EM Induction

Objectives

Lesson outcomes

Define magnetic flux and use Phi = BA when the field is perpendicular to the area.
Use the concept of magnetic flux linkage for coils.
Explain how a changing magnetic flux can induce an e.m.f.
Identify factors that affect the magnitude of an induced e.m.f.

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 introduces magnetic flux, flux linkage, and the basic condition for electromagnetic induction. You will focus on how changing flux produces an induced e.m.f.

What You Need to Know

Use magnetic flux to track how much magnetic field passes through a perpendicular area.
When the field is perpendicular to the area, Phi = BA.
Flux linkage is the magnetic flux linked with all turns of a coil.
An e.m.f. is induced when magnetic flux linkage changes.
A larger rate of change of flux linkage gives a larger induced e.m.f.
Moving a magnet, moving a coil, changing field strength, changing area, or changing angle can all change flux linkage.

How to Work Through It

Identify the area perpendicular to the magnetic field in different diagrams.
Calculate magnetic flux using Phi = BA.
Extend from one loop to a coil using flux linkage.
Explain induction demonstrations in terms of changing flux linkage.

Check Your Understanding

What must change for an e.m.f. to be induced?
Why does moving a magnet faster usually increase the induced e.m.f.?
How does adding turns to a coil affect flux linkage?
When is Phi = BA valid without an angle factor?

Common Mistakes

Saying magnetic flux alone induces an e.m.f.; it must change.
Using the full surface area when only the perpendicular area is relevant.
Forgetting to include the number of turns for flux linkage.
Confusing induced e.m.f. with induced current when the circuit is not complete.

Next Steps

Keep the flux and flux linkage definitions secure before applying Faraday’s and Lenz’s laws.
Practise explaining induction demonstrations using rate of change.