What is Magnetism and Matter Class 12: Complete NCERT Guide

Understanding Magnetism: Definition and Basics

Magnetism is a fundamental force associated with moving electric charges and intrinsic magnetic moments of particles like electrons. In Class 12 Physics, magnetism is introduced as the force exerted by magnets when they attract or repel each other or magnetic materials.

Key points:

• Magnetism originates from the motion of electrons and their spin.
• Magnetic fields are represented by field lines that emerge from the north pole and enter the south pole.
• The unit of magnetic field (magnetic flux density) is Tesla (T).

The chapter begins by explaining these basic concepts to build a foundation for more complex topics like magnetic properties of matter.

Magnetic Field and Magnetic Lines of Force

A magnetic field is a region around a magnet where magnetic forces can be detected. It is a vector quantity, denoted by $\vec{B}$.

Magnetic lines of force:

• Are imaginary lines used to represent the magnetic field.
• Originate from the north pole and terminate at the south pole.
• Never intersect each other.
• Are denser where the magnetic field is stronger.

Formula for magnetic field due to a long straight current-carrying wire:

$$B = \frac{\mu_0 I}{2 \pi r}$$

where:

• $B$ is magnetic field,
• $\mu_0$ is permeability of free space,
• $I$ is current,
• $r$ is distance from wire.

Understanding these lines helps visualize how magnets interact with materials.

Magnetic Properties of Materials: Classification

Materials respond differently when placed in a magnetic field. Based on their response, materials are classified as:

Diamagnetic materials have no permanent magnetic moment and create an induced magnetic field opposite to the applied field.

Paramagnetic materials have unpaired electrons and align weakly with the magnetic field.

Ferromagnetic materials have permanent magnetic domains that align strongly, showing strong attraction.

Magnetisation and Magnetic Intensity

Magnetisation ($\vec{M}$) is the magnetic moment per unit volume of a material and represents how much a material is magnetised in an external magnetic field.

It is given by:

$$\vec{M} = \frac{\text{Total magnetic moment}}{\text{Volume}}$$

Magnetic intensity or magnetic field strength ($\vec{H}$) is defined as the magnetising force applied to the material.

The relation between magnetic induction ($\vec{B}$), magnetisation ($\vec{M}$), and magnetic intensity ($\vec{H}$) is:

$$\vec{B} = \mu_0 (\vec{H} + \vec{M})$$

where $\mu_0$ is the permeability of free space.

This relation helps in understanding how materials affect the magnetic field inside them.

Magnetic Susceptibility and Permeability Explained

Magnetic susceptibility ($\chi$) measures how much a material will become magnetised in an applied magnetic field. It is defined as:

$$\chi = \frac{M}{H}$$

where:

• $M$ is magnetisation,
• $H$ is magnetic field strength.

Magnetic permeability ($\mu$) indicates how easily a magnetic field can pass through a material:

$$\mu = \frac{B}{H} = \mu_0 (1 + \chi)$$

Materials with $\chi > 0$ (paramagnetic and ferromagnetic) increase the magnetic field inside them, while diamagnetic materials have $\chi < 0$ and reduce it.

This concept is crucial for understanding magnetic circuits and devices.

Ferromagnetism: Domains and Hysteresis

Ferromagnetic materials show strong magnetic properties due to the presence of magnetic domains—small regions with aligned magnetic moments.

Key concepts:

• Domains are randomly oriented without an external field.
• When magnetised, domains align, increasing net magnetisation.
• Hysteresis is the lag between magnetisation and applied magnetic field, causing residual magnetism.

Hysteresis loop shows the relationship between $B$ and $H$:

• Coercivity: Field required to reduce magnetisation to zero.
• Retentivity: Residual magnetism after removing the field.

This explains how permanent magnets work and why some materials are used for magnetic storage.

Worked Example: Calculating Magnetic Field Near a Wire

Problem: Calculate the magnetic field at a distance of 5 cm from a long straight wire carrying a current of 3 A.

Solution:

Given:

• $I = 3$ A
• $r = 5$ cm = 0.05 m
• $\mu_0 = 4\pi \times 10^{-7}$ T·m/A

Using formula:

$$B = \frac{\mu_0 I}{2 \pi r} = \frac{4\pi \times 10^{-7} \times 3}{2 \pi \times 0.05}$$

Simplify:

$$B = \frac{12\pi \times 10^{-7}}{2 \pi \times 0.05} = \frac{12 \times 10^{-7}}{0.1} = 1.2 \times 10^{-5} \text{ T}$$

Answer: Magnetic field $B = 1.2 \times 10^{-5}$ Tesla.