AQA A-Level Physics Paper 1, June 2025: Question 29

1 mark · Medium difficulty · Multiple Choice

Determine the total charge that flows through point P in 60 seconds given the variation of two junction currents with time.

Practise this question

Question

A circuit junction diagram shows two branches carrying currents I1 and I2 combining into a single wire that passes through point P. A graph plots current in mA against time in seconds from 0 to 60 s. Current I1 increases linearly from 5 mA at 0 s to 12 mA at 60 s. Current I2 decreases linearly from 19 mA at 0 s to 12 mA at 60 s. Four multiple-choice options are given for the total charge through P: A (1.44 C), B (0.93 C), C (0.72 C), and D (0.42 C).
Question text

29 The diagram shows part of a circuit.

The graph shows how the two currents I1 and I2 vary with time.

What is the total charge that flows through point P in 60 s?

[1 mark]

A 1.44 C

B 0.93 C

C 0.72 C

D 0.42 C

Mark scheme

Show the mark scheme Mark scheme table indicating question 29 has correct answer A (1.44 C) with assessment objective AO1.

29 A 1.44 C AO1

How to answer it

Total Charge Flow at a Circuit Junction

📌 What this question tests

This question assesses your ability to apply Kirchhoff’s First Law (conservation of charge) to determine the total current entering a junction, interpret a current–time (I–t) graph to calculate charge flow (area under the graph), and convert prefix units ( mA to A ) correctly under timed conditions.

Question 29 Breakdown

Multiple Choice (1 Mark)

✅ Correct Answer

A: 1.44 C

Award 1 mark for selecting option A (AO1).

💡 Key Knowledge

  • Kirchhoff’s First Law: The total current entering a junction equals the total current leaving it:
    IP = I₁ + I₂
  • Charge & Current: Charge is the integral of current with respect to time:
    ΔQ = Area under the I–t graph
  • Unit Conversion: Current is given in milliamperes ( mA ):
    1 mA = 1 × 10⁻³ A

📐 Step-by-Step Calculation

There are two efficient methods to solve this question:

Method 1: Elegant Constant Current Insight (Fastest)

  1. Check current at t = 0 s:
    I₁ = 5 mA , I₂ = 19 mA
    IP(0) = 5 + 19 = 24 mA
  2. Check current at t = 60 s:
    I₁ = 12 mA , I₂ = 12 mA
    IP(60) = 12 + 12 = 24 mA
  3. Observe linearity: Both graphs are straight lines with equal and opposite gradients. Therefore, the total current flowing into point P remains constant at 24 mA for the entire 60 s!
  4. Calculate total charge:
    Q = I × t = (24 × 10⁻³ A) × 60 s = 1.44 C

Method 2: Area Under Each Curve (Standard Trapezium Method)

  1. Charge from I₁ (trapezium 1):
    Q₁ = ½ × (5 + 12) × 10⁻³ A × 60 s = 8.5 × 10⁻³ × 60 = 0.51 C
  2. Charge from I₂ (trapezium 2):
    Q₂ = ½ × (19 + 12) × 10⁻³ A × 60 s = 15.5 × 10⁻³ × 60 = 0.93 C
  3. Total charge Q:
    Q = Q₁ + Q₂ = 0.51 C + 0.93 C = 1.44 C

🧠 Exam Technique & Strategy

  • Spot symmetry in multiple-choice questions: Noticing that 5 + 19 = 24 and 12 + 12 = 24 turns a complex dual-trapezium calculation into a single, instant multiplication ( 24 × 60 ).
  • Check units on axes immediately: The y-axis is in mA , not A . Always write the × 10⁻³ factor down first to prevent power-of-ten mistakes.
  • Inspect junction arrows: Verify whether currents are both entering or if one is leaving. Here, both arrows point into the node, meaning they add together.

❌ Common Distractors & Traps

  • Selecting B (0.93 C): This corresponds only to the charge delivered by current I₂ ( 15.5 mA × 60 s ). Students who forget Kirchhoff's Law and only compute the upper trace fall into this trap.
  • Selecting C (0.72 C): Obtained by calculating the charge if current stayed at the final intersection value of 12 mA for the whole duration ( 12 mA × 60 s = 0.72 C ).
  • Selecting D (0.42 C): Obtained by subtracting the two charges ( 0.93 C − 0.51 C = 0.42 C ) instead of adding them, mistaking current directions.

Topics

Physics · Practical skills · 3.5 Electricity · Data analysis

Question and mark scheme from the AQA A-Level Physics examination, Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.