Class 12 Physics – Chapter 1: Electrostatics
Board-Focused, Detailed Notes for Exam Preparation
Attached HOTS and Important FAQs at the end of the notes.
🔴 1. Introduction to Electrostatics
Electrostatics is the branch of physics that studies electric charges at rest. It explains how stationary
charges interact with each other through forces and fields.
In daily life, most of the forces we encounter (friction, tension, normal force) are due to
electromagnetic interactions, not just gravity.
Example:
Friction between two surfaces is due to electromagnetic forces between atoms.
The normal force pushing you up from a surface is electromagnetic.
🔴 2. Electric Charge
📘 Definition:
An electric charge is a fundamental property of matter responsible for electric interactions.
🔹 Types of Charges:
Positive Charge (+): Example – glass rod rubbed with silk.
Negative Charge (−): Example – rubber rod rubbed with fur.
🔹 Properties of Charge:
1. Additivity of Charges: Total charge = algebraic sum of individual charges.
2. Conservation of Charge: Charge can neither be created nor destroyed, only transferred.
3. Quantisation of Charge:
o Charge exists in discrete packets: q=neq = ne, where e=1.6×10−19 Ce = 1.6 \times
10^{-19} \ C
o nn is an integer (±1, ±2, ±3...)
⚡ Triboelectric Charging:
Rubbing transfers electrons from one object to another.
The object losing electrons becomes positively charged.
The object gaining electrons becomes negatively charged.
🔴 3. Basic Laws and Concepts
, ✅ Law of Conservation of Charge:
The total charge in an isolated system is constant. Charge can be transferred but not created or
destroyed.
✅ Quantisation of Charge:
All observable charges are integer multiples of the fundamental charge e=1.6×10−19Ce = 1.6 \times
10^{-19} C
Example:
Number of electrons in 1 coulomb = 11.6×10−19=6.25×1018\frac{1}{1.6 \times 10^{-19}} = 6.25 \
times 10^{18}
🔴 4. Coulomb's Law
📘 Statement:
The electrostatic force between two point charges q1q_1 and q2q_2, separated by a distance rr in
vacuum is:
F=14πε0⋅q1q2r2F = \frac{1}{4 \pi \varepsilon_0} \cdot \frac{q_1 q_2}{r^2}
Where:
FF = Electrostatic force
q1,q2q_1, q_2 = Point charges
rr = Distance between charges
ε0=8.854×10−12 C2/N⋅m2\varepsilon_0 = 8.854 \times 10^{-12} \ C^2/N \cdot m^2
k=14πε0=9×109 Nm2/C2k = \frac{1}{4\pi \varepsilon_0} = 9 \times 10^9 \ Nm^2/C^2
➕ Important Notes:
Like charges repel, unlike charges attract.
Force is along the line joining the charges.
Applies only to point charges or spherical symmetry at large distances.
⚖️
Comparison with Newton's Law of Gravitation:
Feature Coulomb's Law Newton's Law
Depends on Charge Mass
Type of Force Attractive/Repulsive Always Attractive
Medium-dependent? Yes No
Relative strength Very strong Very weak
Board-Focused, Detailed Notes for Exam Preparation
Attached HOTS and Important FAQs at the end of the notes.
🔴 1. Introduction to Electrostatics
Electrostatics is the branch of physics that studies electric charges at rest. It explains how stationary
charges interact with each other through forces and fields.
In daily life, most of the forces we encounter (friction, tension, normal force) are due to
electromagnetic interactions, not just gravity.
Example:
Friction between two surfaces is due to electromagnetic forces between atoms.
The normal force pushing you up from a surface is electromagnetic.
🔴 2. Electric Charge
📘 Definition:
An electric charge is a fundamental property of matter responsible for electric interactions.
🔹 Types of Charges:
Positive Charge (+): Example – glass rod rubbed with silk.
Negative Charge (−): Example – rubber rod rubbed with fur.
🔹 Properties of Charge:
1. Additivity of Charges: Total charge = algebraic sum of individual charges.
2. Conservation of Charge: Charge can neither be created nor destroyed, only transferred.
3. Quantisation of Charge:
o Charge exists in discrete packets: q=neq = ne, where e=1.6×10−19 Ce = 1.6 \times
10^{-19} \ C
o nn is an integer (±1, ±2, ±3...)
⚡ Triboelectric Charging:
Rubbing transfers electrons from one object to another.
The object losing electrons becomes positively charged.
The object gaining electrons becomes negatively charged.
🔴 3. Basic Laws and Concepts
, ✅ Law of Conservation of Charge:
The total charge in an isolated system is constant. Charge can be transferred but not created or
destroyed.
✅ Quantisation of Charge:
All observable charges are integer multiples of the fundamental charge e=1.6×10−19Ce = 1.6 \times
10^{-19} C
Example:
Number of electrons in 1 coulomb = 11.6×10−19=6.25×1018\frac{1}{1.6 \times 10^{-19}} = 6.25 \
times 10^{18}
🔴 4. Coulomb's Law
📘 Statement:
The electrostatic force between two point charges q1q_1 and q2q_2, separated by a distance rr in
vacuum is:
F=14πε0⋅q1q2r2F = \frac{1}{4 \pi \varepsilon_0} \cdot \frac{q_1 q_2}{r^2}
Where:
FF = Electrostatic force
q1,q2q_1, q_2 = Point charges
rr = Distance between charges
ε0=8.854×10−12 C2/N⋅m2\varepsilon_0 = 8.854 \times 10^{-12} \ C^2/N \cdot m^2
k=14πε0=9×109 Nm2/C2k = \frac{1}{4\pi \varepsilon_0} = 9 \times 10^9 \ Nm^2/C^2
➕ Important Notes:
Like charges repel, unlike charges attract.
Force is along the line joining the charges.
Applies only to point charges or spherical symmetry at large distances.
⚖️
Comparison with Newton's Law of Gravitation:
Feature Coulomb's Law Newton's Law
Depends on Charge Mass
Type of Force Attractive/Repulsive Always Attractive
Medium-dependent? Yes No
Relative strength Very strong Very weak
