AQA AS Level Physics Paper 2, November 2021: Question 21

1 mark · Medium difficulty · Multiple Choice

Identify which graph shows the variation of maximum kinetic energy with frequency for a metal with a greater work function.

Practise this question

Question

A multiple-choice physics question showing an initial graph of maximum kinetic energy E_k(max) against frequency f, with threshold frequency intercept P and work function intercept Q. Four alternative graphs labelled A, B, C, and D are presented below with corresponding selection boxes.
Question text

21 The graph shows how the maximum kinetic energy Ek(max) of photoelectrons emitted

from a metal surface varies with the frequency f of the incident radiation.

P is the intercept on the f axis. Q is the intercept on the Ek(max) axis.

Which graph shows the variation of Ek(max) with f for a metal with a greater work function?

[1 mark]

A B

C D

A

B

C

D

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer is option C, accompanied by a small reference diagram showing the correct graph trend.

21 C

How to answer it

Interpreting Photoelectric Effect Graphs

What this question tests

This question assesses your understanding of the photoelectric equation, specifically how changes in the work function of a metal affect the graphical relationship between maximum kinetic energy (Ek(max)) and frequency (f) of incident radiation, including the behavior of intercepts and gradients.

Question 2.1

Identifying the Effect of a Greater Work Function

✅ Correct Answer

C

Awarded 1 mark for selecting option C.

💡 Key Knowledge

  • The photoelectric equation is given by: Ek(max) = hf - φ
  • Comparing this to the equation of a straight line ( y = mx + c ):
    • y-axis = Ek(max)
    • x-axis = frequency f
    • Gradient m = Planck's constant h
    • y-intercept = -φ (negative work function)
  • Threshold frequency f₀ is the x-intercept, where hf₀ = φ .

🧠 Exam Technique

Analyze parameters one by one:

  • Gradient: Since Planck's constant h is a universal constant, the gradient of the line must remain unchanged for all metals. This immediately rules out options with different steepness.
  • Work function ( φ ) increases:
    • The y-intercept ( -φ ) becomes more negative (moves further down the vertical axis).
    • The threshold frequency ( f₀ = φ / h ) increases, meaning the x-intercept P moves further to the right.

❌ Common Errors

  • Confusing intercepts: Students often mistake how a larger work function shifts the threshold frequency, accidentally picking graphs where the x-intercept moves left (decreasing f₀ ).
  • Changing the gradient: Some candidates incorrectly assume that changing the metal alters the steepness of the line, forgetting that the gradient is always Planck's constant h .

Topics

Physics · 3.2 Particles and radiation

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.