Magnetism and Matter — Physics Class 12 Notes (CBSE & HBSE)
Free NCERT Physics notes for Magnetism and Matter (Class 12) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — Magnetism and Matter (CBSE & HBSE)
CBSE focuses on derivation of axial and equatorial fields of bar magnet, earth's magnetism parameters, and classification of magnetic materials with susceptibility. HBSE emphasizes bar magnet properties, gauss's law for magnetism, and definitions of dia, para, and ferromagnetic materials.
Bar Magnet and Earth's Magnetism
Bar Magnet as Magnetic Dipole
A bar magnet acts as a magnetic dipole with North (N) and South (S) poles separated by distance 2l.
Magnetic dipole moment: M = m×2l (A·m²) where m = pole strength (A·m)
Magnetic field on Axial line (at distance r from center, r >> l): B_axial = μ₀·2M/(4π·r³) (directed from S to N, along dipole direction)
Magnetic field on Equatorial line (at distance r from center, r >> l): B_equatorial = μ₀·M/(4π·r³) (directed from N to S, antiparallel to M)
Ratio: B_axial : B_equatorial = 2 : 1 (at same distance)
Gauss's Law for Magnetism
∮B⃗·dA⃗ = 0 (for any closed surface)
Magnetic monopoles do not exist — field lines always form closed loops.
Torque on Bar Magnet in Uniform Field
τ = MB sinθ = M⃗ × B⃗
- Stable equilibrium: θ = 0° (aligned with B)
- Unstable equilibrium: θ = 180°
- Maximum torque: θ = 90°, τ_max = MB
Potential energy: U = −MB cosθ = −M⃗·B⃗
Earth's Magnetism
Earth behaves as a huge bar magnet with its magnetic south pole near geographic north pole.
Three elements of Earth's magnetism:
| Element | Symbol | Definition |
|---|---|---|
| Magnetic Declination | α | Angle between geographic north and magnetic north |
| Angle of Dip (Inclination) | δ | Angle B makes with horizontal at a point |
| Horizontal Component | H | Horizontal component of Earth's total field B_E |
Relations:
- H = B_E cosδ
- V = B_E sinδ (vertical component)
- tan δ = V/H
- B_E = √(H² + V²)
Geographic vs Magnetic Poles
- Geographic poles: axis of Earth's rotation
- Magnetic poles: where field lines are vertical (dip = 90°)
- Magnetic equator: dip = 0°, H = B_E
- Geographic and magnetic north poles do NOT coincide
Diagram Indicator: [Earth with geographic axis vertical and magnetic axis tilted ~11.5°; field lines emerging near geographic south; dip angle δ, horizontal H and vertical V components shown at a location.]
Magnetic Properties: Para, Dia, Ferromagnetic
Magnetization (M)
The magnetization M is the net magnetic moment per unit volume: M = m_net/V (A/m)
Magnetic Intensity (H) and B
B = μ₀(H + M) = μ₀H(1 + χ_m) = μ₀μᵣH = μH
where:
- χ_m = M/H = magnetic susceptibility (dimensionless)
- μᵣ = 1 + χ_m = relative permeability
- μ = μ₀μᵣ = absolute permeability
Classification of Magnetic Materials
1. Diamagnetic Materials
- χ_m: small negative (−10⁻⁶ to −10⁻⁵)
- μᵣ slightly < 1
- Weakly repelled by magnets
- No permanent magnetic moment
- Examples: Bismuth (Bi), Copper (Cu), Silver (Ag), Water, Gold
- Cause: orbital motion opposes applied field
2. Paramagnetic Materials
- χ_m: small positive (+10⁻⁵ to +10⁻³)
- μᵣ slightly > 1
- Weakly attracted by magnets
- Have permanent atomic magnetic moments (randomly oriented)
- Examples: Aluminium, Platinum, Chromium, Oxygen
- Curie's Law: χ_m = C/T (susceptibility decreases with temperature)
3. Ferromagnetic Materials
- χ_m: very large positive (10³ to 10⁵)
- μᵣ >> 1
- Strongly attracted by magnets
- Have magnetic domains (regions of aligned moments)
- Examples: Iron (Fe), Nickel (Ni), Cobalt (Co), Gadolinium
- Above Curie temperature T_C: ferromagnetic → paramagnetic
Comparison Table
| Property | Diamagnetic | Paramagnetic | Ferromagnetic |
|---|---|---|---|
| χ_m | Small, negative | Small, positive | Large, positive |
| μᵣ | < 1 | > 1 | >> 1 |
| Attraction | Repelled | Weakly attracted | Strongly attracted |
| Curie law | Not obeyed | Obeyed (χ∝1/T) | Modified Curie-Weiss |
| Temperature | Independent | Decreases with T | Loses at T_C |
| Example | Bi, Cu, Au | Al, Pt, Cr | Fe, Ni, Co |
Physical Origin
- Diamagnetic: Lenz's law at atomic level — induced moments oppose B
- Paramagnetic: Permanent moments align with B (partial, resisted by thermal motion)
- Ferromagnetic: Domains align with B; exchange interaction between electrons
Diagram Indicator: [Diagrams of three material types: diamagnetic (moments antiparallel), paramagnetic (random moments partially aligning), ferromagnetic (domains with aligned moments). B-H curves for each type.]
Hysteresis and Permanent Magnets
Hysteresis Loop (B-H Curve)
When a ferromagnetic material is subjected to a cyclically varying magnetizing field H, the B-H relationship traces a loop called the hysteresis loop.
Key Points on Hysteresis Loop
| Point/Term | Symbol | Description |
|---|---|---|
| Saturation | B_s | Maximum B when all domains aligned |
| Retentivity | B_r | Residual B when H returns to zero |
| Coercivity | H_c | Reverse H needed to demagnetize (B=0) |
| Energy loss | — | Area enclosed by hysteresis loop per cycle |
Soft vs Hard Magnetic Materials
| Property | Soft Iron (Soft) | Steel/Alnico (Hard) |
|---|---|---|
| Retentivity | Low | High |
| Coercivity | Low | High |
| Hysteresis loss | Low (small loop area) | High (large loop area) |
| Magnetization | Easy | Difficult |
| Demagnetization | Easy | Difficult |
| Use | Electromagnets, transformers, cores | Permanent magnets |
Permanent Magnets
Made of hard magnetic materials (steel, alnico, ferrites, neodymium). Requirements:
- High retentivity (to maintain magnetization)
- High coercivity (to resist demagnetization)
- High saturation magnetization
Examples: Alnico (Al+Ni+Co), SmCo₅, Nd₂Fe₁₄B (strongest)
Electromagnets
Made of soft magnetic materials (soft iron). Requirements:
- High permeability μ
- High saturation magnetization
- Low coercivity (easy to switch on/off)
- Low hysteresis loss
Applications of Magnetic Materials
- Transformer cores: soft iron (low hysteresis loss)
- Computer hard disks: hard magnetic films
- Speakers: Alnico permanent magnets
- Medical MRI: superconducting coils
- Credit cards: hard magnetic coating
Diagram Indicator: [Hysteresis loop B vs H for hard and soft magnetic materials superimposed; labels showing retentivity B_r, coercivity H_c, saturation B_s; the area of soft material loop much smaller than hard material loop.]
Frequently asked questions
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Do these notes follow CBSE and HBSE?
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What does the Magnetism and Matter chapter cover?
Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Magnetism and Matter.