Introduction

Every bar magnet used in a school laboratory, every compass needle, and the Earth itself are all examples of the same underlying physics: a persistent arrangement of circulating electric charge that produces a magnetic field indistinguishable, from a distance, from that of a pair of equal and opposite "magnetic charges" -- poles -- held a fixed distance apart. This sub-topic, MAGNETISM AND MATTER, is the second half of WBCHSE Semester III's Unit 3, and it builds this picture up carefully, in the order the syllabus lists it.

• A current loop as a magnetic dipole (Section 1.2), and the magnetic dipole moment of a revolving electron (Section 1.3) -- the atomic-scale origin of magnetism.
• The bar magnet itself (Section 1.4), and the magnetic field it produces on its axis (Section 1.5) and on its equatorial line (Section 1.6).
• The torque a magnetic dipole feels in a uniform external field (Section 1.7), and magnetic field lines (Section 1.8).
• The bulk magnetic properties of matter: permeability and susceptibility (Section 1.9), intensity of magnetisation (Section 1.10), and retentivity and coercivity (Section 1.11).
• Hysteresis and the B-H loop (Section 1.12), treated qualitatively.
• The Earth's own magnetic field and its elements (Section 1.13).
• Diamagnetic, paramagnetic and ferromagnetic substances (Section 1.14).
• Electromagnets and the factors that decide their strength (Section 1.15).

These ideas are not confined to the physics laboratory. They explain why a compass always points roughly north-south, why MRI scanners and maglev trains need extremely strong electromagnets, why a transformer's iron core is deliberately made "soft" rather than "hard", and why some metals are pulled into a magnetic field while others are gently pushed away from it.