Chapter 5 — Magnetism and Matter

Chapter 5 of the Class 12 Physics NCERT textbook, "Magnetism and Matter", covers the properties of bar magnets, Gauss's law for magnetism, magnetisation, magnetic intensity, and the classification of materials as diamagnetic, paramagnetic, or ferromagnetic.

• The magnet as a dipole — The chapter shows a bar magnet behaves like a magnetic dipole, analogous to a current-carrying solenoid, unifying permanent magnets with the current-based magnetism of the previous chapter.
• No magnetic monopoles — Gauss's law for magnetism captures a striking fact: net magnetic flux through any closed surface is zero, because isolated north or south poles simply do not exist in nature.
• Describing magnetism in materials — Quantities like magnetisation, magnetic intensity, susceptibility, and permeability provide a language for how a material responds to and modifies an applied magnetic field.
• Classifying magnetic materials — Materials fall into diamagnetic, paramagnetic, and ferromagnetic classes based on how they respond to fields, with domain theory explaining why iron-like materials form strong permanent magnets.

1. 01A bar magnet acts as a magnetic dipole; its axial field is B = μ₀·2m / (4πr³) and equatorial field is B = –μ₀m / (4πr³) for r >> l.
2. 02Gauss's law for magnetism states the net magnetic flux through any closed surface is zero (∮ B·dS = 0), because isolated magnetic monopoles do not exist.
3. 03Magnetisation M is the net magnetic moment per unit volume; the total field in a material is B = μ₀(H + M), where H is the magnetic intensity.
4. 04Magnetic susceptibility χ relates M and H by M = χH; relative permeability μr = 1 + χ and permeability μ = μ₀μr.
5. 05Diamagnetic materials (e.g., bismuth, copper, water) are repelled by magnets (χ < 0); paramagnetic materials (e.g., aluminium, oxygen) are weakly attracted (χ small and positive).
6. 06Ferromagnetic materials (e.g., iron, cobalt, nickel) have domain structures and very large χ; hard ferromagnets form permanent magnets while soft ferromagnets lose magnetisation when the field is removed.