In short: Electrostatics (Chapters 1 and 2) is the highest-weightage unit in CBSE Class 12 Physics, carrying 16 marks out of 70. It covers Coulomb's Law, Electric Field, Gauss's Theorem, Electric Potential, and Capacitors. At least 2-3 numericals and 1 derivation question come from this unit every year. Master this unit and you secure 20%+ of your Physics paper.
What This Unit Covers
| Chapter | Key Topics | Expected Marks |
|---|---|---|
| Ch 1: Electric Charges and Fields | Coulomb's Law, Electric Field, Field Lines, Gauss's Theorem | 8-9 |
| Ch 2: Electrostatic Potential and Capacitance | Potential, Potential Energy, Capacitors, Combinations | 7-8 |
| Total | 15-17 |
Chapter 1: Electric Charges and Fields
Coulomb's Law
Force between two point charges:
F = kq₁q₂/r²
Where k = 1/(4πε₀) = 9 × 10⁹ Nm²/C²
Vector form: F₁₂ = kq₁q₂/r² × r̂₁₂
Key points:
- Force is along the line joining the two charges
- Like charges repel, unlike charges attract
- Force follows the superposition principle (net force = vector sum of individual forces)
Electric Field
Due to a point charge: E = kq/r²
Direction: Away from positive charge, towards negative charge.
Electric field due to a dipole:
- On axial line: E = 2kp/r³ (for r >> a)
- On equatorial line: E = kp/r³ (for r >> a)
Where p = qa (dipole moment)
Gauss's Theorem
Statement: The total electric flux through a closed surface equals the net charge enclosed divided by ε₀.
∮ E⃗ · dA⃗ = q_enclosed / ε₀
Applications of Gauss's Theorem (Derivations)
1. Electric field due to an infinite line charge: E = λ/(2πε₀r)
2. Electric field due to an infinite plane sheet: E = σ/(2ε₀)
3. Electric field due to a uniformly charged sphere:
- Outside (r > R): E = kQ/r²
- On surface (r = R): E = kQ/R²
- Inside (r < R): E = 0 (for conducting sphere)
Board exam tip: The derivation of Gauss's theorem application (usually for infinite plane or sphere) is a guaranteed 5-mark question. Practice writing it step by step. See the Physics derivations list.
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Chapter 2: Electrostatic Potential and Capacitance
Electric Potential
Due to point charge: V = kq/r
Due to a dipole:
- Axial point: V = kp/r²
- Equatorial point: V = 0
Relationship: E = -dV/dr (Electric field is negative gradient of potential)
Capacitors
Parallel plate capacitor: C = ε₀A/d
Where A = area of plates, d = distance between plates.
With dielectric: C = Kε₀A/d (K = dielectric constant)
Energy stored: U = ½CV² = ½QV = Q²/(2C)
Capacitor Combinations
Series: 1/C_total = 1/C₁ + 1/C₂ + 1/C₃
Parallel: C_total = C₁ + C₂ + C₃
Solved Numericals
Type 1: Coulomb's Law
Q: Two charges of +3μC and -4μC are placed 30 cm apart. Find the force between them.
Solution:
- q₁ = 3 × 10⁻⁶ C, q₂ = 4 × 10⁻⁶ C, r = 0.3 m
- F = kq₁q₂/r² = 9×10⁹ × 3×10⁻⁶ × 4×10⁻⁶ / (0.3)²
- F = 9×10⁹ × 12×10⁻¹² / 0.09
- F = 108×10⁻³ / 0.09 = 1.2 N (attractive)
Type 2: Capacitor Combination
Q: Three capacitors of 2μF, 3μF, and 6μF are connected in parallel. Find the equivalent capacitance.
Solution:
- C = C₁ + C₂ + C₃ = 2 + 3 + 6 = 11 μF
If in series:
- 1/C = 1/2 + 1/3 + 1/6 = 3/6 + 2/6 + 1/6 = 6/6 = 1
- C = 1 μF
Board Exam Expected Questions
- Derive the expression for electric field due to an infinite plane sheet of charge using Gauss's theorem. (5 marks)
- Two point charges +2μC and -3μC are placed 20 cm apart. Find the electric field at the midpoint. (3 marks)
- Derive the expression for capacitance of a parallel plate capacitor with a dielectric medium. (5 marks)
- Three capacitors of 10μF, 15μF, and 30μF are connected in series across a 60V supply. Find the charge on each capacitor. (3 marks)
- What is Gauss's theorem? State and explain. (2-3 marks)
For complete marks distribution, see the CBSE Class 12 Physics marks weightage. For all formulas, see the Physics derivations list.
Frequently Asked Questions
How many marks does Electrostatics carry?
15-17 marks out of 70 in the theory paper. It is the single highest-weightage unit.
Which derivation is most important?
Gauss's theorem application (electric field due to infinite plane sheet or uniformly charged sphere) appears almost every year as a 5-mark question.
How many numericals should I practice?
At least 30 — 10 Coulomb's Law, 10 Capacitor combinations, 10 Electric field/potential. The types repeat with different numbers.
Is this unit also important for JEE?
Yes. Electrostatics is a major unit in JEE Main and Advanced. Board-level preparation covers 70% of JEE requirements for this topic.
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