Schooling · Class 12 · Physics · Magnetism and Matter · Chapter on a page
CBSE Class 12 · Physics · NCERT Physics Part-I
Magnetism and Matter: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
This note's current text has no applied fact-check on record.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- A bar magnet is equivalent to a…
- When a magnetic dipole of moment m…
- Gauss's law for magnetism states…
- The axial and equatorial fields of a…
- The Earth's magnetic field at any…
- Magnetisation M is the magnetic…
- Magnetic susceptibility χ = M/H…
- Formulas and facts
- Magnetic dipole moment
- Torque on dipole
- Potential energy of dipole
- Axial field of a short magnet
- Equatorial field of a short magnet
- Relation between B, H and M
- Susceptibility and permeability
- Worked examples
- Torque on a bar magnet
- Finding Earth's field components
- Susceptibility and magnetisation
- Watch out for
- Confusing magnetic field B with…
- Treating the angle of dip as the…
- Assuming isolated north or south…
- Using the electric-dipole energy…
- Forgetting that ferromagnets show…
What this chapter is about
This chapter explores how materials respond to magnetic fields and introduces the idea that magnetism, at its core, arises from circulating currents at the atomic level. You have already studied how moving charges and currents produce magnetic fields; here you learn that every magnetic phenomenon in matter can be traced to tiny current loops within atoms.
Key ideas
- A bar magnet is equivalent to a magnetic dipole; its dipole moment m = (pole strength) × (magnetic length), direction from S-pole to N-pole inside the magnet.
- When a magnetic dipole of moment m sits in a uniform field B, it experiences a torque τ = m × B and has potential energy U = −m · B.
- Gauss's law for magnetism states that the net magnetic flux through any closed surface is zero: ∮B · dA = 0. This implies isolated magnetic poles (monopoles) do not exist.
- The axial and equatorial fields of a short bar magnet mirror those of an electric dipole: B_axial ≈ (μ₀/4π)(2m/r³), B_equatorial ≈ (μ₀/4π)(m/r³).
- The Earth's magnetic field at any location is described by three elements: declination (angle between geographic and magnetic meridians), dip or inclination (angle the field makes with horizontal), and horizontal component B_H.
- Magnetisation M is the magnetic moment per unit volume of a material. Magnetic intensity H is defined by B = μ₀(H + M).
- Magnetic susceptibility χ = M/H tells how easily a material is magnetised; permeability μ = B/H and relative permeability μ_r = 1 + χ connect these quantities.
- Materials are classified as diamagnetic (χ small and negative), paramagnetic (χ small and positive), or ferromagnetic (χ large and positive, with hysteresis).
Formulas and facts to remember
- Magnetic dipole moment: m = n I A for a coil of n turns, area A carrying current I.
- Torque on dipole: τ = m B sin θ, maximum when θ = 90°.
- Potential energy of dipole: U = −m B cos θ; minimum when m is along B.
- Axial field of a short magnet: B = (μ₀/4π)(2m/r³).
- Equatorial field of a short magnet: B = (μ₀/4π)(m/r³), direction opposite to m.
- Relation between B, H and M: B = μ₀(H + M) = μ₀ μ_r H.
- Susceptibility and permeability: χ = M/H; μ_r = 1 + χ.
- Curie law (paramagnets): χ ∝ 1/T.
Common mistakes
- Confusing magnetic field B with magnetic intensity H → remember B includes the contribution of the material (M), while H relates to free currents alone.
- Treating the angle of dip as the same everywhere → dip varies with latitude; it is 0° at the magnetic equator and 90° at the magnetic poles.
- Assuming isolated north or south poles can exist → Gauss's law for magnetism forbids monopoles; every magnet has both poles.
- Using the electric-dipole energy formula with a wrong sign → U = −m B cos θ; minimum energy (stable) is when θ = 0.
- Forgetting that ferromagnets show hysteresis → their magnetisation depends on magnetic history, not just the current field.
Quick revision
- A bar magnet is a magnetic dipole with moment m; its field resembles that of an electric dipole at large distances.
- Torque τ = m B sin θ; potential energy U = −m B cos θ.
- Earth's field is described by declination, dip and horizontal component.
- B = μ₀(H + M); χ = M/H; μ_r = 1 + χ.
- Diamagnets have χ < 0; paramagnets have small χ > 0; ferromagnets have large χ > 0 and exhibit hysteresis.
- Gauss's law for magnetism: ∮B · dA = 0 — no magnetic monopoles.