Q.What is meant by magnetic induction?

From a Paperclip to a Magnet: The Intuition

You already know that a magnet can pick up iron nails. But what is actually happening inside that nail when it gets near the magnet? And why does a plastic comb, rubbed on hair, pick up tiny bits of paper — but never iron filings?

The answer lies in magnetization — the process by which a material becomes magnetic.

Think of a piece of iron as a chaotic crowd of tiny compass needles. Each needle is an atomic magnetic moment (a tiny magnet, arising from the spin of electrons). In unmagnetized iron, these needles point in random directions. Their magnetic effects cancel out, so the iron as a whole shows no net magnetism.

Now bring a strong magnet close. Its magnetic field acts like a command: "Line up!" The tiny compass needles inside the iron start rotating, aligning themselves with the external field. The more they align, the stronger the iron's own magnetic field becomes. This alignment is magnetization.

The Precise Definition

Magnetization ($\vec{M}$) is the net magnetic dipole moment per unit volume of a material. It tells you how strongly a material is magnetized — how many tiny atomic magnets are aligned, and in which direction.

If a material has $N$ atoms per unit volume, each with an average magnetic moment $\vec{\mu}_{\text{avg}}$, then:

The SI unit of $\vec{M}$ is amperes per metre (A/m). Why? Because a magnetic dipole moment has units of A·m², and dividing by volume (m³) gives A/m.

How Magnetization Connects to the Magnetic Field

When a material gets magnetized, it produces its own magnetic field. The total magnetic field $\vec{B}$ inside the material is the sum of:

1. The external applied field $\vec{H}$ (caused by free currents, like the current in a solenoid)
2. The material's response — the magnetization $\vec{M}$

The fundamental relation is:

where $\mu_0 = 4\pi \times 10^{-7} \, \text{T·m/A}$ is the permeability of free space.

The Three Kinds of Magnetic Materials

Not all materials respond the same way to an external field. The magnetization $\vec{M}$ is proportional to $\vec{H}$ for most materials (at least for small fields):

where $\chi_m$ is the magnetic susceptibility — a dimensionless number that tells you how easily a material magnetizes.

A Simple Example

Take a long iron rod placed inside a solenoid carrying current $I$. The solenoid produces a uniform field $\vec{H}$ inside. The iron rod becomes magnetized: its atomic moments align, producing $\vec{M}$ in the same direction as $\vec{H}$.

If $\chi_m$ for iron is about $5000$, then $\vec{M} = 5000 \, \vec{H}$. The total field inside the rod becomes:

That is why an iron core can amplify the magnetic field of a solenoid by thousands of times.

The Bottom Line

Magnetization is the measure of how much a material becomes magnetic when placed in an external field. It arises from the alignment of atomic magnetic dipoles. For linear materials, $\vec{M} = \chi_m \vec{H}$. For ferromagnets, the response is nonlinear, strong, and can be permanent — that is how you get a bar magnet from a piece of iron.

Magnetization and the classification of materials as diamagnetic, paramagnetic and ferromagnetic is a core topic of the NCERT Class 12 Physics chapter on magnetism and matter, tested regularly in CBSE boards and JEE Main. Students searching "diamagnetic paramagnetic ferromagnetic materials class 12 physics difference" will find this magnetic-susceptibility-based comparison matches the standard NCERT table.