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Photoelectric Effect (A level)

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For A level physics (AQA 7408, OCR A and Edexcel). Shine light on a metal plate in a vacuum photocell: change the frequency or wavelength, the intensity, the metal and the stopping potential, and watch photons release electrons only above the threshold frequency. Then practise E = hf, the photoelectric equation, the Ek(max)–f graph, the photon model and photon flux.

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Worked solution

    Key facts

    E = hf = hc/λ

    hf = φ + Ek(max)   and   eVs = Ek(max)

    threshold: hf₀ = φ, so f₀ = φ/h and λ₀ = hc/φ

    1 eV = 1.60×10−19 J

    photons per second = P/(hf) = Pλ/(hc);   I = (electrons per second) × e

    The photon model: light arrives as photons of energy hf. One photon is absorbed by one electron, all at once. The work function φ is the minimum energy needed to release an electron from the surface. Intensity sets the number of photons per second, not the energy of each, so Ek(max) depends only on f and φ. A graph of Ek(max) against f is a straight line with gradient h and intercept −φ.

    Data-sheet values: h = 6.63×10−34 J s, c = 3.00×108 m s−1, e = 1.60×10−19 C, me = 9.11×10−31 kg. Work functions used: Cs 2.1 eV, Na 2.3 eV, Ca 2.9 eV, Mg 3.7 eV, Zn 4.3 eV, Cu 4.7 eV (typical values; real surfaces vary).

    Calculated answers within 2% are marked correct. Values read from the explorer or a graph are marked correct within the tolerance given in the question.

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