Magnetism and Matter is a significant chapter in the Class 12 Physics syllabus, offering a deep understanding of magnetic phenomena in materials and the Earth’s magnetism. This topic bridges the gap between electromagnetic theory and physical properties of matter. It helps students understand how magnetic fields interact with materials, how magnets work, and how magnetic properties vary among different substances. The chapter also explores the theoretical and mathematical aspects of magnetism, which play an important role in many scientific and technological applications.
Introduction to Magnetism
Magnetism is a physical phenomenon caused by the motion of electric charges. It gives rise to magnetic fields and magnetic forces. All materials respond to magnetic fields in different ways, depending on their atomic structure and electron arrangements. Magnetism and Matter, as a chapter, explains how magnetic properties arise in materials and how they are measured and categorized. The chapter also discusses the magnetic field due to a bar magnet and introduces the concept of the magnetic dipole moment.
Bar Magnet
Magnetic Dipole
A bar magnet is a simple magnetic dipole. It has two poles the north and the south separated by a distance. The strength of a magnetic dipole is characterized by its magnetic dipole moment (M), given by:
- M = m à 2l
Here,mis the pole strength, and2lis the distance between the poles. The magnetic dipole moment is a vector quantity and points from the south pole to the north pole.
Magnetic Field Lines
The magnetic field lines of a bar magnet originate from the north pole and end at the south pole. These lines are continuous loops and help visualize the strength and direction of the magnetic field. The closer the lines, the stronger the field.
Torque on a Magnetic Dipole
When a magnetic dipole is placed in a uniform magnetic field, it experiences a torque. The torque Ï is given by:
- Ï = M Ã B
Where M is the magnetic moment and B is the magnetic field. This torque tends to align the dipole with the magnetic field.
The Earth’s Magnetism
Earth behaves like a giant bar magnet with a magnetic field similar to that of a dipole. The magnetic field of the Earth is due to the motion of molten iron and nickel in its outer core. This field plays a crucial role in navigation and protects the planet from solar radiation.
Elements of Earth’s Magnetic Field
- Magnetic Declination: The angle between geographic north and magnetic north.
- Magnetic Inclination (Dip): The angle made by the magnetic field with the horizontal at a point on Earth.
- Horizontal Component (Bh): The component of Earth’s magnetic field in the horizontal direction.
Magnetization and Magnetic Materials
Magnetization (M)
Magnetization is the magnetic moment per unit volume of a material. It shows how strongly a material is magnetized under the influence of an external magnetic field. It is given by:
- M = m / V
Where m is the magnetic moment and V is the volume of the material.
Magnetic Intensity (H)
It is the external magnetic field applied to a material. It is measured in A/m (ampere per meter).
Magnetic Susceptibility (Ï)
Magnetic susceptibility is the ratio of magnetization (M) to magnetic field intensity (H):
- Ï = M / H
It is a dimensionless quantity and indicates how easily a material can be magnetized.
Magnetic Permeability (μ)
Permeability is the ability of a material to support the formation of a magnetic field within itself. It is related to magnetic susceptibility as:
- μ = μâ (1 + Ï)
Where μâ is the permeability of free space.
Classification of Magnetic Materials
Materials are classified into different types based on their behavior in a magnetic field:
- Diamagnetic materials: These have negative susceptibility. They are weakly repelled by magnetic fields. Examples: Bismuth, Copper.
- Paramagnetic materials: These have small positive susceptibility. They are weakly attracted to magnetic fields. Examples: Aluminum, Platinum.
- Ferromagnetic materials: These have large positive susceptibility. They are strongly attracted to magnetic fields and can retain magnetism. Examples: Iron, Nickel, Cobalt.
Hysteresis
Hysteresis refers to the lag between magnetization and the applied magnetic field. When a magnetic material is magnetized and then the external field is removed, it does not return to its original state immediately. The graph between magnetization and magnetic field intensity forms a loop called the hysteresis loop.
Key Points of the Hysteresis Loop
- Retentivity: The magnetization retained by the material when the external field is removed.
- Coercivity: The negative magnetic field required to bring the magnetization to zero.
Materials with high retentivity and coercivity are used to make permanent magnets. Soft iron, with low coercivity, is used in electromagnets.
Magnetic Properties of Solids
The magnetic behavior of a solid depends on the alignment of its atomic magnetic moments. In ferromagnetic substances, these moments align parallel even without an external field, making them strongly magnetic. In paramagnetic substances, moments align only in the presence of an external field. Diamagnetic materials show weak opposition to magnetic fields due to changes in electron orbits.
Magnetic Field Due to a Magnetic Dipole
The magnetic field at any point due to a dipole can be calculated using mathematical formulas. For example, at a point along the axial line of a magnetic dipole:
- B = (μâ / 4Ï) à (2M / r³)
And at a point along the equatorial line:
- B = (μâ / 4Ï) à (M / r³)
Where M is the magnetic moment and r is the distance from the center of the dipole.
Magnetic Potential Energy
When a magnetic dipole is placed in a magnetic field, it possesses potential energy. This energy is given by:
- U = M · B
This equation is analogous to electric potential energy and shows that the dipole tends to align itself to minimize energy.
Applications of Magnetism in Daily Life
Understanding magnetism and matter has practical importance in various fields. Some applications include:
- Electric motors and generators
- Transformers and inductors
- Magnetic storage devices like hard drives
- Magnetic levitation trains
- Medical imaging using MRI
Class 12 Magnetism and Matter is a foundational topic that combines theoretical concepts with practical significance. It explains how materials respond to magnetic fields, how the Earth’s magnetic field influences daily life, and how different substances behave magnetically. Mastering this chapter helps students develop analytical thinking and prepares them for advanced physics topics in higher education. Consistent practice, understanding definitions, and visualizing field patterns are key strategies for success in this chapter.