# 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

## 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

- [induced current](https://www.gradenine.co.uk/glossary/induced-current)

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-circular-conducting-loop-is-held-33d7105a) · Published by Druglandscape Ltd.