Electric Charges and Fields is Chapter 1 of CBSE Class 12 Physics (NCERT). It carries approximately 15–16 marks in board exams and forms the foundation for electrostatics. This guide covers all concepts with clear explanations and 10 practice MCQs.
Properties of Electric Charge
- Quantization: q = ne (n is integer, e = 1.6 × 10⁻¹⁹ C)
- Conservation: Total charge in an isolated system is constant
- Additivity: Total charge = algebraic sum of all charges
Electric charge is a scalar quantity. SI Unit: Coulomb (C)
Coulomb’s Law
F = kq₁q₂/r² = q₁q₂/(4πε₀r²)
- k = 9 × 10⁹ N·m²/C²
- ε₀ = 8.85 × 10⁻¹² C²/N·m²
- Force is central and along line joining charges
Superposition Principle: Net force = vector sum of all individual Coulomb forces
Electric Field
E = F/q₀ (force per unit positive test charge)
Due to point charge Q: E = kQ/r²
Electric Field Lines:
- Start from +Q, end at −Q
- Never intersect (would imply two field directions at one point)
- Closer lines = stronger field
- Perpendicular to conducting surface
Electric Flux and Gauss’s Law
Electric Flux: Φ = E·A·cosθ
Gauss’s Law: Φ = q_enclosed / ε₀
Total flux through closed surface = enclosed charge / ε₀
Applications:
| Configuration | Electric Field |
|---|---|
| Infinite line charge (λ) | E = λ/(2πε₀r) |
| Infinite plane sheet (σ) | E = σ/(2ε₀) |
| Inside conductor | E = 0 |
| Outside conducting sphere | E = kQ/r² |
Electric Dipole
Two charges ±q separated by distance 2a. Dipole moment: p = q × 2a (C·m)
| Location | Field |
|---|---|
| Axial line | E = 2kp/r³ |
| Equatorial line | E = kp/r³ |
CBSE Board Exam Tips
- 1 mark: State Gauss’s law / SI unit of electric flux
- 2 marks: Draw field lines for dipole; State Coulomb’s law
- 3 marks: Apply Gauss’s law for infinite sheet; Superposition problems
- 5 marks: E on axial and equatorial line of dipole; Gauss’s law proof
Practice MCQs — Electric Charges and Fields
Practice Quiz — 10 CLAT-Style Questions
Click an option to reveal the answer and explanation.
Last updated: April 2026 | Ready For Boards — Expert CBSE Board Exam Preparation
