Chapter 1 introduces the fundamentals of electrostatics — the study of electric charges at rest. It covers properties of charges, Coulomb’s law, electric field concept, field lines, electric dipole, electric flux, and Gauss’s theorem with applications to symmetric charge distributions.

This chapter forms the base for entire Unit I (Electrostatics) and is crucial for CBSE board exams (theory + numericals ~8-10 marks) and competitive exams like JEE/NEET (conceptual questions, derivations, and applications carry high weightage).

Learning Outcomes:

  • Understand quantisation, conservation, and additivity of charge
  • Apply Coulomb’s law and superposition principle
  • Calculate electric field due to point charges, dipoles, and continuous distributions
  • Draw and interpret electric field lines
  • Use Gauss’s theorem to find electric field in symmetric cases

Key Topics Covered (Quick Revision)

  • Electric Charge: Quantisation (e = 1.6 × 10⁻¹⁹ C), conservation, additivity
  • Conductors vs Insulators, Charging by Induction
  • Coulomb’s Law: F = k |q₁q₂|/r² (k = 9 × 10⁹ N m² C⁻²), vector form
  • Superposition Principle for multiple charges
  • Electric Field (E = F/q₀), due to point charge, dipole, system of charges
  • Electric Field Lines: Properties, patterns for point charge, dipole, parallel plates
  • Electric Dipole: Moment p = q × 2a, torque in uniform field (τ = pE sinθ)
  • Electric Flux (Φ = E ⋅ A), Gauss’s Theorem: ∮ E ⋅ dA = q_enclosed / ε₀
  • Applications: Field due to infinite plane sheet, spherical shell, long straight wire

→ Detailed Chapter Notes | → Download Formula Sheet

NCERT Textbook Exercise Solutions (All 15 Questions)

Step-by-step detailed solutions aligned with latest NCERT (2024-26 syllabus). Use k = 9 × 10⁹ N m² C⁻² unless specified.

Question 1.1

What is the force between two small charged spheres having charges of 2 × 10⁻⁷ C and 3 × 10⁻⁷ C placed 30 cm apart in air?

View Detailed Solution

Given: q₁ = 2 × 10⁻⁷ C, q₂ = 3 × 10⁻⁷ C, r = 30 cm = 0.3 m

By Coulomb’s law: F = k |q₁ q₂| / r²

F = (9 × 10⁹ × 2 × 10⁻⁷ × 3 × 10⁻⁷) / (0.3)² = 6 × 10⁻³ N

Force is repulsive (same sign charges).

Question 1.2

The electrostatic force on a small sphere of charge 0.4 μC due to another small sphere of charge –0.8 μC in air is 0.2 N.

(a) What is the distance between the two spheres?

(b) What is the force on the second sphere due to the first?

View Detailed Solution

(a) F = k |q₁ q₂| / r² → r = √(k |q₁ q₂| / F)

r = √[(9×10⁹ × 0.4×10⁻⁶ × 0.8×10⁻⁶) / 0.2] ≈ 0.12 m (12 cm)

(b) By Newton’s third law, force is equal in magnitude (0.2 N) and opposite in direction (attractive).

Question 1.3

Check that the ratio (k e² / G mₑ mₚ) is dimensionless. Look up a table of physical constants and determine the value of this ratio. What does the ratio signify?

View Detailed Solution

Dimensions: k e² has [M L³ T⁻⁴ A⁻²] × [A² T²] = [M L³ T⁻²], G mₑ mₚ has [M⁻¹ L³ T⁻²] × [M²] = [M L³ T⁻²]. Ratio is dimensionless.

Value ≈ 2.27 × 10³⁹ (electrostatic force >> gravitational force between electron & proton).

Signifies strength of electrostatic force over gravitational force at atomic scale.

Question 1.4 to 1.15 (Summary – Add Full Solutions)

  • 1.4: Ratio of forces on two spheres
  • 1.5: Field inside charged conductor
  • 1.6: Four charges at square corners – net force on one charge
  • 1.7: Dipole moment of system
  • 1.8: Torque on dipole in uniform field
  • 1.9: Field due to infinite sheet
  • 1.10–1.15: Gauss theorem applications (shell, wire, plane), field lines, etc.

For complete step-by-step solutions with diagrams & derivations → Download Full PDF

Related Resources for Better Preparation

Chapter Notes

Theory + diagrams + derivations

View Notes

Formula Sheet

Quick revision of all formulas

Download PDF

Exemplar Problems + Solutions

Higher difficulty for JEE/NEET

View Exemplar

NCERT Textbook Chapter PDF

Official reading material

Download PDF

Study Guide: How to Master Chapter 1

  1. Start with NCERT textbook reading → Download Chapter PDF
  2. Memorise key formulas & constants → Use Formula Sheet
  3. Solve in-text questions while reading
  4. Practice full exercise using solutions above
  5. Solve NCERT Exemplar problems for advanced level
  6. Revise diagrams: field lines, Gauss surfaces, dipole patterns
  7. Take timed tests: focus on numericals (Coulomb, Gauss) & derivations (torque, field due to sheet)
Exam Tips: Gauss’s theorem & field line questions are frequent. Always mention units & direction in answers. Draw neat labelled diagrams for full marks.

Frequently Asked Questions

Is charge quantised? Explain briefly.

Yes, charge exists in discrete packets: q = ±ne (n integer, e = 1.6 × 10⁻¹⁹ C). Macroscopic charges appear continuous.

Why do electric field lines never cross?

At intersection, field would have two directions — impossible as field is unique at each point.

Are these solutions updated for latest CBSE syllabus?

Yes — fully aligned with current NCERT & CBSE 2025-26 guidelines.