AQA A-Level Physics Paper 1, June 2022: Question 1

✅ Correct Answer

238/92 U → 4/2 α + 234/90 Th

(Condone use of He instead of α, and allow 4/2 He)

💡 Key Knowledge

• Conservation Laws: Nucleon number (top) and proton number (bottom) must balance on both sides.
• Alpha particles are helium nuclei represented as 4/2 α or 4/2 He .

🧠 Exam Technique

Always double-check arithmetic: 238 = 4 + 234 and 92 = 2 + 90 . Ensure the "+" sign separating the products is clearly written to secure full marks.

❌ Common Errors

Failing to include the "+" sign between product particles, or incorrectly balancing the nucleon and proton numbers for thorium (Th).

✅ Correct Answer

• A neutron changes to a proton (or a down quark changes to an up quark).
• The exchange particle is the W⁻ boson because the interaction involves the weak interaction / weak nuclear force and conserves charge.

💡 Key Knowledge

Beta-minus decay involves a down quark turning into an up quark ( d → u + W⁻ ). Because charge must be conserved, the carrier of the weak force must be negatively charged ( W⁻ ).

🧠 Exam Technique

Structure your answer in two clear parts: 1. Identify the underlying quark/nucleon process. 2. Name the particle ( W⁻ ) and explicitly link it to the weak interaction and charge conservation.

❌ Common Errors

Just stating "W boson" without specifying the negative sign ( W⁻ ), or incorrectly attributing the process to the strong nuclear force.

✅ Correct Answer

FOR: Line C is present in both hydrogen and helium spectra (OR Line E is present in both sodium and helium spectra).

AGAINST: Line D is present in the helium spectrum but is missing from both the hydrogen and sodium spectra.

💡 Key Knowledge

Line spectra are unique fingerprints of elements. For a mixture hypothesis to be true, all lines of the mystery element must be accounted for by the constituent elements, and no unexplainable lines should appear.

🧠 Exam Technique

Directly reference the lettered lines from the diagram. Clearly split your argument into a "FOR" statement and an "AGAINST" statement to secure both available marks.

❌ Common Errors

Discussing lines A, B, or F which do not provide direct comparative evidence for this specific logical test, or failing to explicitly state why a line proves the hypothesis false.

✅ Correct Answer

Change in energy level = 2.11 to 2.14 eV (Accept 2.11 eV)

📐 Step-by-Step Calculation

1. Determine wavelength from Figure 1: Line E is located between 580 nm and 590 nm (approx. 585 nm or 5.85 × 10⁻⁷ m).
2. Calculate photon energy in Joules:
   E = hc / λ
   E = (6.63 × 10⁻³⁴ J s × 3.00 × 10⁸ m s⁻¹) / (5.85 × 10⁻⁷ m)
   E = 3.40 × 10⁻¹⁹ J
3. Convert Joules to electronvolts (eV):
   E = (3.40 × 10⁻¹⁹ J) / (1.60 × 10⁻¹⁹ J eV⁻¹) = 2.13 eV

❌ Common Trap

Forgetting to convert nanometres to metres ( × 10⁻⁹ ) or forgetting the final division by the elementary charge ( 1.60 × 10⁻¹⁹ ) when converting from Joules to eV.

✅ Correct Answer

• Photons act as energy carriers.
• In an absorption spectrum, an atom absorbs a photon and an electron moves/is excited to a higher energy state/level.
• In an emission spectrum, an atom de-excites as an electron moves to a lower energy state/level, emitting a photon.

💡 Key Knowledge

Atomic energy levels are discrete. Photons are absorbed or released only when their energy matches the exact difference between two energy levels ( ΔE = hf ).

🧠 Exam Technique

Use precise physics terminology: mention photons, energy states/levels, excitation (higher level), and de-excitation (lower level). Avoid vague descriptions like "light bouncing off electrons."

❌ Common Errors

Confusing the direction of electron transitions, failing to mention photons as the mechanism of energy transfer, or using prohibited terms like "shells" instead of "energy levels/states".