Introduction

You have already studied a short bar magnet as a magnetic dipole — a pair of poles of equal and opposite pole strength, separated along an axis, producing a magnetic field at points along that axis and along the perpendicular equatorial line. You have also observed that such a magnet, when suspended freely, always settles along the geographic North-South direction, which is precisely why a compass can be used for navigation.

This chapter builds on that foundation in two directions. First, it studies the behaviour of a short bar magnet kept in two mutually perpendicular magnetic fields — the torque and potential energy that result when it sits in an external uniform field, and the small angular oscillations it performs about its equilibrium orientation when disturbed. Second, it asks a deeper question that the earlier, purely macroscopic picture of a bar magnet does not answer: why do some materials behave as magnets (or become magnetised) while most everyday materials do not? Answering this requires going down to the atomic scale, where an orbiting and spinning electron is itself a tiny source of magnetic moment. Building on that atomic picture, the chapter classifies every material's magnetic response into three broad types — diamagnetism, paramagnetism, and ferromagnetism — studies how ferromagnetism depends on temperature and on magnetic history (hysteresis), and closes with three practical applications that follow directly from this understanding: permanent magnets, electromagnets, and magnetic shielding.