What is Magnetism and Matter Class 12: Complete Physics Guide

Understanding Magnetism: Definition and Basics

Magnetism is a fundamental physical phenomenon where certain materials exert attractive or repulsive forces on others. It originates from the motion of electric charges and the intrinsic magnetic moments of elementary particles like electrons.

Key points:

• Magnetism is caused by moving charges or spin of electrons.
• It results in magnetic fields, which can attract or repel materials.
• The magnetic field is a vector field represented by magnetic field lines.

In Class 12 NCERT Physics, magnetism is studied to understand how materials respond to magnetic fields and the nature of these fields themselves.

Magnetic Properties of Matter: Types of Magnetic Materials

Materials respond differently to magnetic fields based on their atomic structure. The three main types of magnetic materials are:

Diamagnetic materials have no permanent magnetic moment, while paramagnetic materials have unpaired electrons causing weak attraction. Ferromagnetic materials exhibit strong magnetism due to aligned magnetic domains.

Magnetic Field and Magnetic Lines of Force

The magnetic field ($\vec{B}$) represents the magnetic influence around a magnet or current-carrying conductor.

• Magnetic field lines emerge from the north pole and enter the south pole.
• Lines never intersect and are denser where the field is stronger.
• The direction of $\vec{B}$ at any point is tangent to the field line there.

For example, the magnetic field around a bar magnet forms closed loops, illustrating the continuous nature of magnetic fields.

Magnetisation and Magnetic Susceptibility

Magnetisation ($\vec{M}$) is the magnetic moment per unit volume of a material. It indicates how much a material becomes magnetised when placed in a magnetic field.

• $\vec{M} = \frac{\text{Total magnetic moment}}{\text{Volume}}$
• Magnetic susceptibility ($\chi$) measures the degree of magnetisation:

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

where $\vec{H}$ is the magnetic field intensity.

• Positive $\chi$ indicates attraction (paramagnetic/ferromagnetic).
• Negative $\chi$ indicates repulsion (diamagnetic).

Understanding these helps predict how materials behave in magnetic fields.

Earth’s Magnetism and Magnetic Poles

Earth behaves like a giant magnet with magnetic poles near the geographic poles.

• The magnetic field of Earth protects us from solar winds.
• Magnetic poles are not fixed and slowly shift over time.
• Compass needles align with Earth's magnetic field, pointing towards the magnetic north pole.

This natural magnetism is a practical example of magnetism studied in Class 12 NCERT Physics.

Worked Example: Calculating Magnetisation

Problem: A material has a total magnetic moment of $2 \times 10^{-3}$ A·m$^2$ and volume $10^{-6}$ m$^3$. Calculate its magnetisation.

Solution:

Magnetisation $M = \frac{\text{Total magnetic moment}}{\text{Volume}} = \frac{2 \times 10^{-3}}{10^{-6}} = 2 \times 10^{3}$ A/m

Thus, the magnetisation is $2000$ A/m.

This formula is essential for Class 12 students to solve numerical problems on magnetism.