module 4): photoelectric effect; stopping potential; work function OCR A Physics AS, Module 4: Electrons, waves and photons — 4.3.1 The photoelectric effect (threshold frequency, stopping potential, work function) OCR A Physics AS, Module 1: Practical skills — controlling variables and interpreting measurements in photoelectric experiments

In a lab, two otherwise identical vacuum photocells X and Y are illuminated with the same monochromatic light. The lamp position is adjusted so that, at zero applied voltage, both cells have the same initial photocurrent. The stopping potentials measured to just reduce the current to zero are |Vstop,X| = 0.70 V and |Vstop,Y| = 0.40 V. Which statement must be true?

  1. A Both cathode materials have the same work function because their initial photocurrents were matched.
  2. B X has the larger work function because it needs the larger stopping potential.
  3. C Y has the larger work function because it needs the smaller stopping potential. (correct)
  4. D Increasing the light intensity on Y would increase its stopping potential to match X.

Answer

The correct answer is C.

Correct: C — Y has the larger work function because it needs the smaller stopping potential. A Matching the initial photocurrent only shows the photon arrival rate was adjusted via intensity, not that the work functions are equal. B A larger stopping potential means photoelectrons had higher maximum kinetic energy, which implies a smaller work function for X, not larger. C For the same light frequency, a larger work function leaves less energy for the electrons, so a smaller stopping potential is sufficient to halt them. D Stopping potential depends on photon energy and work function, not light intensity, so increasing intensity would not raise Y’s stopping potential.

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