AQA AS Level Physics Paper 1, June 2019: Question 1

✅ Correct Answer

Specific charge = 4.8 × 10⁷ C kg⁻¹

📐 Step-by-Step Calculation

1. Recall the definition: Specific charge = Charge / Mass
2. Find the total charge: The nucleus has 1 proton (charge = +1.60 × 10⁻¹⁹ C) and 1 neutron (charge = 0). Total charge = 1.60 × 10⁻¹⁹ C .
3. Find the total mass: Mass of 1 proton + 1 neutron = (1.67 × 10⁻²⁷ + 1.67 × 10⁻²⁷) kg (using standard approximate nucleon mass).
4. Compute: (1.60 × 10⁻¹⁹) / (3.34 × 10⁻²⁷) = 4.79 × 10⁷ C kg⁻¹ , rounding to 2 significant figures gives 4.8 × 10⁷ C kg⁻¹ .

❌ Common Errors

• Forgetting to include the mass of the neutron in the denominator.
• Mixing up charge values or using electron mass instead of nucleon mass.

🧠 Exam Technique

Even if you don't calculate the final decimal value correctly, writing down the formula charge / mass or substituting numbers correctly secures the first method mark (M1).

✅ Correct Answer

Tick: pion (or pi meson)

💡 Key Knowledge

• Muons are leptons and interact via the weak force, gravity, and electromagnetic force.
• Photons are the exchange particles of the electromagnetic force.
• Pions are the exchange particles that mediate the strong nuclear force between nucleons in the low-energy residual model.
• W⁺ bosons are exchange particles of the weak nuclear force.

✅ Correct Answer

Must cover three key behavioral points regarding distance:

• Attractive at short ranges (up to about 3 fm).
• Repulsive at very short ranges (closer than 0.5 fm).
• Maintains stability by preventing nucleons from collapsing inward or drifting apart (equilibrium position).

💡 Key Knowledge

• Range: Extremely short-range force; practically zero past 3 to 4 femtometres (fm).
• Repulsion core: Prevents total nuclear collapse when nucleons are forced tightly together.
• Equilibrium: The separation distance where attractive and balanced forces hold the nucleus steady.

❌ Common Errors

Mentioning electrostatic/electric forces between the proton and neutron as the reason for stability will lose the final mark, as the strong nuclear force alone governs this nuclear behavior.

✅ Correct Answer

Decay mode that produces a nucleus of a different element: beta-minus (β⁻) decay.

🧠 Exam Technique & Reasoning

• Alpha decay: Tritium (mass number 3) is far too light to undergo alpha decay (which requires a nucleus with large mass/nucleon numbers, typically A > 100).
• Electron capture: Occurs in proton-rich nuclei. Tritium has 1 proton and 2 neutrons (neutron-rich), so it cannot capture an inner-shell electron.
• Beta-minus decay: A neutron turns into a proton, emitting an electron and an antineutrino. The proton number ($Z$) changes from 1 to 2, transforming hydrogen into helium ( ⁴He or helium-3 depending on exact balancing, but definitively changing the element).

❌ Common Errors

Failing to explicitly link the chosen decay mode to the conclusion that a change of element occurs (i.e., changing proton number $Z$).