Revision

Objectives

Lesson outcomes

Retrieve the key definitions, equations, and direction rules for magnetic fields.
Practise mixed questions involving forces, Hall probes, flux, and induction.
Identify weak areas before the magnetic fields and induction test.

Syllabus

CIE 9702 syllabus points

19 linked

20.1.1 understand that a magnetic field is an example of a field of force produced either by moving charges or by permanent magnets
20.1.2 represent a magnetic field by field lines
20.2.1 understand that a force might act on a current-carrying conductor placed in a magnetic field
20.2.2 recall and use the equation F = BIL sin θ, with directions as interpreted by Fleming’s left-hand rule
20.2.3 define magnetic flux density as the force acting per unit current per unit length on a wire placed at right‑angles to the magnetic field
20.3.1 determine the direction of the force on a charge moving in a magnetic field
20.3.2 recall and use F = BQv sin θ
20.3.3 understand the origin of the Hall voltage and derive and use the expression VH = BI / (ntq), where t = thickness
20.3.4 understand the use of a Hall probe to measure magnetic flux density
20.3.5 describe the motion of a charged particle moving in a uniform magnetic field perpendicular to the direction of motion of the particle
20.3.6 explain how electric and magnetic fields can be used in velocity selection
20.4.1 sketch magnetic field patterns due to the currents in a long straight wire, a flat circular coil and a long solenoid
20.4.2 understand that the magnetic field due to the current in a solenoid is increased by a ferrous core
20.4.3 explain the origin of the forces between current-carrying conductors and determine the direction of the forces
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 density
the force acting per unit current per unit length on a wire placed at right angles to the magnetic field.
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 consolidates the B4 magnetic fields and induction sequence before assessment. You should be able to move between force equations, direction rules, Hall voltage, magnetic flux, and induction explanations.

What You Need to Know

Magnetic force questions often depend on geometry and direction as much as equation choice.
Moving charges, current-carrying conductors, and Hall probes all use magnetic force ideas.
Flux and flux linkage are the foundation for electromagnetic induction.
Faraday’s law gives magnitude; Lenz’s law gives the opposing direction.
Practical and explanation questions often require precise language about the change causing the induced e.m.f.

How to Work Through It

Start with retrieval of all chapter 20 equations and direction rules.
Sort practice questions by skill: force, field pattern, Hall effect, flux, or induction.
Complete a mixed set without prompts so you practise choosing the model yourself.
Finish with a short target list for final revision before the test.

Check Your Understanding

Can you choose between F = BIL sin theta and F = BQv sin theta?
Can you explain Lenz’s law without saying only that it “opposes the field”?
Can you use Hall voltage and flux equations with correct SI units?
Can you sketch the required field patterns clearly?

Common Mistakes

Memorising equations without checking the angle or direction.
Losing marks through vague induction explanations.
Forgetting the difference between flux and flux linkage.
Mixing up force rules for currents and charges.

Next Steps

Use the test lesson to check whether calculation choice and explanation quality are secure.
Revisit any lesson where your revision questions are still unreliable.