AS Daily A Level Physics question
In a school lab, a clean metal surface in a vacuum tube is illuminated with monochromatic UV light of wavelength 365 nm. The stopping potential needed to reduce the photocurrent to zero is measured as 0.80 V. Using hc ≈ 1240 eV·nm, which is the best estimate for the metal’s work function?
Answer
The correct answer is B.
Correct: B — 2.6 eV. Photon energy ≈ 1240/365 ≈ 3.4 eV; the stopping potential of 0.80 V corresponds to 0.80 eV of maximum kinetic energy, so the work function ≈ 3.4 − 0.8 = 2.6 eV. A 0.8 eV — Confuses the kinetic energy at stopping with the energy needed to liberate an electron. B 2.6 eV — Subtracts the electron’s kinetic energy (from the stopping potential) from the photon energy to find the work function. C 3.4 eV — Ignores that some photon energy appears as electron kinetic energy. D 4.2 eV — Incorrectly adds the stopping potential energy to the photon energy.