Last Updated: April 2026
Electric Charges and Fields — Chapter 1 of NCERT Class 12 Physics — is the foundation of the entire Electrostatics unit, which constitutes approximately 15–16 marks in the CBSE Board Exam. This chapter is also highly tested in CUET, JEE, and NEET, making it one of the most important chapters across all competitive exams.
This comprehensive guide covers all key concepts, important formulae, derivations, and board-level questions from Chapter 1 of CBSE Class 12 Physics.
Chapter Overview
| Parameter | Details |
|---|---|
| Chapter | Chapter 1 — Electric Charges and Fields |
| Class | Class 12 Physics (NCERT) |
| Board | CBSE / ISC / Bihar Board / Most State Boards |
| Marks Weightage | 8–9 marks (Electrostatics unit total ~15–16 marks) |
| Key Topics | Electric charge, Coulomb’s law, Electric field, Gauss’s law, Field lines |
| Difficulty | Moderate |
Topic 1: Electric Charge
Properties of Electric Charge
- Additive: Total charge = algebraic sum of all charges
- Conserved: Charge can neither be created nor destroyed (only transferred)
- Quantized: Charge always exists in integral multiples of e (e = 1.6 × 10⁻¹⁹ C)
Quantization of Charge: q = ne, where n = ±1, ±2, ±3… and e = 1.6 × 10⁻¹⁹ C
1 Coulomb = charge carried by 6.25 × 10¹⁸ electrons
Methods of Charging
- Friction: Electrons transferred between bodies (e.g., glass rod + silk cloth)
- Conduction: Direct contact with charged body
- Induction: Charged body brought near conductor without contact
Topic 2: Coulomb’s Law
Statement: The electrostatic force between two point charges is directly proportional to the product of charges and inversely proportional to the square of the distance between them.
F = k × q₁ × q₂ / r²
where:
- k = Coulomb’s constant = 1/(4πε₀) = 9 × 10⁹ N m²/C²
- ε₀ = Permittivity of free space = 8.85 × 10⁻¹² C²/Nm²
- q₁, q₂ = charges in Coulombs
- r = distance between charges in metres
Key Points about Coulomb’s Law
- It is an inverse square law (like Newton’s gravitational law)
- Valid for point charges and spherical charge distributions
- In a medium: F_medium = F_vacuum / K, where K = dielectric constant
- Follows Newton’s Third Law: forces are equal and opposite
- Principle of Superposition: Net force = vector sum of individual forces
Topic 3: Electric Field
Definition: Electric field at a point is the force experienced per unit positive test charge placed at that point (without disturbing the original charge distribution).
E = F/q₀ = kq/r²
Unit: N/C or V/m | Dimension: [MLT⁻³A⁻¹]
Electric Field Lines — Properties
| Property | Explanation |
|---|---|
| Direction | Start from positive charge; end at negative charge |
| Never intersect | If they did, field would have two directions at one point (impossible) |
| Density | Closer lines = stronger field |
| Uniform field | Parallel, equally spaced straight lines |
| Isolated positive charge | Lines radiate outward in all directions |
| Closed loops? | No — electrostatic field lines are not closed loops |
Electric Field Due to Special Charge Distributions
| Configuration | Electric Field Formula |
|---|---|
| Point charge q at distance r | E = kq/r² |
| Electric dipole — on axial line | E = 2kp/r³ (r >> a) |
| Electric dipole — on equatorial line | E = kp/r³ (opposite to dipole moment) |
| Infinite line charge (linear charge density λ) | E = λ/(2πε₀r) |
| Infinite plane sheet (surface charge density σ) | E = σ/(2ε₀) — uniform, perpendicular to surface |
Topic 4: Gauss’s Law
Statement: The total electric flux through any closed surface is equal to the net charge enclosed divided by ε₀.
Φ = ∮ E⃗ · dA⃗ = q_enclosed / ε₀
Electric Flux (Φ): Φ = E × A × cosθ
Unit: Nm²/C or Vm
Applications of Gauss’s Law
- Inside conductor in equilibrium: E = 0 (no field inside)
- On surface of conductor: E = σ/ε₀ (normal to surface)
- Inside uniformly charged sphere: E = ρr/3ε₀ (r < R)
- Outside uniformly charged sphere: E = kQ/r² (r > R)
Important Board Exam Questions
- State and prove Coulomb’s Law. (3 marks)
- State Gauss’s Law and use it to find electric field due to an infinite plane sheet of charge. (5 marks)
- Draw electric field lines for (a) two equal positive charges (b) equal and opposite charges. (2 marks)
- A charge of 4μC is placed at origin. Find the electric field at point (3, 4, 0) cm. (3 marks)
- Explain the term electric dipole moment. Derive the expression for field on the axial line. (5 marks)
Practice Quiz: CBSE Class 12 Physics Chapter 1
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Frequently Asked Questions — CBSE Class 12 Physics Ch 1
How many marks are from Chapter 1 in CBSE Class 12 Physics board exam?
Chapter 1 (Electric Charges and Fields) is part of Unit I: Electrostatics, which carries 15–16 marks in CBSE Class 12 Physics. Chapter 1 alone contributes approximately 8–9 marks, and Chapter 2 (Electrostatic Potential and Capacitance) contributes the rest. Typically there will be one 5-mark question and two 2-3 mark questions from this unit.
What derivations from Chapter 1 are important for the board exam?
Important derivations from Chapter 1: (1) Electric field due to an infinite plane sheet using Gauss’s Law, (2) Electric field on the axial line of an electric dipole, (3) Electric field on the equatorial line of an electric dipole. Of these, the plane sheet and dipole field derivations appear most frequently in CBSE board exams (5-mark questions).
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