Introduction
In the IB Physics syllabus, Lenz’s Law is one of the key principles of electromagnetism. It explains how induced currents always act to oppose the change that produced them. Together with Faraday’s Law of Induction, Lenz’s Law forms the foundation of many IB HL and SL exam questions on electricity and magnetism.
For IB students, mastering Lenz’s Law is essential not only for the exams but also for understanding real-world technologies like electric generators, transformers, and induction cooktops.
Quick Start Checklist
To prepare for Lenz’s Law questions in IB Physics:
- Memorize the definition: The direction of induced current opposes the change causing it.
- Practice applying the right-hand rule to magnetic fields.
- Study how it connects to Faraday’s Law (ε = –dΦ/dt).
- Review applications: generators, transformers, electromagnetic braking.
- Use RevisionDojo resources to strengthen your electromagnetism practice.
The Definition of Lenz’s Law
Lenz’s Law states:
The induced electromotive force (emf) or current in a closed circuit is always in a direction that opposes the change in magnetic flux that produced it.
This is represented mathematically in Faraday’s Law:
ε = – dΦ/dt
Where:
- ε = induced emf
- Φ = magnetic flux (BAcosθ)
- The negative sign represents Lenz’s Law (opposition to change).
Lenz’s Law in IB Physics Exams
Lenz’s Law appears in HL and SL exam questions where students are asked to:
- Determine the direction of induced currents.
- Apply the concept to explain why a magnet resists entering a coil.
- Link emf calculations to flux changes.
- Describe applications like power generation or braking systems.
Example Problem
A bar magnet is pushed into a coil with its north pole facing the coil.
Question: What is the direction of the induced current?
Answer: The coil produces a north pole facing the magnet, opposing its motion. Using the right-hand rule, the induced current is counterclockwise.
Applications of Lenz’s Law
- Electric Generators: Induced currents resist changes in magnetic flux as the coil rotates.
- Transformers: Secondary coils oppose the change from the primary coil.
- Electromagnetic Braking: Trains and roller coasters use Lenz’s Law to slow down without friction.
- Induction Cooking: Electric currents oppose changes in magnetic fields to heat pans.
Common Misconceptions
- “Lenz’s Law always stops motion.”
Not true—it resists the change but doesn’t eliminate it. External work can overcome the resistance. - “It only applies to magnets.”
Wrong. It applies to any change in magnetic flux, including moving wires and coils. - “It’s separate from Faraday’s Law.”
Incorrect. Lenz’s Law is built into Faraday’s Law via the negative sign.
Tips for Success
- Visualize flux changes: Draw diagrams showing whether flux increases or decreases.
- Use the right-hand rule: Thumb = motion, fingers = magnetic field, palm = force direction.
- Always think in opposites: The induced current resists, not supports, the change.
- Practice past paper diagrams: These appear frequently in IB Physics Paper 2.
- Connect to real-world examples: Helps with application-style exam questions.
Frequently Asked Questions (FAQs)
1. Is Lenz’s Law in the IB Physics booklet?
Yes. It appears in the electromagnetism section alongside Faraday’s Law.
2. Do HL and SL students study Lenz’s Law differently?
The principle is the same, but HL students see it in more complex applications, like alternating current generators and transformers.
3. How does Lenz’s Law relate to energy conservation?
It ensures that induced currents don’t create energy from nothing—the opposition is a direct consequence of energy conservation.
4. What’s the most common IB exam question on Lenz’s Law?
Determining the direction of induced currents in coils when a magnet moves toward or away.
5. Can I lose marks for skipping explanation?
Yes. IB examiners award marks for reasoning—always explain how Lenz’s Law opposes the change.
Conclusion
Lenz’s Law is more than a physics rule—it’s a principle of energy conservation applied to electromagnetism. In IB Physics HL and SL, it helps explain how induced currents resist change and powers real-world technology from trains to transformers. By practicing with diagrams, applying the right-hand rule, and connecting it to Faraday’s Law, you’ll be well prepared for IB exams.
RevisionDojo provides expert guidance to help you master electromagnetism and all IB Physics topics with confidence.
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