AQA A-Level Physics Paper 1, June 2024: Question 9

✅ Correct Answer

Graph A is the correct choice.

1 mark awarded for selecting option A.

💡 Key Knowledge

• The photoelectric equation is arranged in the form of a straight-line equation ( y = mx + c ): Eₖ₍ₘₐₓ⟩ = hf - φ .
• The gradient of the line is equal to the Planck constant ( h ), meaning all such lines for any metal must be perfectly parallel.
• The threshold frequency ( f₀ ) is the intercept on the frequency axis, where Eₖ₍ₘₐₓ⟩ = 0 , calculated as f₀ = φ / h .
• Since metal X has a greater work function than metal Y ( φₓ > φᵧ ), metal X requires a higher threshold frequency ( f₀,ₓ > f₀,ᵧ ).

🧠 Exam Technique

• Check gradients first: Eliminate any options where the lines are not parallel (eliminates B and D, since h is a universal constant).
• Analyze intercepts: Look at the threshold frequency axis ( f ). Since φₓ > φᵧ , metal X must intercept the frequency axis further to the right (at a higher frequency value) than metal Y. This points directly to graph A.

❌ Common Errors

• Confusing work function with threshold: Students sometimes mix up which metal should have a higher intercept. Remember that a larger work function means you need a higher frequency of light to release electrons, shifting the line to the right.
• Non-parallel lines: Selecting graphs with different steepness (like graph B) demonstrates a fundamental misunderstanding that the Planck constant changes depending on the metal used.