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

More on This Chapter

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

Last Updated: September 23, 2026

Common Mistakes Students Make in Electric Charges and Fields

Quick visual: a worked diagram from the full Solutions page, for reference.

Frequently Asked Questions

What is Coulomb Law, and what does it describe?
Coulomb Law describes the force between two point charges, stating that the force is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

What is an electric field, and why is the concept useful?
An electric field is the region around a charge where another charge experiences a force; it lets us describe the effect of a charge at every point in space without needing to place a test charge there physically.

What to Revise First (and Last) in This Chapter

Written by Satish

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