Class 12 Physics Notes

Magnetism and Matter Class 12 Physics Notes

Complete, exam-ready notes on magnetism and matter: magnetic dipoles and the bar magnet, magnetic field due to a dipole, Earth's magnetism, and the magnetic properties of materials (diamagnetic, paramagnetic and ferromagnetic) — written for CBSE boards, JEE and NEET revision.

Class12SubjectPhysicsCoversCBSE · JEE · NEET

Written byDeep Narayan· Science & Mathematics EducatorReviewed byPushpanjali

What is magnetism in one line?

Magnetism arises from the magnetic moments of atoms and electrons; in matter it is described by how materials respond to a magnetic field — as diamagnetic, paramagnetic or ferromagnetic depending on their atomic structure.

Bar Magnet and Magnetic Dipole

Magnetic dipole moment

m = NIA

For a current loop of N turns with current I and area A, the magnetic moment is NIA. The magnetic field at a point on the axis of a short bar magnet (distance r, far field) is $B_{\text{axis}} = \frac{\mu_0}{4\pi}\frac{2m}{r^3}$ , and on the equator $B_{\text{equator}} = \frac{\mu_0}{4\pi}\frac{m}{r^3}$ .

m = NIA,\quad B_{\text{axis}} = \frac{\mu_0}{4\pi}\frac{2m}{r^3}

Magnetic moment and axial field

Torque and Work on a Magnetic Dipole

Torque on a dipole in a uniform field:
$\tau = mB\sin\theta$
(vector:
$\vec{\tau} = \vec{m}\times\vec{B}$
).
Potential energy:
$U = -mB\cos\theta$
(minimum at θ = 0, aligned).
A compass needle aligns with the field because torque tends to align the moment with B.

\vec{\tau} = \vec{m}\times\vec{B},\quad U = -mB\cos\theta

Torque and energy of a dipole

Earth's Magnetism

Earth behaves like a magnetic dipole with the magnetic south pole near the geographic north.
Magnetic declination: angle between true north and magnetic north.
Angle of dip: angle the total field makes with the horizontal.
Horizontal component:
$B_H = B\cos\delta$
, vertical component:
$B_V = B\sin\delta$
.
At the magnetic poles the dip is 90°; at the equator it is 0°.

Earth's field variation

Earth's magnetic field is not constant — it varies with time (secular variation), drifts over decades, and has reversed polarity many times in geological history.

Magnetic Properties of Materials

$\text{Diamagnetic:}$
weakly repelled by a field; no unpaired electrons; susceptibility small and negative (e.g. bismuth, copper, water).
$\text{Paramagnetic:}$
weakly attracted; unpaired electrons; small positive susceptibility that decreases with temperature (e.g. aluminium, oxygen).
$\text{Ferromagnetic:}$
strongly attracted; domains align in the field; large positive susceptibility, exhibits hysteresis (e.g. iron, cobalt, nickel).

Curie temperature

Above the Curie temperature, a ferromagnetic material loses its ferromagnetism and becomes paramagnetic (e.g. iron at about 1043 K). Below it, domains can align spontaneously.

Hysteresis

A ferromagnetic material's magnetisation lags behind the applied field, tracing a closed loop called the hysteresis curve. The area of the loop represents energy loss per cycle.

Soft magnetic materials (e.g. iron): narrow hysteresis loop, low energy loss — used in transformer cores.
Hard magnetic materials (e.g. steel): wide loop, retain magnetisation — used for permanent magnets.

Solved Examples

Example: A bar magnet of moment $m = 1\,\text{A m}^2$ is placed at 30° to a $B = 0.2\,\text{T}$ field. Find the torque.

Solution: $\tau = mB\sin\theta = 1\times0.2\times\sin30 = 0.2\times0.5 = 0.1\,\text{N m}$ .

Revision

Key formulas at a glance

Memorise these before attempting numericals — most exam questions hinge on one of them.

Dipole moment

m = NIA

Axial field

B = \frac{\mu_0}{4\pi}\frac{2m}{r^3}

Equatorial field

B = \frac{\mu_0}{4\pi}\frac{m}{r^3}

Torque

\tau = mB\sin\theta

Potential energy

U = -mB\cos\theta

Earth's field components

B_H = B\cos\delta,\ B_V = B\sin\delta

Exam tips

How this chapter is asked

Where this topic appears in CBSE, JEE Main and NEET papers.

Torque aligns dipole moment with the field (U = −mB cosθ).
Diamagnetic: negative susceptibility; ferromagnetic: large positive.
Curie temperature: ferro → para transition.
Very far axial field ∝ 2m/r³, equatorial ∝ m/r³.
Hard magnets have wide hysteresis loops; soft magnets narrow.

FAQ

Common questions

What is the difference between diamagnetic, paramagnetic and ferromagnetic materials?

Diamagnetic materials are weakly repelled (no unpaired electrons); paramagnetic are weakly attracted (unpaired electrons, temp-dependent); ferromagnetic are strongly attracted with aligned domains and hysteresis.

What is hysteresis?

The lag of magnetisation behind the applied field in ferromagnetic materials, producing a closed loop whose area represents energy lost per magnetisation cycle.

What is the Curie temperature?

The temperature above which a ferromagnetic material loses its permanent magnetism and behaves as paramagnetic, because thermal agitation destroys the alignment of domains.

Why does a compass needle point north?

The needle is a magnet whose moment aligns with Earth's magnetic field, pointing toward the Earth's magnetic south pole near the geographic north.

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