Magnetism and Matter Class 12 Notes | CBSE Physics Chapter 5

Magnetism and Matter is Chapter 5 of CBSE Class 12 Physics. This chapter covers the magnetic properties of materials — how different materials respond to magnetic fields. You will learn about bar magnets, the earth’s magnetism, diamagnetic, paramagnetic, and ferromagnetic materials, and key terms like magnetic susceptibility and permeability.

This chapter typically carries 3–4 marks. Classification of magnetic materials, earth’s magnetic elements, and definitions are commonly tested.

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Key Concepts

1. Bar Magnet as an Equivalent Solenoid

A bar magnet behaves like a solenoid with a fixed magnetic moment. The magnetic field lines of a bar magnet are identical to those of a solenoid.

Magnetic dipole moment: m = NIA (for a solenoid/loop)

Axial field (far away): B = (μ₀/4π)(2m/r³)

Equatorial field (far away): B = (μ₀/4π)(m/r³)

2. Torque on a Magnetic Dipole

τ = m × B = mB sin θ

Potential energy: U = −m · B = −mB cos θ

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3. Earth’s Magnetism

The Earth behaves like a giant bar magnet. The geographic north pole is near the magnetic south pole (field lines enter there).

Elements of Earth’s Magnetic Field

Element	Symbol	Description
Declination	δ	Angle between geographic north and magnetic north at a place
Dip (Inclination)	I	Angle between the total magnetic field and the horizontal
Horizontal component	BH	BH = B cos I (component along the surface)

Total field: B² = BH² + BV²; tan I = BV/BH

At magnetic poles: I = 90° (BH = 0); At magnetic equator: I = 0° (BV = 0)

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4. Classification of Magnetic Materials

Property	Diamagnetic	Paramagnetic	Ferromagnetic
Response to field	Weakly repelled	Weakly attracted	Strongly attracted
Susceptibility (χ)	Small, negative	Small, positive	Very large, positive
Relative permeability (μr)	Slightly < 1	Slightly > 1	>> 1 (10² to 10⁵)
Examples	Bismuth, copper, diamond, water	Aluminium, sodium, oxygen, platinum	Iron, cobalt, nickel, gadolinium
Temperature effect	Independent	χ ∝ 1/T (Curie’s law)	Above Curie temp → paramagnetic

Key Terms

• Magnetisation (M): Magnetic moment per unit volume; M = χH
• Magnetic susceptibility (χ): Measures how easily a material is magnetised
• Magnetic permeability (μ): μ = μ₀(1 + χ) = μ₀μr
• Curie temperature: Temperature above which ferromagnetic becomes paramagnetic
• Hysteresis: Lagging of B behind H when a ferromagnet is magnetised and demagnetised

Hysteresis Loop

• Retentivity: B remaining when H is reduced to zero
• Coercivity: Reverse H needed to reduce B to zero
• Area of hysteresis loop = energy lost per cycle
• Soft iron: narrow loop (electromagnets, transformers)
• Steel: wide loop (permanent magnets)

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Important Definitions

Term	Definition
Magnetic dipole moment	m = NIA — product of current, area, and number of turns
Declination	Angle between geographic and magnetic meridians at a place
Dip	Angle of Earth’s field with horizontal
Diamagnetic	Materials weakly repelled by magnets (χ < 0)
Paramagnetic	Materials weakly attracted by magnets (χ > 0, small)
Ferromagnetic	Materials strongly attracted by magnets (χ >> 0)
Curie temperature	Temperature above which ferromagnets lose their ferromagnetism
Hysteresis	Lagging of magnetisation behind the applied field in ferromagnets

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Solved Examples

Example 1

At a place, BH = 0.3 G and dip angle = 60°. Find the total magnetic field.

Answer: BH = B cos I → B = BH/cos I = 0.3/cos 60° = 0.3/0.5 = 0.6 G

Example 2

A bar magnet of magnetic moment 0.5 A·m² is placed at 30° to a uniform field of 0.2 T. Find the torque.

Answer: τ = mB sin θ = 0.5 × 0.2 × sin 30° = 0.1 × 0.5 = 0.05 N·m

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Important Questions for Board Exams

1-Mark

1. Define magnetic declination.
2. What is the Curie temperature?

3-Mark

1. Classify magnetic materials into three categories. Give properties and examples of each.
2. Define the three elements of Earth’s magnetic field. How are they related?
3. What is a hysteresis loop? What do retentivity and coercivity represent?

5-Mark

1. Distinguish between diamagnetic, paramagnetic, and ferromagnetic materials with examples. Explain the concept of Curie temperature and hysteresis.

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Quick Revision Points

• Bar magnet ≈ magnetic dipole; m = NIA; τ = mB sin θ; U = −mB cos θ
• Earth’s field: Declination (δ), Dip (I), Horizontal component BH = B cos I
• Diamagnetic: χ < 0, repelled (Cu, Bi, H₂O); Paramagnetic: χ > 0, weak attraction (Al, O₂)
• Ferromagnetic: χ >> 0, strong attraction (Fe, Co, Ni); above Curie temp → paramagnetic
• Curie’s law (paramagnetic): χ ∝ 1/T
• Hysteresis: retentivity (B at H=0), coercivity (H to make B=0)
• Soft iron: easy to magnetise/demagnetise (electromagnets); Steel: permanent magnets

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Previous: Ch 4 — Moving Charges and Magnetism
Next: Ch 6 — Electromagnetic Induction