EM Induction + Lenz's Law

Objectives

Lesson outcomes

Use Faraday's law to relate induced e.m.f. to rate of change of flux linkage.
Use Lenz's law to determine the direction of an induced current or e.m.f.
Explain induction experiments using opposition to the change producing the e.m.f.
Connect Lenz's law to conservation of energy.

Syllabus

CIE 9702 syllabus points

5 linked

20.5.1 define magnetic flux as the product of the magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density
20.5.2 recall and use Φ = BA
20.5.3 understand and use the concept of magnetic flux linkage
20.5.4 understand and explain experiments that demonstrate: • that a changing magnetic flux can induce an e.m.f. in a circuit • that the induced e.m.f. is in such a direction as to oppose the change producing it • the factors affecting the magnitude of the induced e.m.f.
20.5.5 recall and use Faraday’s and Lenz’s laws of electromagnetic induction

Definitions

Required definitions

Magnetic flux
magnetic flux density multiplied by the cross-sectional area perpendicular to the field.

Lesson Notes

Student guidance and lesson notes

Overview

This lesson adds direction to electromagnetic induction. You will use Faraday’s law for magnitude and Lenz’s law to decide which way the induced effect acts.

What You Need to Know

Faraday’s law links induced e.m.f. to the rate of change of magnetic flux linkage.
Lenz’s law states that the induced e.m.f. acts in a direction that opposes the change producing it.
The minus sign in Faraday’s law represents this opposition.
The induced current direction depends on the change in flux, not just the direction of the original magnetic field.
Lenz’s law is consistent with conservation of energy: work must be done to keep changing the flux.

How to Work Through It

Decide whether flux linkage is increasing or decreasing.
Work out what magnetic effect would oppose that change.
Use a direction rule to find the induced current needed to produce that effect.
Practise Faraday’s law calculations using change in flux linkage over time.

Check Your Understanding

What change is the induced current opposing in a given diagram?
Why does a magnet falling through a coil slow down?
How does the induced e.m.f. change if the same flux change happens in half the time?
Why would Lenz’s law be impossible if it helped the original change?

Common Mistakes

Opposing the magnetic field instead of opposing the change in flux.
Choosing current direction before identifying whether flux is increasing or decreasing.
Dropping the number of turns from a Faraday’s law calculation.
Treating the minus sign as a simple arithmetic negative rather than a direction statement.

Next Steps

Use Lenz’s law when interpreting transformers, eddy currents, and braking effects.
Bring both magnitude and direction reasoning into the revision lesson.