A2 Daily A Level Physics question
In a lab, students measure the stopping potential for photoelectrons from a clean zinc plate using ultraviolet light of frequency 1.20×10^15 Hz. The stopping potential is about 0.67 V. Zinc’s work function is 4.3 eV. If they increase the light frequency by 10% while keeping the intensity the same, what stopping potential should they expect (nearest value)? Assume the surface remains clean and contact potentials are negligible.
Answer
The correct answer is C.
Correct: C — About 1.17 V (about a 75% increase). Photon energy scales with frequency, but a fixed amount (the work function, 4.3 eV) is needed for emission; the remainder sets the stopping potential, so V2 ≈ 1.1×0.67 V + 0.1×4.3 V ≈ 0.74 V + 0.43 V ≈ 1.17 V. A The 10% increase idea ignores the fixed energy needed to escape; because of the offset, the rise is much larger than 10%. B Stopping potential depends on the maximum electron energy, set by photon energy and work function, not on intensity (intensity mainly affects current). C This uses the correct reasoning: the extra 10% applies to the whole photon energy, then subtract the fixed work function, giving ≈1.17 V. D This adds only 10% of the work function and forgets that the kinetic-energy term (and thus the 0.67 V part) also scales with the 10% increase in photon energy.