Magnetism and Matter is an important chapter in Class 12 Physics that explains how materials behave in magnetic fields. Unlike the previous chapter (Moving Charges and Magnetism), which focuses on currents producing magnetic fields, this chapter focuses on magnetic properties of materials and permanent magnets.
This chapter is very important for:
- CBSE Board Exams (theory + numericals)
- JEE (conceptual + application)
- NEET (direct concept-based questions)
👉 Core Idea: Matter responds differently to magnetic fields depending on its internal structure.
1. Bar Magnet and Its Properties
Concept Explanation
A bar magnet behaves like a magnetic dipole with two poles: North and South.
Key Properties
- Like poles repel, unlike poles attract
- Poles always exist in pairs (no isolated pole)
- Magnetic field lines go from North to South outside the magnet
Diagram (Conceptual)
N —————- S
→ → → → → → → → →
← ← ← ← ← ← ← ← ←
2. Magnetic Field Lines (Deep Concept)
Definition
Magnetic field lines represent the direction and strength of magnetic field.
Important Properties
- Always form closed loops
- Never intersect
- Density represents field strength
3. Magnetic Dipole and Dipole Moment
Definition
A magnetic dipole consists of two equal and opposite magnetic poles separated by a distance.
Dipole Moment
M = m × 2l
Where:
- m = pole strength
- 2l = distance
Physical Meaning
It represents strength of magnet.
4. Torque on Magnetic Dipole
Formula
τ = MB sinθ
Concept
Magnet tries to align with magnetic field.
5. Magnetic Field Due to Bar Magnet
Axial Line
B = (μ₀/4π) × (2M / r³)
Equatorial Line
B = (μ₀/4π) × (M / r³)
6. Earth’s Magnetism (Very Important Theory)
Concept
Earth behaves like a giant magnet.
Magnetic Elements
- Declination (D)
- Inclination (I)
- Horizontal component (H)
Relation
B = √(H² + V²)
7. Types of Magnetic Materials (Concept-Based)
1. Diamagnetic
- Weakly repelled
- No permanent dipoles
Examples: Bismuth, Copper
2. Paramagnetic
- Weakly attracted
- Temporary dipoles
Examples: Aluminium
3. Ferromagnetic
- Strongly attracted
- Permanent dipoles
Examples: Iron, Cobalt
8. Magnetisation and Magnetic Intensity
Magnetisation (M)
Magnetic moment per unit volume
Magnetic Field Intensity (H)
External magnetic field
9. Susceptibility and Permeability
Magnetic Susceptibility
χ = M/H
Permeability
μ = μ₀(1 + χ)
10. Hysteresis (Important Concept)
Definition
Lagging of magnetisation behind magnetic field.
Graph Insight
- Loop represents energy loss
Application
Used in transformer cores
Important Numericals (Exam-Oriented)
Numerical 1
Find magnetic field on axial line at 10 cm from magnet.
Use formula:
B = (μ₀/4π)(2M/r³)
Numerical 2
Find dipole moment if pole strength = 2 A·m and length = 0.5 m
M = m × 2l = 2 × 1 = 2 A·m²
Numerical 3
Find susceptibility if M = 4 A/m and H = 2 A/m
χ = 4/2 = 2
Important Formula Sheet
- M = m × 2l
- τ = MB sinθ
- χ = M/H
- μ = μ₀(1 + χ)
JEE / NEET Focus
- Material properties
- Hysteresis
- Earth magnetism
CBSE Board Strategy
- Write definitions clearly
- Draw diagrams
- Practice numericals
Conclusion (Teaching Insight)
This chapter helps us understand how materials behave in magnetic fields. Concept clarity is more important than memorizing formulas. Focus on understanding types of materials and magnetic properties.