AQA A-Level Physics Paper 2, June 2023: Question 20

1 mark · Easy difficulty · Multiple Choice

Calculate the magnetic flux density from the change in magnetic force when the current in a wire is increased.

Practise this question

Question

Multiple choice question 20: A wire is at right angles to a uniform magnetic field and carries an electric current. The wire is 150 mm in length. When the current in the wire is increased by 4.0 A, the force acting on the wire increases by 3.6 × 10⁻³ N. What is the magnetic flux density of the field? Four options: A: 6.0 × 10⁻⁶ T, B: 6.0 × 10⁻³ T, C: 1.7 × 10² T, D: 1.7 × 10⁵ T.
Question text

20 A wire is at right angles to a uniform magnetic field and carries an electric current.

The wire is 150 mm in length.

When the current in the wire is increased by 4.0 A, the force acting on the wire increases

by 3.6 × 10−3 N.

What is the magnetic flux density of the field?

[1 mark]

A 6.0 × 10−6 T

B 6.0 × 10−3 T

C 1.7 × 102 T

D 1.7 × 105 T

Mark scheme

Show the mark scheme Mark scheme table row for question 20 showing the correct answer as option B with value 6.0 × 10⁻³ T.

20 B 6.0 × 10−3 T

How to answer it

Magnetic Flux Density & The Motor Effect

📋 What this question tests

This question assesses your ability to apply the magnetic force relationship for a current-carrying conductor in a magnetic field:

  • Understanding and applying the formula F = B I L (and its rate-of-change form ΔF = B ΔI L ).
  • Correctly rearranging equations to make magnetic flux density ( B ) the subject.
  • Converting non-standard metric units ( mm to m ) into SI units before calculating.
  • Handling calculations with numbers in standard scientific notation.
Question 20 • Multiple Choice

Determining Magnetic Flux Density from Changes in Current & Force

AQA A-Level Physics • Magnetic Fields • 1 Mark

✅ Correct Answer

Option B: 6.0 × 10⁻³ T

The calculation yields exactly 0.0060 T or 6.0 × 10⁻³ T when length is converted correctly to metres.

Award 1 mark for selecting option B.

💡 Key Knowledge

  • The Motor Effect: A wire carrying a current perpendicular to a uniform magnetic field experiences a force given by:
    F = B I L sin(θ)
  • Since the wire is at right angles, θ = 90° and sin(90°) = 1 , simplifying to F = B I L .
  • Linear Relationship: Because B and L are constant, change in force is proportional to change in current:
    ΔF = B × ΔI × L

📐 Step-by-Step Calculation

  1. Convert all values to SI base units:
    Length, L = 150 mm = 150 × 10⁻³ m = 0.150 m
    Change in current, ΔI = 4.0 A
    Change in force, ΔF = 3.6 × 10⁻³ N
  2. Rearrange for magnetic flux density (B):
    B = ΔF / (ΔI × L)
  3. Substitute the values:
    B = (3.6 × 10⁻³) / (4.0 × 0.150)
    B = (3.6 × 10⁻³) / 0.60
  4. Evaluate:
    B = 6.0 × 10⁻³ T

🧠 Exam Technique & Distractor Analysis

AQA carefully crafts multiple-choice distractors to trap students who make standard errors:

  • Why A is wrong (6.0 × 10⁻⁶ T): Obtained if you forget to convert millimetres to metres and use L = 150 directly in the denominator.
  • Why C is wrong (1.7 × 10² T): Obtained if you invert the formula as B = (ΔI × L) / ΔF .
  • Why D is wrong (1.7 × 10⁵ T): Obtained if you both invert the formula AND fail to convert millimetres to metres.

❌ Common Pitfalls to Avoid

  • Prefix oversight: Millimetres ( mm ) must always be converted to metres ( m ) by multiplying by 10⁻³ before calculating.
  • Confusing absolute current with change in current: The question states the current is increased by 4.0 A, not that the total current is 4.0 A. Using ΔF = B ΔI L directly resolves this without needing initial values.
  • Incorrect rearrangement: Ensure you divide the force by both the current and the length. Check units: T = N A⁻¹ m⁻¹ .

Topics

Physics · 3.7 Fields and their consequences (A-level only)

Question and mark scheme from the AQA A-Level Physics examination, Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.