MAGNETISM AND MATTER

5.1 Introduction

Magnetism is a fundamental force observed universally, from galaxies to the atomic level. The term 'magnet' originates from a Greek island, Magnesia, where magnetic ores were found. Historical discoveries by scientists like Oersted and Ampere established that moving charges create magnetic fields. This chapter investigates magnetism comprehensively.

Basic Concepts

1. Earth’s magnetism: The Earth behaves like a giant magnet, exhibiting a magnetic field directed from geographic south to north.
2. Magnet poles: When a bar magnet is suspended, it aligns north-south, with the end pointing north termed the north pole and the opposite end the south pole.
3. Magnetic forces: Same poles repel each other, while opposite poles attract. Crucially, magnetic poles cannot be isolated; breaking a magnet yields two smaller magnets, both with a north and a south pole.
4. Materials: Common materials like iron can be magnetized, and the chapter elaborates on various types of magnetism related to different materials:
   • Paramagnetism: Weak attraction to external magnetic fields.
   • Diamagnetism: Repels external magnetic fields.
   • Ferromagnetism: Strong attraction and ability for materials to maintain magnetic properties.

5.2 Magnetic Field and Bar Magnets

Magnetic Field Lines

Iron filings visualized around a magnet reveal its magnetic field lines, illustrating how they form closed loops and do not intersect. Important properties:

1. Field lines are continuous.
2. Direction at any point is given by the tangent to the field line.
3. Density represents strength; closer lines indicate stronger fields.

Bar Magnet vs. Solenoid

A bar magnet can be viewed as a collection of current loops, resembling a solenoid. Both produce similar magnetic fields, which can be calculated based on the magnetic moment. The torque experienced by a magnetic dipole in a uniform field is given by τ = m × B, where m is the magnetic moment. The potential energy in a magnetic field is noted as U = –m.B.

5.3 Gauss's Law of Magnetism

Gauss's law for magnetism states that the net magnetic flux through any closed surface is zero, which indicates that magnetic field lines neither start nor end at a point, contradicting electric charge behavior. This reinforces the non-existence of isolated magnetic monopoles.

5.4 Magnetization and Magnetic Intensity

Definitions

1. Magnetization (M): Net magnetic moment per unit volume.
2. Magnetic intensity (H): External effects on materials. Given B = μ(H + M) where B is the magnetic field.
3. The relationship between M and H is expressed through magnetic susceptibility (χ). This helps classify materials:
   • Diamagnetic: χ < 0
   • Paramagnetic: 0 < χ < ε
   • Ferromagnetic: χ >> 1

5.5 Magnetic Properties of Materials

Types of Magnetic Materials

1. Diamagnetism: Materials repel magnetic fields, often have zero net magnetic moment normally, with induced moments opposing external fields.
   • e.g., bismuth, copper, and superconductors exhibit perfect diamagnetism (Meissner effect).
2. Paramagnetism: Materials are weakly attracted to magnetic fields due to aligned atomic dipoles, influenced by thermal motion.
   • e.g., aluminum and oxygen.
3. Ferromagnetism: Materials display strong magnetization due to cooperative alignment in domains, resulting in permanent magnetization post external field removal.
   • e.g., iron, cobalt, and hard ferromagnets like Alnico.

Summary Points

• The study of magnetism is foundational and dates back to ancient times.
• A bar magnet's orientation can be described mathematically through torque, field equations, and energy formulas.
• Understanding magnetism includes recognizing the unique properties and behaviors of various materials under magnetic influence.
• The classification of materials expands beyond basic categories, introducing advanced concepts in material science and physics related to magnetism.

5.6 Conclusion

The chapter concludes with exercises to reinforce understanding, extending concepts to practical applications. The intricate nature of magnetic phenomena unveils its critical role across physics and engineering.

1. Magnetic poles: Bar magnets have north and south poles; they cannot be isolated.
2. Magnetic fields: Field lines represent strength and direction; they form closed loops.
3. Gauss's law: The net magnetic flux through a closed surface is zero.
4. Types of magnetism: Includes diamagnetism (repels), paramagnetism (attracts weakly), and ferromagnetism (strong attraction, can retain magnetization).
5. Magnetization (M): Defined as the net magnetic moment per unit volume.
6. Magnetic intensity (H): Influences the magnetic properties of materials.
7. Susceptibility (χ): Determines material classification; diamagnetic (χ < 0), paramagnetic (χ > 0), ferromagnetic (χ >> 1).
8. Superconductors: Exhibit perfect diamagnetism and conductivity; expel magnetic fields (Meissner effect).