AQA AS Level Physics Paper 2, June 2022: Question 32

1 mark · Medium difficulty · Multiple Choice

Calculate the potential difference between points P and Q in a circuit containing two series resistors and a junction with incoming currents.

Practise this question

Question

A circuit diagram showing a vertical branch with point P at the top, a 20 ohm resistor, a junction where a 4 A current enters from the left, a 30 ohm resistor below the junction, and point Q at the bottom. A current of 3 A flows downwards through point P and the 20 ohm resistor. Multiple-choice options A (60 V), B (70 V), C (180 V), and D (270 V) are listed below with selection ovals.
Question text

32 The diagram shows the currents in part of a circuit.

What is the potential difference between points P and Q?

[1 mark]

A 60 V

B 70 V

C 180 V

D 270 V

Mark scheme

Show the mark scheme The mark scheme table shows question 32 has the correct answer D (AO1), corresponding to 270 V.

32 D (AO1) 270 V

How to answer it

Calculating Potential Difference Across Circuit Junctions

AQA AS Level Physics • Electricity

What this question tests

This question assesses your ability to combine Kirchhoff's First Law (conservation of current at a junction) with Ohm's Law (V = IR) to determine the total potential difference across multiple series components carrying different branch currents.

Question Breakdown & Solution

✅ Correct Answer

Option D (270 V) is the correct choice.

Mark Awarded: 1 / 1 (AO1 - Demonstration of knowledge)

💡 Key Knowledge

  • Kirchhoff's 1st Law: The sum of currents entering a junction equals the sum of currents leaving it. Charge is conserved.
  • Ohm's Law: Potential difference equals current multiplied by resistance ( V = IR ).
  • Series Additivity: The total potential difference between two distant points is the sum of the potential differences across individual components along the path.

📐 Step-by-Step Calculation

  1. Find the current through the top resistor (20 Ω): The diagram clearly shows a current of 3 A entering point P and flowing down through the 20 Ω resistor.
  2. Calculate voltage across the top resistor: V₁ = I₁ × R₁ = 3 A × 20 Ω = 60 V .
  3. Find the current through the bottom resistor (30 Ω) using Kirchhoff's 1st Law: A current of 3 A comes from the top, and 4 A enters from the side branch. At the junction, they combine: I₂ = 3 A + 4 A = 7 A flowing down through the 30 Ω resistor.
  4. Calculate voltage across the bottom resistor: V₂ = I₂ × R₂ = 7 A × 30 Ω = 210 V .
  5. Find total potential difference between P and Q: V_total = V₁ + V₂ = 60 V + 210 V = 270 V .

🧠 Exam Technique

For multiple-choice circuit analysis questions:

  • Annotate the diagram directly on your exam paper with calculated currents at every node.
  • Check current directions carefully to ensure you are adding currents correctly at junctions rather than subtracting them by mistake.

❌ Common Errors & Distractors

  • Using only one current: Students sometimes assume the 4 A current applies everywhere, leading to incorrect calculations like 4 × (20 + 30) = 200 V .
  • Forgetting the junction rule: Failing to add the 3 A and 4 A together results in taking just 7 × 20 or similar mix-ups, leading to distractors like A, B, or C.

Topics

Physics · 3.5 Electricity

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.