AQA AS Level Physics Paper 1, June 2025: Question 1
9 marks · Medium difficulty · Short Answer
Calculate the specific charge of a tritium nucleus, explain its value relative to other hydrogen isotopes, deduce the quark structure of a neutral kaon from a reaction, and identify the decay products of a positive pion.
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Question text
01 Hydrogen has three naturally occurring isotopes. One of these isotopes is
hydrogen-3 (3H) .
01.1 Calculate the specific charge of a nucleus of hydrogen-3.
[2 marks]
specific charge = C kg−1
01.2 The specific charge of the hydrogen-3 nucleus is less than the specific charges of the
nuclei of the other two isotopes.
Suggest why.
[2 marks]
Kaons (K) and pions (π) are produced when a beam of antiprotons is incident on a
sample of hydrogen gas.
*02*One reaction that occurs is
p + p → K− + π+ + K0
01.3 Deduce, with reference to appropriate conservation laws, the quark structure of the
K0 produced in this reaction.
[3 marks]
quark structure =
01.4 One possible decay of a π+ particle is
π+ → A + B
where A is a charged lepton.
Suggest, with reference to appropriate conservation laws, the name of A and the
name of B.
[2 marks]
name of A
name of B
Mark scheme
Show the mark scheme
Question Answers Additional Comments/Guidance Mark AO
01.1 Any one from Condone one power of ten error in first bullet 2 1 × AO1
point.
charge 1 × AO2
• use of specific charge =
mass
Third bullet point: Condone any mass for a
• charge = 1.6 × 10−19 -27
nucleon than rounds to 1.7 x 10 kg to 2
• mass = 3 × 1.67(4) × 10−27 significant figures.
For example, 3 x 1.7 x 10-27 would be
acceptable
Alternative for MP1:
Use of proton specific charge ÷ 3
3.1 × 107 to 3.2 × 107 (C kg−1)
Answer in range gains both marks
MP1: Allow correct comparison of hydrogen-3
01.2 Idea that nuclei of the hydrogen isotopes have same charge / 2 1 × AO1
same number of protons (as hydrogen-3) with either hydrogen-2 or hydrogen-1
1 × AO2
MP2: Allow correct comparison of hydrogen-3
Idea that nuclei of other isotopes have less mass / fewer with either hydrogen-2 or hydrogen-1
nucleons / fewer neutrons (than hydrogen-3) Do not accept incorrect statements about
numbers of neutrons or protons as reason for
6 lighter nuclei.
Accept reverse argument
01.3 a statement that strangeness is conserved Accept a demonstration of conservation of 3 AO3
strangeness (with numbers seen).
Expect to see: 0 + 0 → -1 + 0 + 1
Fo MP1, condone 0 + 0 → 1 + 0 + -1 (MAX 2
where this error seen)
MP2 An incorrect statement in any bullet
point does not gain this mark.
Any one from
Max 1 where no other mark is scored for:
• K− has a quark structure of us / K− has strangeness of -1
00 • idea that baryon number must be
• K (must) contain an s quark / K has strangeness of (+)1 conserved so all produced particles are
+ + 7
• π quark structure is up antidown / π is not a strange particle mesons
/ π+ has a strangeness of 0 • neutral kaon has a quark structure of
• kaons are produced by the strong interaction 𝑑𝑑̅𝑠𝑠
treat mention of conservation of lepton number
s d or charge as neutral
01.4 Any one from For demonstrations expect to see: 2 AO3
• charge must be conserved OR demonstrates conservation (+) 1 (e) → (+) 1 (e) + 0
of charge
• lepton number must be conserved OR demonstrates
80 → -1 + (+) 1
conservation of lepton number
Treat BP1 and BP2 as independent marks.
first option: A = positron and B = electron neutrino
Max 1 for A and B in the incorrect order.
OR
Second option: A = positive muon and B = muon neutrino
Condone clearly recognisable symbols in place
of names.
Total 9
How to answer it
Isotopes, Specific Charge & Particle Interactions
This question assesses core subatomic physics from the AQA AS specification:
- Specific charge calculations: Understanding the formula specific charge = charge / mass for a nucleus (not an atom).
- Nuclear composition: Explaining variations in specific charge between isotopes of hydrogen (¹H, ²H, ³H).
- Strong interaction & strangeness conservation: Deducing the quark composition of neutral kaons (K⁰) produced in strong interactions.
- Weak particle decays: Applying conservation of charge (Q) and lepton family number (L) to pion decays.
Specific Charge of a Hydrogen-3 Nucleus
Calculate the specific charge of a nucleus of hydrogen-3 (³₁H) [2 marks]
📐 Step-by-Step Calculation
Step 1: Identify nuclear charge (Q)
A hydrogen-3 nucleus has 1 proton and 2 neutrons.
Q = 1 × 1.60 × 10⁻¹⁹ C = 1.60 × 10⁻¹⁹ C
Step 2: Identify nuclear mass (m)
Total nucleons = 3.
m = 3 × 1.67 × 10⁻²⁷ kg = 5.01 × 10⁻²⁷ kg
Step 3: Calculate specific charge
Specific charge = Q / m
= (1.60 × 10⁻¹⁹) / (5.01 × 10⁻²⁷)
= 3.19 × 10⁷ C kg⁻¹ (allow 3.1 × 10⁷ to 3.2 × 10⁷ C kg⁻¹)
✅ Mark Scheme Breakdown
- Mark 1 (Method): Use of charge / mass with Q = 1.6 × 10⁻¹⁹ C and m = 3 × 1.67(4) × 10⁻²⁷ kg (condones 1 power of ten error).
- Mark 2 (Accuracy): Final answer in range 3.1 × 10⁷ to 3.2 × 10⁷ C kg⁻¹.
❌ Common Errors
- Including orbital electrons: The question asks for the nucleus, not the neutral atom. Including electron mass or finding net atomic charge (zero) scores 0.
- Wrong mass: Using the mass of only 1 proton instead of 3 nucleons.
- Forgetting units: Always write C kg⁻¹ if not pre-printed.
🧠 Exam Technique
Always highlight whether the question asks for a nucleus, an ion, or an atom. The phrase "nucleus of hydrogen-3" immediately tells you mass = 3 × nucleon mass, and charge = +1e.
Comparing Specific Charges of Isotopes
Suggest why the specific charge of hydrogen-3 is less than the other two isotopes [2 marks]
✅ Model Answer
- Mark 1: All hydrogen isotopes have the same nuclear charge / same number of protons (+1e).
- Mark 2: The other isotopes (hydrogen-1 and hydrogen-2) have less mass / fewer nucleons / fewer neutrons than hydrogen-3.
Reverse argument accepted: Hydrogen-3 has the greatest mass / most nucleons for the same charge.
💡 Key Knowledge
Formula: Specific charge = Q / m
- ¹₁H: Q = 1e , m ≈ 1u → Ratio ≈ 1
- ²₁H: Q = 1e , m ≈ 2u → Ratio ≈ 1/2
- ³₁H: Q = 1e , m ≈ 3u → Ratio ≈ 1/3
Since Q is constant, as mass increases, specific charge decreases.
❌ Examiner Trap
Do not give vague statements like "they have different numbers of particles" without specifying protons/neutrons. Never state that lighter isotopes have "fewer protons"—all hydrogen isotopes must have exactly 1 proton!
Conservation Laws & Quark Structure of K⁰
Deduce, with reference to conservation laws, the quark structure of K⁰ in p + p̄ → K⁻ + π⁺ + K⁰ [3 marks]
✅ Model Deduction (Full 3 Marks)
- Mark 1 (Conservation Law): The reaction is a strong interaction, so strangeness (S) is conserved.
- Mark 2 (Strangeness analysis):
Initial strangeness: p (0) + p̄ (0) = 0 .
Products: K⁻ has S = -1 (quark structure sū), π⁺ has S = 0 (ud̄).
Therefore, to conserve strangeness ( 0 + 0 → -1 + 0 + S_K⁰ ), K⁰ must have strangeness S = +1 (must contain an s̄ antiquark). - Mark 3 (Quark Structure): K⁰ has charge Q = 0. Since the anti-strange quark s̄ has charge +1/3 e, it must pair with a down quark d (charge -1/3 e).
Quark structure = s̄d (or ds̄).
💡 Properties of Kaons
- Kaons are mesons (quark-antiquark pair) containing strange quarks.
- Strange quark (s): Q = -1/3 e , S = -1
- Anti-strange quark (s̄): Q = +1/3 e , S = +1
- K⁺ = us̄ (S = +1)
- K⁻ = sū (S = -1)
- K⁰ = ds̄ or s̄d (S = +1)
- K̄⁰ = sd̄ (S = -1)
❌ Common Misconceptions
- Mixing up S: Confusing the sign of strangeness. Remember: strange quark s has S = -1 , anti-strange s̄ has S = +1 .
- Writing sd̄: That is the antikaon K̄⁰ , which has S = -1. Writing sd̄ scores 0 for the final mark.
- Not naming the conservation law: You must explicitly state that strangeness is conserved (or show the numerical equation 0 + 0 → -1 + 0 + 1 ).
🧠 Exam Technique
When an exam question says "with reference to appropriate conservation laws", your first line should always name the law: "Strangeness is conserved in strong interactions". This secures the first mark immediately.
Pion Decay: π⁺ → A + B
Suggest, with reference to conservation laws, the name of charged lepton A and particle B [2 marks]
✅ Model Answer (Choose Either Option)
Option 1 (Muon pathway - most common in nature):
- Name of A: anti-muon (or positive muon / μ⁺)
- Name of B: muon neutrino (νμ)
Option 2 (Positron pathway):
- Name of A: positron (or anti-electron / e⁺)
- Name of B: electron neutrino (νe)
📐 Checking Conservation Laws
1. Conservation of Charge (Q):
π⁺ (Q = +1) → A (Q = +1) + B (Q = 0)
A must carry +1 charge; B must be neutral.
2. Conservation of Lepton Number (L):
Initial L = 0 (π⁺ is a meson, not a lepton)
A is an anti-lepton (e⁺ or μ⁺), so L = -1 .
Therefore, B must have L = +1 to balance: 0 → -1 + (+1) .
Hence B must be a neutrino of the matching generation.
❌ Common Errors
- Naming an anti-neutrino: Suggesting an antineutrino gives L = -1 + (-1) = -2 , violating lepton conservation!
- Swapping A and B: The question specifies A is a charged lepton. Putting neutrino for A loses a mark.
- Mismatched lepton families: Pairing a positron with a muon neutrino violates individual lepton flavour conservation (Le and Lμ).
🧠 Top Tip for Full Marks
Explicitly write the numbers for the conservation laws in your written explanation:
Charge: +1 → +1 + 0
Lepton number: 0 → -1 + 1
This guarantees the justification mark even if your explanation text is brief.
Topics
Physics · 3.2 Particles and radiation
Question and mark scheme from the AQA AS Level Physics examination, Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.