Revision Notes: Class 12 Physics Chapter 1 Electric Charges and Fields

Class 12 Physics Chapter 1 – Electric Charges and Fields: Quick Revision Notes

• Coulomb’s law: F = kq₁q₂/r², k = 1/(4πε₀) ≈ 9×10⁹ N·m²/C²
• Principle of superposition: net force/field from multiple charges = vector sum of individual contributions
• Electric field: E = F/q₀ (force per unit positive test charge); field of a point charge: E = kq/r²
• Field lines: start on + charge, end on − charge (or infinity); never cross; density indicates field strength; perpendicular to a conductor’s surface
• Electric dipole moment: p = q×2a (from −q to +q); field on axial line ≈ 2kp/r³; field on equatorial line ≈ kp/r³ (for r»a)
• Torque on a dipole: τ = pE sinθ = p×E (vector); potential energy U = −pE cosθ = −p·E
• Continuous charge distributions: linear density λ = q/l, surface density σ = q/A, volume density ρ = q/V
• Electric flux: φ = E·A = EA cosθ (flat surface in uniform field)
• Gauss’s law: φ(closed surface) = Q_enclosed/ε₀ — depends only on enclosed charge, not surface shape or size
• Field of an infinite line charge: E = λ/(2πε₀r)
• Field of an infinite charged sheet: E = σ/(2ε₀); between two oppositely charged parallel plates: E = σ/ε₀
• Field due to a charged spherical shell: outside (r>R): E = kQ/r² (acts as if all charge at centre); inside (r<R): E = 0
• Charge is quantised (q = ne) and conserved (never created/destroyed, only transferred)
• Not currently CBSE-examinable: Q1.13 and Q1.25–Q1.34 of the textbook exercise (2023-24 rationalisation) — still useful for JEE/NEET

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NCERT Solutions | Extra Questions (HOTS) | Class 12 Physics Book These Class 12 Physics Chapter 1 notes are ideal for quick revision just before exams.

Written by Satish

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