Magnetism and Matter — Part 2: Magnetic materials, electromagnets and the B–H loop: visual lesson, ISC Class 12 Physics

Lesson

Magnetic Materials

Part 2 — what happens to matter placed in a magnetic field, and how we choose materials for magnets and cores.

In this lesson you will

Define magnetisation, magnetic intensity, susceptibility and relative permeability, and use B = μ₀(H + M)
Classify dia-, para- and ferromagnetic substances and apply Curie’s law and the Curie temperature
Explain electromagnets and the B–H loop, and choose materials for permanent magnets, electromagnets and transformer cores

Concept

Magnetisation M and magnetic intensity H

Magnetisation $M$ is the net magnetic moment per unit volume of a material, $M = m/V$ (A m⁻¹). Magnetic intensity $H$ is the part of the field due to the free currents; inside a long solenoid $H = nI$ (A m⁻¹). The total field is $B = \mu_0(H + M)$

With no material, $M = 0$ and $B = \mu_0 H$ .

Table of magnetisation, magnetic intensity, susceptibility and relative permeability with their meanings and units
Quantity	Meaning	Unit
Magnetisation $M$	magnetic moment per unit volume, $m/V$	A m⁻¹
Magnetic intensity $H$	set by the free currents, e.g. $nI$ in a solenoid	A m⁻¹
Susceptibility $\chi$	$M/H$	none
Relative permeability $\mu_r$	$\mu/\mu_0 = 1 + \chi$	none

Concept

Susceptibility and relative permeability

For most materials $M$ is proportional to $H$ : $M = \chi H$ , where $\chi$ is the magnetic susceptibility (no unit). Then $B = \mu_0(1 + \chi)H = \mu_0\mu_r H, \qquad \mu_r = 1 + \chi$ $\mu_r$ is the relative permeability; the permeability is $\mu = \mu_0\mu_r$ .

$\chi < 0$ : diamagnetic. $\chi$ small and $> 0$ : paramagnetic. $\chi \gg 1$ : ferromagnetic.

Table of magnetisation, magnetic intensity, susceptibility and relative permeability with their meanings and units
Quantity	Meaning	Unit
Magnetisation $M$	magnetic moment per unit volume, $m/V$	A m⁻¹
Magnetic intensity $H$	set by the free currents, e.g. $nI$ in a solenoid	A m⁻¹
Susceptibility $\chi$	$M/H$	none
Relative permeability $\mu_r$	$\mu/\mu_0 = 1 + \chi$	none

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