A circular conducting loop is held stationary in a region where a uniform magnetic field is directed perpendicular to the plane of the loop. The magnetic field strength decreases uniformly to zero over a short time interval. Explain why a current flows in the loop during this interval and determine the direction of this induced current relative to the decreasing field.

IB DP Physics Higher Level (2023 syllabus) — D.4 Induction (HL only) · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A decreasing magnetic field changes the magnetic flux through the loop, so the flux is a function of time.

By Faraday’s law a time‑varying flux induces an electromotive force (emf) around the loop.

The loop has a finite resistance; the induced emf drives a current through the loop.

Lenz’s law states that the induced current will oppose the change in flux. Since the field is decreasing, the induced current produces a magnetic field in the same direction as the original field. Using the right‑hand rule, the current therefore flows anticlockwise when the loop is viewed from the side where the original field points.

Examiner tips

  • Use the chain: change in flux → emf → current; mention resistance. State Lenz’s law explicitly and link it to the direction. Use the right‑hand rule to describe the sense of the current.
  • common_mistakes
  • :
  • Saying the current flows in the same direction as the field rather than specifying anticlockwise. Forgetting to mention the loop’s resistance. Mixing up the sense of the induced field (opposing vs. same as the decreasing field).

Mark scheme (4 marks)

  1. The changing magnetic field produces a changing magnetic flux through the loop.
  2. By Faraday's law, a changing flux induces an electromotive force (emf) in the loop.
  3. Because the loop has finite resistance, this induced emf drives a current around the loop.
  4. By Lenz's law, the induced current flows in the direction that opposes the decrease in flux, so it flows in the same sense as the original field (i.e. the induced current creates a magnetic field in the same direction as the original decreasing field, and by the right-hand rule the current flows anticlockwise when viewed from the direction the original field points).

Key terms in this question

induced current

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