AQA AS Level Physics Paper 2, November 2021: Question 31

✅ Correct Answer

D: 23.0 mA

💡 Key Knowledge

• Parallel Circuits: Components connected in parallel across an ideal power supply (negligible internal resistance) share the same potential difference equal to the supply emf ( 12 V ).
• Kirchhoff's First Law: The total current leaving the battery equals the sum of the currents flowing through each parallel branch ( I_total = I_1 + I_2 ).
• Graph Reading: Find the current value on the y-axis corresponding to V = 12 V for both characteristic curves.

🧠 Exam Technique

• Locate 12 V on the horizontal x-axis.
• Trace vertically up to intersect both the solid curve and the dashed curve.
• Read across to the vertical y-axis to find the current for each component at 12 V .
• Sum the two current values carefully, paying attention to the unit scale.

📐 Step-by-Step Calculation

1. Identify supply voltage: V = 12 V
2. Read current for component 1 (solid line at 12 V): 14.0 mA
3. Read current for component 2 (dashed line at 12 V): 9.0 mA
4. Apply parallel current rule:
   I_battery = 14.0 mA + 9.0 mA = 23.0 mA

❌ Common Errors & Traps

• Intersection confusion: Students often incorrectly look at where the two lines cross each other ( ~7 V ) instead of evaluating both components at the battery voltage ( 12 V ). This yields distractor option A ( 7.9 mA ).
• Reading only one component: Forgetting to add the currents together, leading to answers like 14.5 mA (distractor B) or 14.0 mA .
• Scale misinterpretation: Misreading the minor grid squares on the y-axis where each small square represents 0.2 mA .