Magnetism and matter is a significant chapter in Class 12 Physics that deals with the magnetic properties of materials, Earth’s magnetism, and how magnetic effects are produced by electric currents. This chapter plays a critical role in board examinations and competitive exams like JEE and NEET. Understanding this topic helps students appreciate the connection between electricity and magnetism and provides a solid foundation for future studies in electromagnetism and modern physics. Class 12 Magnetism and Matter notes are designed to present these complex topics in a simplified and structured way that enhances both comprehension and retention.
Magnetism and Its Types
What is Magnetism?
Magnetism is a force of attraction or repulsion that acts at a distance due to the motion of electric charges. It is a property exhibited by certain materials when exposed to a magnetic field.
Types of Magnetic Materials
- Diamagnetic materials: Weakly repelled by magnetic fields. Examples include bismuth, copper, silver.
- Paramagnetic materials: Weakly attracted to magnetic fields. Examples include aluminum, platinum.
- Ferromagnetic materials: Strongly attracted and can retain magnetism. Examples include iron, cobalt, nickel.
Magnetic Properties of Materials
Magnetic Permeability and Susceptibility
These properties describe how a material responds to an external magnetic field.
- Magnetic Permeability (μ): The ability of a material to support the formation of a magnetic field within itself.
- Magnetic Susceptibility (Ï): A measure of how easily a material can be magnetized. It is the ratio of magnetization (M) to the applied magnetic field (H): Ï = M/H.
Classification Based on Susceptibility
- Diamagnetic: Ï< 0
- Paramagnetic: Ï >0 (small value)
- Ferromagnetic: Ï >>0 (large value)
Bar Magnet and Magnetic Field Lines
Properties of a Bar Magnet
A bar magnet has two poles north and south. It generates a magnetic field around it and shows attractive behavior when placed near magnetic materials.
- Like poles repel, unlike poles attract.
- Magnetic poles always exist in pairs.
- The field is strongest at the poles.
Magnetic Field Lines
These are imaginary lines used to represent the direction and strength of a magnetic field.
- They emerge from the north pole and enter the south pole.
- They never intersect.
- The closer the lines, the stronger the field.
Magnetic Dipole and Dipole Moment
Magnetic Dipole
A magnetic dipole consists of two equal and opposite magnetic charges separated by a small distance. A bar magnet behaves like a magnetic dipole.
Magnetic Dipole Moment (M)
It is the product of the pole strength (m) and the separation between the poles (2l):
M = m à 2l
The SI unit of magnetic dipole moment is A·m² (ampere square meter).
Magnetism in Earth
Earth’s Magnetism
The Earth itself acts like a giant magnet with a magnetic field similar to that produced by a bar magnet tilted slightly from the geographic axis.
Components of Earth’s Magnetic Field
- Magnetic Declination: The angle between geographic north and magnetic north.
- Magnetic Inclination (Dip): The angle made by the Earth’s magnetic field with the horizontal.
- Horizontal Component (Bh): The horizontal part of the Earth’s magnetic field vector.
Magnetometers
Instruments used to measure magnetic field strength and direction. They are often used in geological surveys and navigation systems.
Magnetic Force and Torque
Torque on a Magnetic Dipole
When a magnetic dipole is placed in a uniform magnetic field, it experiences a torque that tries to align it with the field. The torque (Ï) is given by:
Ï = M à B = MB sinθ
Where M is the dipole moment, B is the magnetic field, and θ is the angle between M and B.
Potential Energy of a Magnetic Dipole
The potential energy (U) of a magnetic dipole in a magnetic field is:
U = -M · B = -MB cosθ
Magnetization and Magnetic Intensity
Magnetization (M)
It is the magnetic moment per unit volume of a magnetized material. It represents the density of magnetic dipoles in a material.
Magnetizing Field (H)
The external magnetic field applied to magnetize a material. It is measured in amperes per meter (A/m).
Relation Between B, H, and M
The total magnetic field (B) inside a material is given by:
B = μâ(H + M)
Where μâ is the permeability of free space.
Hysteresis and Magnetic Materials
Hysteresis Loop
A graph of magnetization (M) versus magnetic field (H) shows how a material responds to magnetization and demagnetization. The area within the loop represents energy loss.
Retentivity and Coercivity
- Retentivity: The ability of a material to retain magnetization after the removal of the magnetic field.
- Coercivity: The intensity of magnetic field required to reduce magnetization to zero.
Applications of Hysteresis
Understanding hysteresis helps in choosing appropriate magnetic materials for transformers, memory storage, and electromagnets.
Magnetic Properties of Substances
- Soft Magnetic Materials: Easily magnetized and demagnetized. Used in transformers (e.g., soft iron).
- Hard Magnetic Materials: Retain magnetism for a long time. Used in permanent magnets (e.g., steel).
Tips to Study Magnetism and Matter
- Use diagrams: Visualizing magnetic field lines and dipole orientations improves memory retention.
- Practice numerical problems: Focus on torque, magnetic moment, and hysteresis-based questions.
- Revise definitions: Learn key terms like susceptibility, permeability, and coercivity.
- Understand applications: Relate concepts to real-world devices like compasses and MRI machines.
Importance of Class 12 Magnetism and Matter Notes
The Magnetism and Matter chapter in Class 12 Physics bridges the gap between theoretical magnetism and practical applications in electrical engineering and electronics. Understanding magnetic properties, dipole concepts, Earth’s magnetism, and the hysteresis phenomenon helps students appreciate both natural and artificial magnetic systems. Well-organized notes simplify the chapter’s content, reinforce classroom learning, and improve exam performance. With regular revision and application-based learning, students can master this topic effectively and lay a strong foundation for advanced studies and technical careers.