A scientist investigates the photoelectric effect using a clean caesium metal surface. When light of a certain frequency is shone on the surface, electrons are emitted. The scientist then doubles the intensity of the light while keeping the frequency the same. Describe and explain what happens to the number of electrons emitted per second and the maximum kinetic energy of the emitted electrons when the intensity is doubled.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The number of electrons emitted per second increases when the intensity is doubled
Higher intensity means more photons arrive per second
Each photon can eject at most one electron, so more photons give more electrons
The maximum kinetic energy of the emitted electrons does not change
The maximum kinetic energy depends only on the photon frequency (or energy), not on intensity
Higher intensity means more photons arrive per second
Each photon can eject at most one electron, so more photons give more electrons
The maximum kinetic energy of the emitted electrons does not change
The maximum kinetic energy depends only on the photon frequency (or energy), not on intensity
Examiner tips
- Use the command words ‘describe’ and ‘explain’ – describe the change in number, explain why intensity changes photon flux, then explain why kinetic energy is unchanged because it depends on frequency
Common mistakes
- Confusing intensity with frequency, claiming kinetic energy increases, or stating that all electrons gain energy rather than only the maximum
Mark scheme (5 marks)
- The number of electrons emitted per second increases when intensity is doubled
- Higher intensity means more photons arriving per second
- Each photon interacts with one electron, so more photons cause more electrons to be emitted
- The maximum kinetic energy of the emitted electrons does not change
- Maximum kinetic energy depends only on the frequency (or energy) of each photon, not on intensity
Key terms in this question
intensity · photoelectric effect · maximum kinetic energy · frequency
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Describe and explain" question →
- Decode the mark scheme abbreviations →
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