A student investigates the photoelectric effect using two different metals, P and Q. Metal P has a higher work function than metal Q. The student shines light of the same frequency onto both metals. Describe and explain the differences in the photoelectric emission from the two metals.

AQA A-Level Physics (7408) — 3.2.2 EM Radiation and Quantum Phenomena · Describe and explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Two metals, P and Q, are used in a photoelectric effect investigation. Metal P has a higher work function than metal Q. Light of the same frequency is shone onto each metal in turn.

Model answer (5 marks)

Electrons are emitted only from the metal whose work function is less than or equal to the photon energy. Metal Q, having the lower work function, will emit electrons and is more likely to do so than metal P. Each photon transfers all of its energy to a single electron. Therefore the maximum kinetic energy of electrons emitted from Q is greater than that from P (if P emits at all), because less energy is consumed to overcome the work function. The number of electrons emitted per second from a given metal depends on the number of incident photons (i.e. the light intensity) and not on the work function.

Examiner tips

  • Use the phrase "only from the metal whose work function is less than or equal to the photon energy" to secure the first mark.
  • Mention that Q emits electrons and P may not, to gain the second mark.
  • State that each photon gives all its energy to one electron – third mark.
  • Explain that Q’s electrons have higher KE because less energy is used for the work function – fourth mark.

Common mistakes

  • Confusing work function with electron affinity.
  • Claiming that the number of electrons depends on work function rather than light intensity.
  • Using the wrong unit for photon energy (e.g. joules instead of eV).

Mark scheme (5 marks)

  1. Electrons are only emitted from the metal whose work function is less than or equal to the energy of the photon
  2. Metal Q (lower work function) will emit electrons / is more likely to emit electrons from the given light
  3. Each photon gives all of its energy to one electron
  4. The maximum kinetic energy of emitted electrons from Q is greater than it would be from P (if P emits at all), because less energy is used to overcome the work function
  5. The number of electrons emitted per second (from the same metal) does not depend on the work function but on the number of photons / intensity of the light

Key terms in this question

work function · photoelectric effect · emission

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