Magnetism and Matter Revision Notes for NEET Physics PDF Download
🧲 MAGNETISM & MATTER · Magnetic Materials & Earth's Magnetism
Bar magnet • Magnetic field lines • Magnetisation • Hysteresis • Para/Dia/Ferro • Earth's magnetism • NEET problems
🧲 BAR MAGNET & FIELD LINES
Magnetic dipole moment: m = pole strength × length (A·m²).
Field along axis: B = (μ₀/4π) × (2m/r³)
Field on equatorial line: B = (μ₀/4π) × (m/r³)
Direction: from south to north inside, north to south outside.
📈 Field lines of a bar magnet
📊 MAGNETISATION & MAGNETIC INTENSITY
Magnetisation M = net magnetic moment per unit volume (A/m).
Magnetic field intensity H = B/μ₀ – M (or H = B/μ₀ in vacuum).
Relation: B = μ₀(H + M).
Magnetic susceptibility χ = M/H (dimensionless).
Relative permeability μr = B/(μ₀H) = 1 + χ.
🔍 CLASSIFICATION OF MAGNETIC MATERIALS
Diamagnetic
χ < 0, μr < 1 (small negative).
Examples: Bi, Cu, Water, Diamond.
Field lines repelled; no permanent dipole.
Paramagnetic
χ > 0 small, μr > 1 (slightly positive).
Examples: Al, Pt, Mn, O₂.
χ ∝ 1/T (Curie's law). Field lines attracted.
Ferromagnetic
χ >> 0, μr >> 1 (large positive).
Examples: Fe, Ni, Co, Gadolinium.
Domain structure, hysteresis, Curie temperature.
📉 HYSTERESIS & ENERGY LOSS
B-H curve for ferromagnetic materials shows lag between magnetisation and applied field.
Retentivity (Br): residual magnetisation when H=0.
Coercivity (Hc): reverse field needed to demagnetise.
Area of hysteresis loop = energy loss per cycle (heat).
🖍️ B-H hysteresis loop
🌍 EARTH'S MAGNETISM
Earth acts like a giant bar magnet with magnetic south near geographic north (and vice versa).
Elements of Earth's field:
• Declination (δ): angle between geographic and magnetic meridians.
• Inclination (dip angle θ): angle made by B with horizontal.
• Horizontal component BH = B cosθ, vertical component BV = B sinθ.
At equator: θ = 0°, BH = B, BV = 0.
At poles: θ = 90°, BH = 0, BV = B.
📐 Earth's magnetic field & dip angle
💾 MAGNETIC POTENTIAL ENERGY
Potential energy of a magnetic dipole in uniform B: U = –m·B = –mB cosθ.
Torque on dipole: τ = m × B → magnitude τ = mB sinθ.
Work done to rotate dipole from θ₁ to θ₂: W = mB (cosθ₁ – cosθ₂).
💡 NEET TIPS & SHORTCUTS
- Diamagnetic materials have negative susceptibility, independent of temperature.
- Paramagnetic susceptibility follows Curie's law: χ ∝ 1/T.
- Ferromagnetic materials have domains; above Curie temperature they become paramagnetic.
- For a bar magnet, field at axial point is double that at equatorial point at same distance.
⚠️ COMMON MISTAKES
- Confusing magnetic field lines direction inside vs outside magnet.
- Using χ = μr – 1 incorrectly for diamagnetic (χ negative).
- Forgetting that earth's magnetic north is actually a magnetic south pole.
- Applying torque formula without proper sign for potential energy.
📌 QUICK REVISION CARD
Magnetic dipole moment: m = pole strength × length
Axial field (bar magnet): B = μ₀ m/(2πr³)
Equatorial field: B = μ₀ m/(4πr³)
B = μ₀(H + M)
χ = M/H, μr = 1 + χ
Torque: τ = mB sinθ
Potential: U = –mB cosθ