AQA A-Level Physics Paper 1, June 2024: Question 10

1 mark ยท Medium difficulty ยท Multiple Choice

Determine how the maximum kinetic energy of photoelectrons and the photoelectric current change when the frequency of incident light is increased while the incident power remains constant.

Practise this question

Question

Multiple choice question 10 asking what happens to maximum kinetic energy E_k(max) and photoelectric current I when the frequency of monochromatic light incident on a metal surface is increased, given that the incident power rate is constant. Four options A, B, C, D present combinations of 'increases', 'decreases', and 'no change'.
Question text

10 Monochromatic light is incident on a metal surface in a vacuum and photoelectrons are

emitted from the surface. The photoelectric current I is the rate of flow of charge from the

surface.

The maximum kinetic energy of the photoelectrons is Ek(max).

Ek(max) and I are measured.

The frequency of the light is then increased. There is no change to the rate at which

energy is incident on the surface.

What happens to Ek(max) and I when the frequency is increased?

[1 mark]

Ek(max) I

A increases decreases

B increases no change

C no change no change

D no change decreases

Mark scheme

Show the mark scheme Mark scheme for question 10 indicating the correct option is A, where E_k(max) increases and I decreases.

10 A increases decreases AO2

How to answer it

Photoelectric Effect: Frequency and Current

๐Ÿ“Œ What this question tests

This question assesses your understanding of the photoelectric equation ( Eโ‚–(max) = hf - ฯ† ) and how photon energy, frequency, and photon flux affect both the maximum kinetic energy of emitted photoelectrons and the photoelectric current. It specifically tests your ability to decouple the effects of changing light frequency while keeping the total incident light energy per second constant.

Question 10 [1 mark]

Multiple Choice: Effect of Increasing Frequency on Eโ‚–(max) and I

โœ… Correct Answer: Option A

Eโ‚–(max) increases
I decreases

Awarded 1 mark for selecting option A.

๐Ÿ’ก Key Knowledge

  • Einstein's Photoelectric Equation: Eโ‚–(max) = hf - ฯ†
  • Energy of an individual photon depends directly on its frequency ( E = hf ).
  • Photoelectric current ( I ) is the rate of flow of charge, which is directly proportional to the number of electrons emitted per second.
  • Number of emitted electrons per second depends on the number of incident photons per second (if intensity/rate of energy transfer is constant).

๐Ÿง  Exam Technique

  • Analyze one variable at a time: First look at frequency ( f ). Since f increases, hf increases. With work function ( ฯ† ) remaining constant, Eโ‚–(max) must increase. This immediately eliminates options C and D.
  • Use the energy constraint: The question states there is no change to the rate at which energy is incident on the surface ( P = n ร— hf , where n is the number of photons per second).
  • If power ( P ) is constant and frequency ( f ) increases, each individual photon carries more energy ( hf ). Therefore, the number of photons arriving per second ( n ) must decrease. Fewer incident photons mean fewer emitted photoelectrons, causing current ( I ) to decrease.

โŒ Common Errors

  • Assuming current increases with frequency: Students often incorrectly memorize that "turning up the light" increases current without reading carefully that frequency was increased while energy transfer rate remained constant.
  • Confusing intensity with frequency: Assuming higher frequency means higher intensity. Intensity is tied to the number of photons per second, not individual photon energy.

Topics

Physics ยท 3.2 Particles and radiation

Question and mark scheme from the AQA A-Level Physics examination, Paper 1, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.