CBSE Class 12 Physics: Magnetism & Matter — Complete Notes 2026

Bar Magnet Properties

• Poles: north and south; like poles repel, unlike poles attract; poles cannot be isolated
• Magnetic field lines: start from N to S outside magnet; closed loops through magnet (S to N inside)
• Pole strength m: force between poles F = (μ₀/4π) × m₁m₂/r²; magnetic moment M = m × 2l

Bar Magnet as Equivalent Solenoid

• A bar magnet is equivalent to a solenoid carrying current; both have similar field patterns
• Torque on magnetic dipole: τ = MB sin θ; potential energy U = −MB cos θ
• Equilibrium: θ = 0° (stable, minimum energy), θ = 180° (unstable, maximum energy)

Earth's Magnetism

• Earth acts as giant bar magnet; geographic poles ≠ magnetic poles (offset by ~11.5°)
• Magnetic declination: angle between geographic north and magnetic north; varies by location
• Magnetic dip (inclination): angle made by earth's magnetic field with horizontal; 90° at poles, 0° at magnetic equator

Elements of Earth's Magnetic Field

• Magnetic declination (θ): measured by compass; essential for navigation
• Angle of dip (δ): 0° at magnetic equator, 90° at magnetic poles; India: 20°–30°
• Horizontal component Bₕ: Bₕ = B cos δ; Vertical component Bᵥ = B sin δ; B = √(Bₕ² + Bᵥ²)

Classification of Magnetic Materials

• Diamagnetic: slightly repelled by magnets; μᵣ < 1; χ is small negative; copper, bismuth, water
• Paramagnetic: slightly attracted; μᵣ > 1; χ is small positive; aluminum, oxygen, chromium
• Ferromagnetic: strongly attracted; μᵣ >> 1; χ very large; iron, cobalt, nickel; retain magnetisation

Hysteresis

• Ferromagnets: relationship between B and H is non-linear; forms a loop on B-H graph (hysteresis loop)
• Retentivity: residual magnetism when H = 0; coercivity: reverse field needed to demagnetise
• Soft iron (low coercivity): electromagnet cores; hard iron (high coercivity, high retentivity): permanent magnets

CBSE Board Focus

• Magnetism: 4–6 marks in CBSE Class 12; elements of Earth's magnetism with numerical most common
• Hysteresis loop: draw and label; explain use of soft and hard iron; 3-mark question
• Compare dia/para/ferro in tabular form: susceptibility, relative permeability, examples