AQA A-Level Physics Paper 2, June 2025: Question 19

1 mark · Medium difficulty · Multiple Choice

Identify which ramps result in the smallest and largest time for identical magnets to slide down, based on the ramps' resistivity and thickness.

Practise this question

Question

Question 19 shows four ramps labelled P, Q, R, and S with identical magnets sliding down each. A table gives the resistivity and thickness: ramp P has resistivity rho and thickness d; ramp Q has resistivity rho and thickness 2d; ramp R has resistivity 2rho and thickness d; ramp S has resistivity 2rho and thickness 2d. Friction is the same for each ramp. A multiple-choice table asks to identify which row correctly gives the ramp with the smallest time t and largest time t, with options A (R, S), B (R, Q), C (Q, S), and D (Q, P).

Mark scheme

Show the mark scheme Mark scheme for question 19 displays the answer key: question number 19, correct answer B with 'R' for smallest t and 'Q' for largest t, assessed under AO2.

How to answer it

Electromagnetic Braking & Resistivity of Conducting Ramps

What this question tests

This question assesses your ability to combine Faraday's and Lenz's laws with the formula for electrical resistance ( R = ρL / A ). You need to understand how eddy currents produce an opposing electromagnetic braking force, and how ramp dimensions and material properties alter that braking force and the resulting transit time.

Question 19 Analysis

Multiple Choice: Determining Extremes of Sliding Time (t)

✅ Correct Answer: B (Smallest t: R, Largest t: Q)

Smallest time ( t ) corresponds to the magnet moving fastest, which requires the least electromagnetic braking. Ramp R has the highest resistance, yielding the smallest eddy currents.

Largest time ( t ) corresponds to the magnet moving slowest, which requires the most electromagnetic braking. Ramp Q has the lowest resistance, yielding the largest eddy currents.

Award: 1 Mark (AO2)

🔧 Step-by-Step Derivation

  1. Induced EMF: As each identical magnet slides at a given speed, the rate of change of magnetic flux induces an emf ε in the ramp loops.
  2. Cross-sectional Area: The cross-sectional area through which eddy current loops circulate is directly proportional to thickness d ( A ∝ d ).
  3. Loop Resistance: Using R = ρL / A , the electrical resistance of the eddy current path scales as:
    R ∝ ρ / d
  4. Eddy Current & Braking Force:
    Induced current I = ε / R ∝ d / ρ .
    By Lenz's law, retarding force Fdrag ∝ I ∝ d / ρ .
  5. Evaluate Ratio ( d / ρ ):
    • P: d / ρ = 1.0
    • Q: 2d / ρ = 2.0 → Max drag → Largest t
    • R: d / (2ρ) = 0.5 → Min drag → Smallest t
    • S: 2d / (2ρ) = 1.0

💡 Key Physics Principles

  • Lenz's Law: An induced current always flows in such a direction that its magnetic effect opposes the change that produces it.
  • Eddy Currents: Circular loops of electrical current induced within bulk conductors by a changing magnetic field.
  • Terminal Velocity / Retarding Force: Greater retarding force counteracts the component of weight down the slope ( mg sin θ ), resulting in lower average velocity and longer slide times.

🧠 Exam Technique: Elimination Strategy

Notice that ramps P and S have identical ratios:

(d / ρ)P = 1 and (2d / 2ρ)S = 1

Because multiple choice questions have a unique single answer, neither P nor S could possibly be an extreme on its own without tying with the other. This immediately eliminates options A, C, and D in seconds!

❌ Common Traps & Misconceptions

  • Inverting the Time Relationship: Confusing largest braking force with smallest time. Remember: more braking force = slower movement = longer time.
  • Misinterpreting Resistance: Assuming thicker plates have higher resistance. Increasing thickness d increases the cross-sectional area of the eddy current paths, which reduces electrical resistance.
  • Confusing Resistivity (ρ) with Resistance (R): Higher resistivity increases resistance, decreasing eddy currents and reducing magnetic damping.

Topics

Physics · 3.5 Electricity · 3.7 Fields and their consequences (A-level only)

Question and mark scheme from the AQA A-Level Physics examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.