AQA AS Level Physics Paper 2, November 2021: Question 29

1 mark · Medium difficulty · Multiple Choice

Calculate the magnitude of the current at X using Kirchhoff's first law from the given junction network.

Practise this question

Question

Multiple-choice question 29 featuring a circuit diagram with two junctions connected in series. The first junction has a wire X entering from the left, a 2 A current entering from the top, and a 4 A current leaving upwards from the bottom. The second junction has a 3 A current entering from the right, a 4 A current leaving upwards, and a 1 A current entering from the bottom. Options for the magnitude of current at X are A zero, B 2 A, C 3 A, and D 6 A.
Question text

29 The diagram shows the currents in a set of wires.

What is the magnitude of the current at X?

[1 mark]

A zero

B 2 A

C 3 A

D 6 A

Mark scheme

Show the mark scheme The mark scheme table indicates the correct answer for question 29 is D, corresponding to 6 A.

29 D 6 A

How to answer it

Kirchhoff's First Law and Circuit Junctions

AQA AS Level Physics • Multiple Choice

What this question tests

This question assesses your understanding and application of Kirchhoff's First Law (conservation of charge at an electrical junction) by requiring you to systematically work your way through connected junctions from right to left to find an unknown current.

Question 29 Analysis

Evaluating Junctions Step-by-Step

✅ Correct Answer: D (6 A)

The magnitude of the current at wire X is 6 A.

💡 Key Knowledge

  • Kirchhoff's First Law: The sum of currents entering any junction must equal the sum of currents leaving that junction.
  • This is a direct consequence of the conservation of charge.

🧠 Exam Technique

When dealing with multi-junction problems, always start at the junction where you have the most known values and work backwards sequentially toward your target variable.

❌ Common Errors

Students often try to look at the entire network at once or guess based on visual symmetry, leading to crossed signs. Always write down the balance equation for each individual node clearly.

📐 Step-by-Step Calculation

  1. Analyze the right-hand junction:
    Entering currents = 3 A (from right) + 1 A (from bottom) = 4 A .
    Leaving current = 4 A (going upwards).
    Therefore, the horizontal connecting wire carries current moving leftwards away from this junction with a magnitude of 4 A (since total entering = total leaving: 3 + 1 = 4 upwards, meaning 4 flows into the connecting wire to the left).
  2. Analyze the left-hand junction (at X):
    Entering currents = 2 A (from top) + 4 A (arriving via the connecting wire from the right junction).
    Total current entering the junction = 2 A + 4 A = 6 A .
    Since no other currents are leaving except down wire X, the magnitude of the current at X must balance this total.
  3. Conclusion: Current at X = 6 A .
Mark Allocation: 1 mark for selecting option D. Top-level responses execute multi-node tracking quickly and accurately without sign confusion.

Topics

Physics · 3.5 Electricity

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.