AQA A-Level Physics Paper 1, June 2022: Question 29

Calculating the Number of Charge Carriers (Electrons)

What this question tests

This question assesses your ability to apply the definitions of electric current ( I = Q / t ) and quantised charge ( Q = ne ) in combination with unit conversions (milliseconds and minutes into SI base units).

Question 29 • Multiple Choice • [1 Mark]

Exam Question Breakdown

The current in a metallic conductor is 1.5 mA . How many electrons pass a point in the conductor in two minutes?

✅ Correct Answer: A

1.1 × 10¹⁸

💡 Key Knowledge

Current is the rate of flow of charge: I = Q / t , rearranged to Q = I × t .
Total charge is quantised by the elementary charge: Q = n × e , where e = 1.60 × 10⁻¹⁹ C .
Always convert time into seconds and current into amperes before calculating.

🧠 Exam Technique

Look out for standard prefixes: milli ( × 10⁻³ ) and minutes to seconds ( × 60 ). Missing either will result in a distractor option.
Combine equations algebraically before plugging in numbers to reduce intermediate rounding errors: n = (I × t) / e .

❌ Common Errors

Forgetting time conversion: Leaving time as 2 instead of 120 seconds.
Milli prefix omission: Using 1.5 instead of 1.5 × 10⁻³ A .
Inverted formula: Dividing e by Q instead of total charge by the elementary charge.

📐 Step-by-Step Calculation

Convert current to SI units: I = 1.5 mA = 1.5 × 10⁻³ A
Convert time to SI units: t = 2 minutes = 2 × 60 = 120 s
Calculate total charge ( Q ):
Q = I × t = (1.5 × 10⁻³ A) × (120 s) = 0.18 C
Determine number of electrons ( n ):
n = Q / e = 0.18 / (1.60 × 10⁻¹⁹) = 1.125 × 10¹⁸
Round to appropriate significant figures: 1.1 × 10¹⁸ (matching Option A).

Examiner Note: Option A is the correct key. Top-level responses quickly convert units mentally or write out explicit conversion factors, avoiding common traps involving time units ( min to s ).

AQA A-Level Physics Paper 1, June 2022: Question 29

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