NCERT Solutions for Class 12 Physics Chapter 5 Magnetism and Matter
NCERT Solutions for Class 12 Physics Chapter 5 Magnetism and Matter includes quick notes, key formulas, derivations, and step-by-step solutions covering magnetic dipoles, Gauss's law for magnetism, Earth's magnetic field, and magnetic classification of materials (diamagnetic, paramagnetic, and ferromagnetic).
Overview of Topics & Subtopics (Click to Jump)
1. The Bar Magnet & Magnetic Dipole Moment
Short Note:
• Magnetic Dipole Moment (m): Vector quantity pointing from South pole to North pole. For a bar magnet of pole strength q_m and length 2l: m = q_m × 2l. SI unit: A·m² or J/T.
• Bar Magnet as an Equivalent Solenoid: A bar magnet creates a magnetic field identical to a finite solenoid with N turns per unit length carrying current I.
• Magnetic Field due to Bar Magnet (distance r >> l):
- Axial Line: B_axial = (μ₀ / 4π) × (2 m / r³) (along direction of dipole moment m).
- Equatorial Line: B_equatorial = (μ₀ / 4π) × (m / r³) (opposite to direction of dipole moment m).
• Torque & Potential Energy in Uniform B-field:
- Torque: τ = m × B = m B sin θ.
- Potential Energy: U = -m · B = -m B cos θ.
2. Gauss's Law in Magnetism
Short Note:
• Statement: The total magnetic flux through any closed surface is always zero: Φ_B = ∮ B · dA = 0.
• Physical Significance: Magnetic monopoles (isolated magnetic N or S poles) do not exist in nature. Magnetic field lines always form continuous closed loops.
• Comparison with Electrostatics: In electrostatics, ∮ E · dA = q_enclosed / ε₀ because isolated electric charges exist, whereas magnetic monopoles do not.
3. Earth's Magnetism & Elements of Terrestrial Magnetism
Short Note:
• Earth's Magnetic Field: Earth acts as a giant magnetic dipole tilted at an angle of roughly 11.3° to its geographic rotational axis. Magnetic South pole is located near geographic North pole.
• Elements of Earth's Magnetic Field:
1. Magnetic Declination (D): The angle between geographic meridian and magnetic meridian at a place.
2. Magnetic Dip or Inclination (I or δ): Angle made by total earth's magnetic field vector B with horizontal direction. Dip is 0° at equator and 90° at magnetic poles.
3. Horizontal Component (B_H): Component of earth's total field along horizontal direction: B_H = B cos δ; Vertical component B_V = B sin δ.
• Relation: tan δ = B_V / B_H and B = √(B_H² + B_V²).
4. Magnetic Properties of Materials (Dia, Para & Ferromagnetism)
Short Note:
• Key Magnetic Quantities:
- Magnetization (M): Magnetic moment per unit volume: M = m / V (SI unit: A/m).
- Magnetic Intensity (H): H = B₀ / μ₀ (SI unit: A/m).
- Magnetic Susceptibility (χ_m): Measure of response of material to applied field: χ_m = M / H.
- Relative Permeability (μ_r): μ_r = 1 + χ_m = μ / μ₀.
• Curie's Law for Paramagnetism: Susceptibility varies inversely with absolute temperature T: χ_m = C / T.
• Curie Temperature (T_c): Critical temperature above which a ferromagnetic material transitions into a paramagnetic material.
Intext & Conceptual Questions
• Statement: Gauss's law for magnetism states that the net magnetic flux through any closed Gaussian surface is always zero, i.e., ∮ B · dA = 0.
• Significance: It implies that magnetic field lines never originate or terminate on isolated sources; they form continuous closed loops. This confirms that isolated magnetic monopoles do not exist in nature. Every magnetic pole always exists as an inseparable north-south pair (dipole).
| Property | Diamagnetic | Paramagnetic | Ferromagnetic |
|---|---|---|---|
| Behavior in External Field | Feebly repelled by magnets; moves towards weaker field. | Feebly attracted by magnets; moves towards stronger field. | Strongly attracted by magnets; moves strongly to stronger field. |
| Susceptibility (χ_m) | Small and negative (-1 ≤ χ < 0). | Small and positive (0 < χ < ε). | Very large and positive (χ >> 1000). |
| Relative Permeability (μ_r) | Slightly less than 1 (0 ≤ μ_r < 1). | Slightly greater than 1 (μ_r > 1). | Much greater than 1 (μ_r >> 1). |
| Temperature Effect | Independent of temperature. | Inversely proportional (χ ∝ 1/T). | Decreases above Curie temperature (T_c). |
| Examples | Bismuth, Copper, Water, Gold, Air. | Aluminum, Platinum, Chromium, Oxygen. | Iron, Nickel, Cobalt, Alnico. |
Textbook Chapter End Exercise Questions
Given:
• Angle (θ) = 30°
• Uniform magnetic field (B) = 0.25 T
• Torque (τ) = 4.5 × 10⁻² J
Formula:
τ = m B sin θ
Calculation:
4.5 × 10⁻² = m × 0.25 × sin(30°)
4.5 × 10⁻² = m × 0.25 × 0.5
4.5 × 10⁻² = m × 0.125
m = (4.5 × 10⁻²) / 0.125 = 0.36 J/T (or A·m²)
The magnitude of the magnetic dipole moment of the magnet is 0.36 J/T (or 0.36 A·m²).
| Property | Soft Iron (Used for Electromagnets) | Steel (Used for Permanent Magnets) |
|---|---|---|
| Retentivity | High retentivity (retains strong field). | High retentivity. |
| Coercivity | Low coercivity (easily demagnetized when current stops). | High coercivity (resists demagnetization). |
| Hysteresis Loss | Small hysteresis loop area (low energy dissipation). | Large hysteresis loop area. |
| Suitability | Ideal for core of transformers, solenoids & electromagnets. | Ideal for making strong, long-lasting permanent magnets. |
Read More & Chapter Resources
These short notes are designed for quick revision. Read the main points once, then use the related material below to practise the same chapter. Revising a concept and checking it with questions can make your preparation more focused.