Chapter 5 Magnetism And Matter

Class 12 Physics: Chapter 5 - Magnetism and Matter

Introduction

This chapter explores the fundamental concepts of magnetism, magnetic properties of materials, Earth's magnetism, and the magnetic effects of current. Key concepts include magnetic dipoles, magnetic field intensity, and magnetic susceptibility.

Topics Covered

1. Magnetism and Gauss's Law
2. Magnetic Dipole
3. Magnetisation and Magnetic Intensity
4. Magnetic Properties of Materials
5. Earth's Magnetism
6. Magnetic Declination and Dip
7. Magnetic Hysteresis

Magnetism and Gauss's Law

Magnetism is a class of physical phenomena mediated by magnetic fields. Gauss's law for magnetism states that the net magnetic flux through any closed surface is zero. This implies that magnetic monopoles do not exist.

∮ B · dA = 0

where B is the magnetic field, and dA is the differential area vector on a closed surface.

Magnetic Dipole

A magnetic dipole is a system that generates a magnetic field, typically represented by a bar magnet or a current loop. The magnetic moment (m) of a dipole in a uniform magnetic field (B) experiences a torque (τ) given by:

τ = m × B

The potential energy (U) of a magnetic dipole in a magnetic field is given by:

U = -m · B

Magnetisation and Magnetic Intensity

Magnetisation (M) is the vector field representing the density of permanent or induced magnetic dipole moments in a magnetic material. Magnetic Intensity (H) is the measure of the magnetising force.

B = μ₀(H + M)

where B is the magnetic field, μ₀ is the permeability of free space, H is the magnetic intensity, and M is the magnetisation.

Magnetic Properties of Materials

Materials can be classified based on their magnetic properties into diamagnetic, paramagnetic, and ferromagnetic materials:

Diamagnetic Materials

These materials create an induced magnetic field in a direction opposite to an externally applied magnetic field and are repelled by a magnetic field.

Paramagnetic Materials

These materials create an induced magnetic field in the same direction as an externally applied magnetic field and are attracted to a magnetic field.

Ferromagnetic Materials

These materials have a strong attraction to magnetic fields and can retain their magnetic properties after the external field is removed.

Earth's Magnetism

The Earth behaves like a giant bar magnet with a magnetic field similar to that of a dipole. The magnetic field is due to the movement of molten iron alloys in the Earth's outer core.

Magnetic Declination and Dip

Magnetic declination is the angle between geographic north and magnetic north. Magnetic dip (or inclination) is the angle that the Earth's magnetic field makes with the horizontal at any point.

Magnetic Hysteresis

Magnetic hysteresis refers to the lag between the change in magnetisation of a material and the change in the magnetic field. The hysteresis loop represents this lag and shows the relationship between B and H.

Important Terms and Meanings

Magnetic Dipole

Magnetic Dipole: A pair of equal and opposite magnetic charges or poles separated by a distance, creating a magnetic field.

Magnetisation

Magnetisation: The process of inducing a magnetic dipole moment in a material.

Magnetic Intensity

Magnetic Intensity: A measure of the strength of a magnetic field in a given direction.

Diamagnetic

Diamagnetic: Materials that create an induced magnetic field in the opposite direction to an applied magnetic field and are weakly repelled by a magnetic field.

Paramagnetic

Paramagnetic: Materials that create an induced magnetic field in the same direction as an applied magnetic field and are weakly attracted to a magnetic field.

Ferromagnetic

Ferromagnetic: Materials that exhibit strong attraction to magnetic fields and can retain their magnetic properties.

Magnetic Declination

Magnetic Declination: The angle between geographic north and magnetic north.

Magnetic Dip

Magnetic Dip: The angle that the Earth's magnetic field makes with the horizontal at any point.

Hysteresis

Hysteresis: The lag between the change in magnetisation of a material and the change in the magnetic field.

Frequently Asked Questions (FAQ)

1. What is magnetism?

Magnetism is a class of physical phenomena mediated by magnetic fields, where materials can attract or repel each other.

2. What does Gauss's law for magnetism state?

Gauss's law for magnetism states that the net magnetic flux through any closed surface is zero, implying the non-existence of magnetic monopoles.

3. What is a magnetic dipole?

A magnetic dipole consists of two equal and opposite magnetic charges or poles separated by a distance, creating a magnetic field.

4. How is the magnetic moment related to torque?

The magnetic moment (m) of a dipole in a uniform magnetic field (B) experiences a torque (τ) given by τ = m × B.

5. What is magnetisation?

Magnetisation (M) is the vector field representing the density of permanent or induced magnetic dipole moments in a material.

6. What is magnetic intensity?

Magnetic Intensity (H) is a measure of the magnetising force applied to a material.

7. How is the magnetic field inside a material represented?

The magnetic field inside a material is represented by B = μ₀(H + M), where μ₀ is the permeability of free space, H is the magnetic intensity, and M is the magnetisation.

8. What are diamagnetic materials?

Diamagnetic materials create an induced magnetic field in the opposite direction to an applied magnetic field and are weakly repelled by a magnetic field.

9. What are paramagnetic materials?

Paramagnetic materials create an induced magnetic field in the same direction as an applied magnetic field and are weakly attracted to a magnetic field.

10. What are ferromagnetic materials?

Ferromagnetic materials exhibit strong attraction to magnetic fields and can retain their magnetic properties after the external field is removed.

11. How does the Earth generate its magnetic field?

The Earth's magnetic field is generated by the movement of molten iron alloys in its outer core, which creates electric currents.

12. What is magnetic declination?

Magnetic declination is the angle between geographic north and magnetic north at a given location.

13. What is magnetic dip?

Magnetic dip (or inclination) is the angle that the Earth's magnetic field makes with the horizontal at any point.

14. What is magnetic hysteresis?

Magnetic hysteresis refers to the lag between the change in magnetisation of a material and the change in the magnetic field, depicted by the hysteresis loop.

15. Why do diamagnetic materials repel magnetic fields?

Diamagnetic materials repel magnetic fields because their induced magnetic moments oppose the external magnetic field.

16. Why do paramagnetic materials attract magnetic fields?

Paramagnetic materials attract magnetic fields because their induced magnetic moments align with the external magnetic field.

17. What is the significance of the hysteresis loop?

The hysteresis loop shows the relationship between the magnetic field (H) and the magnetisation (B) of a material, indicating energy loss due to hysteresis.

18. How does a ferromagnetic material retain its magnetism?

Ferromagnetic materials retain their magnetism due to the alignment of magnetic domains, which remain aligned even after the external field is removed.

19. What is the role of magnetic domains in ferromagnetic materials?

Magnetic domains in ferromagnetic materials are regions where the magnetic moments are aligned in the same direction, contributing to strong magnetisation.

20. How does temperature affect ferromagnetic materials?

Temperature affects ferromagnetic materials by causing thermal agitation, which can disrupt the alignment of magnetic domains and reduce magnetisation at higher temperatures.