Magnetism and Matter — Part 2: Magnetic properties of materials: visual lesson, CBSE Class 12 Physics

Lesson

Magnetic properties of materials

Part 2 — why iron is pulled strongly into a magnetic field, aluminium only slightly, and bismuth is pushed out.

In this lesson you will

Define magnetisation, magnetic intensity, susceptibility and relative permeability
Classify materials as diamagnetic, paramagnetic and ferromagnetic, with examples
Explain the effect of temperature on magnetic properties (Curie law, Curie temperature)

Concept

Magnetisation, intensity, susceptibility

Magnetisation $\vec M$ : net magnetic moment per unit volume (A m $^{-1}$ ). For a material in an applied field: $\vec B = \mu_0(\vec H + \vec M),\qquad \vec M = \chi\vec H,\qquad \vec B = \mu_0\mu_r\vec H,\qquad \mu_r = 1 + \chi$ $\vec H$ (magnetic intensity, A m $^{-1}$ ) is set by the free currents, e.g. $H = nI$ in a solenoid.

Table of magnetisation, magnetic intensity, susceptibility and relative permeability with symbols, units and meanings
Quantity	Symbol	Unit	Meaning
Magnetisation	M	A m⁻¹	Moment per unit volume
Magnetic intensity	H	A m⁻¹	Set by free currents
Susceptibility	χ	none	M/H
Relative permeability	μr	none	1 + χ

Worked example

Worked example 4: magnetisation of a bar

An iron bar 10 cm × 1.0 cm × 1.0 cm has a magnetic moment of 8.0 A m². Find its magnetisation.

Step 1 — volume $V = 0.10 \times 0.010 \times 0.010 = 1.0 \times 10^{-5}$ m³.
Step 2 — $M = \dfrac{m}{V} = \dfrac{8.0}{1.0 \times 10^{-5}} = 8.0 \times 10^{5}$ A m $^{-1}$ .
Comment — a strongly magnetised ferromagnet; the same volume of a paramagnet in a lab field would have M millions of times smaller.

Worked table: a volume of ten to the minus five cubic metre and a moment of 8 ampere square metre give a magnetisation of 8 times ten to the five amperes per metre
Quantity	Value
Volume	$1.0 \times 10^{-5}$ m³
Moment	8.0 A m²
Magnetisation	$8.0 \times 10^{5}$ A m $^{-1}$

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