# Explain how Lenz's law is consistent with the principle of conservation of energy, using the example of a conducting ring falling vertically through a region of uniform horizontal magnetic field.

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

> A conducting ring falls vertically downward and enters a region of uniform horizontal magnetic field directed perpendicular to the plane of the ring. An induced current flows in the ring as it enters the field region.

## Mark scheme (4 marks)

1. As the ring enters the field, the magnetic flux through the ring increases, inducing an EMF and hence a current in the ring.
2. By Lenz's law, the direction of the induced current is such that its magnetic effect opposes the change causing it — here, opposing the increase in flux / opposing the downward motion of the ring.
3. The opposing (upward) force on the current-carrying ring means that work must be done by gravity against this retarding force to maintain the ring's motion, providing the energy input to the system.
4. If Lenz's law were reversed (current aided the motion), the ring would accelerate without any energy input, violating conservation of energy; thus Lenz's law ensures energy is conserved by requiring an external energy source for every induced current.

## Key terms

- [Lenz's law](https://www.gradenine.co.uk/glossary/lenz-s-law)
- [conservation of energy](https://www.gradenine.co.uk/glossary/conservation-of-energy)

## 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/explain-how-lenz-s-law-is-consistent-8a8476f0) · Published by Druglandscape Ltd.