Master RBSE Class 12 Physics Chapter 5 (Magnetism and Matter). Get complete NCERT Solutions, key concepts, formulas, and important questions for top marks in your Rajasthan Board (RBSE) 2025 exams.
Introduction: Unlocking the World of Magnetism
Welcome, Class 12 RBSE students! Chapter 5 of your Physics syllabus, “Magnetism and Matter,” is a fascinating dive into the world of magnetic fields, their sources, and the magnetic properties of materials. This chapter, based on the NCERT textbook, is crucial for your board exams and forms a vital foundation for higher studies.
This comprehensive guide provides a complete breakdown of the core concepts, essential formulas, and detailed NCERT Solutions for Class 12 Physics Chapter 5 (Magnetism and Matter), specifically tailored for the Rajasthan Board (RBSE) examination format. Let’s aim for the top rank!
1. Core Concepts Class 12 Physics Chapter 5 : The Pillars of Magnetism and Matter
To excel in this chapter, you must have a clear understanding of the following key concepts:
| Topic | Key Concepts to Master | Essential Formula(s) |
| Bar Magnet | Magnetic field lines (properties), Bar magnet as an equivalent solenoid (analogy), Magnetic dipole moment (M | B |
| Dipole in a Uniform Field | Torque (τ | τ |
| Earth’s Magnetism | The three magnetic elements of Earth: Magnetic Declination (α), Angle of Dip (δ), Horizontal Component (BH). | BH=Becosδ BV=Besinδ tanδ=BHBV |
| Magnetisation | Magnetisation (I | B |
| Magnetic Materials | Classification: Diamagnetic, Paramagnetic, and Ferromagnetic substances (properties and examples). | Curie’s Law for Paramagnetism: χm∝T1 |
| Permanent Magnets | Hard ferromagnetic materials (high retentivity, high coercivity), Hysteresis loop concept. |
2. Detailed NCERT Solutions for Chapter 5 Exercises (RBSE Pattern)
The exercises in this chapter primarily involve applying formulas related to torque, potential energy, and Earth’s magnetic elements, along with conceptual questions on magnetic properties.
Type 1: Numerical Problems (Torque, Potential Energy, Magnetic Field)
Example 1: Torque on a Bar Magnet (NCERT Exercise 5.3)
Question: A short bar magnet is placed with its axis at 30∘ with a uniform external magnetic field of 0.25 T. It experiences a torque of magnitude 4.5×10−2 J. What is the magnitude of the magnetic moment of the magnet?
Solution: Given:
- Angle, θ=30∘
- Magnetic Field, B=0.25 T
- Torque, τ=4.5×10−2 J
Formula Used:
τ=MBsinθ
Calculation:
M=BsinθτM=(0.25 T)×sin(30∘)4.5×10−2 JM=0.25×0.54.5×10−2M=0.1254.5×10−2M=0.36 J/T
Answer: The magnitude of the magnetic moment (M) is 0.36 J/T.
Example 2: Potential Energy (NCERT Exercise 5.4)
Question: A bar magnet with M=0.32 J/T is in a uniform magnetic field B=0.15 T. What is the potential energy in (a) stable equilibrium and (b) unstable equilibrium?
Solution: Formula Used: U=−MBcosθ (a) Stable Equilibrium: The magnetic moment is parallel to the field, so θ=0∘.
Ua=−MBcos(0∘)=−(0.32)(0.15)(1)=−0.048 J
(b) Unstable Equilibrium: The magnetic moment is antiparallel to the field, so θ=180∘.
Ub=−MBcos(180∘)=−(0.32)(0.15)(−1)=+0.048 J
Type 2: Conceptual and Theoretical Questions
NCERT Exercise 5.1(a): The Magnetic Elements Question: Name the three independent quantities conventionally used to specify the Earth’s magnetic field. Answer: The three magnetic elements are:
- Magnetic Declination (α or D)
- Angle of Dip (δ or I)
- Horizontal Component of Earth’s Magnetic Field (BH or H)
NCERT Exercise 5.2(b): Earth’s Core Question: The Earth’s core contains iron. Yet, geologists do not regard this as a source of the Earth’s magnetism. Why? Answer: The Earth’s core is in a molten state and at a very high temperature. This temperature is much higher than the Curie Temperature of iron. Above the Curie Temperature, a ferromagnetic material like iron loses its magnetism and behaves as a paramagnetic substance. Thus, the solid iron core cannot be the primary source of Earth’s stable magnetic field. (The source is believed to be electric currents in the molten outer core).
3. RBSE Board Exam Important Questions
Focus on these frequently asked topics for guaranteed marks:
- Derivation: Derive the expressions for Magnetic Field on the Axial and Equatorial lines of a bar magnet.
- Definitions: Define Magnetic Declination, Angle of Dip, Magnetisation (I), and Magnetic Susceptibility (χm).
- Conceptual Distinction: Differentiate clearly between Diamagnetic, Paramagnetic, and Ferromagnetic materials based on μr, χm, and their behaviour in a non-uniform magnetic field.
- Numerical: Practice numerical problems involving the formulas τ=MBsinθ and U=−MBcosθ.
| Chapter 5 | (Open) |
By understanding these core concepts and practicing the detailed NCERT solutions, you will be fully prepared to tackle any question from RBSE Class 12 Physics Chapter 5 and secure a top rank!