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AS & A-Level Physics 20 — Magnetism and Induction

AS & A-Level Physics 20 — Magnetism and Induction

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Independent Deckloop revision aligned with the Cambridge International AS & A Level Physics (9702) syllabus, 2025–2027. Not affiliated with or endorsed by Cambridge International Education. Chapter 20 of 26: Magnetism and induction. Concepts, worked applications and misconception checks.

Physics EN A-Level
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Sources and Nature of Magnetic Fields

A magnetic field is a region of space where a magnetic force can be experienced. This force acts on other magnets and on moving electric charges. Fundamentally, all magnetic fields originate from two sources: permanent magnets, which have an inherent magnetic field due to the alignment of atomic magnetic moments, and moving electric charges, such as an electric current flowing through a wire. The interaction between these fields or between a field and a moving charge is what gives rise to magnetic forces. Unlike electric fields, which can be produced by stationary charges, magnetic fields require motion of charges or the presence of a permanent magnet.

Key points

  • A magnetic field is a region where magnetic forces are exerted.
  • Magnetic fields are produced by permanent magnets.
  • Magnetic fields are also produced by moving electric charges (electric currents).
  • Magnetic forces act on other magnets and on moving charges.

Worked example

Question

Identify two distinct physical scenarios where a magnetic field is present and describe its origin.

Solution

1. Scenario 1: A compass needle deflecting near a bar magnet.
2. Origin 1: The bar magnet is a permanent magnet, possessing an inherent magnetic field due to the alignment of magnetic domains within its material.
3. Scenario 2: A small plotting compass showing deflection when placed near a wire connected to a battery.
4. Origin 2: The electric current flowing through the wire consists of moving charges (electrons), which generate a magnetic field around the wire.

A magnetic field exists around a permanent bar magnet due to its intrinsic magnetic properties. A magnetic field also exists around a current-carrying wire, generated by the flow of electric charges.

Common pitfalls

  • Confusing magnetic fields with electric fields; electric fields are produced by stationary charges, while magnetic fields require moving charges or permanent magnets.
  • Believing that magnetic fields only originate from permanent magnets, overlooking the magnetic effect of electric currents.

Prerequisites

  • Understanding of electric charge is fundamental to moving charges.
  • Knowledge of electric current as a flow of charge carriers is essential.
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