Class 12 Physics Chapter 1 Electric Charges and Fields is the first and most important chapter of Electrostatics. This chapter builds the foundation for upcoming chapters like Electrostatic Potential & Capacitance and helps you solve many numerical problems in boards and competitive exams.
In this blog, you will learn:
✅ Concepts in simple English
✅ All important definitions
✅ Key formulas and results
✅ Electric field lines & flux
✅ Gauss’s law and its applications
✅ Important questions for CBSE + JEE/NEET
Electric Charges and Fields Class 12 Notes PDF | Formulas, Derivations, MCQ & PYQ
1. Electric Charge
✅ What is Electric Charge?
Electric charge is a fundamental property of matter due to which it experiences an electric force in the presence of another charge.
Types of Charges
There are two types of electric charges:
- Positive charge (+)
- Negative charge (–)
Basic Rule
- Like charges repel each other
- Unlike charges attract each other
Example:
- Proton → Positive charge
- Electron → Negative charge
2. Properties of Electric Charge
(A) Quantisation of Charge
Electric charge is always present in discrete packets, not continuous.
q = ne
Where:
- = charge on the body
- = integer (1, 2, 3, …)
- (elementary charge)
✅ This means charge is always a multiple of .
(B) Conservation of Charge
Charge can neither be created nor destroyed. It can only be transferred from one body to another.
q_{total} = constant
(C) Additivity of Charge
The total charge on a system is the algebraic sum of all charges.
q_{total} = q_1 + q_2 + q_3 + ....
3. Methods of Charging a Body
(A) Charging by Friction (Rubbing)
When two objects are rubbed together, electrons transfer from one body to another.
Example:
Glass rod rubbed with silk → glass becomes positive and silk becomes negative.
(B) Charging by Conduction (Contact)
When a charged body touches a neutral body, charge flows from the charged body to the neutral body.
👉 After conduction, the neutral body gets the same type of charge.
(C) Charging by Induction (Very Important)
In induction, a body can be charged without touching it.
Steps:
- Bring a charged rod near a neutral conductor
- Charges inside the conductor separate
- Connect the conductor to earth (earthing)
- Remove earthing
- Remove the charged rod
✅ The conductor becomes charged with opposite sign.
4. Coulomb’s Law
Coulomb’s law states that the electrostatic force between two point charges is:
- Directly proportional to the product of charges
- Inversely proportional to the square of the distance between them
F = \frac{1}{4\pi \epsilon_0}\frac{q_1q_2}{r^2}
Where:
- = force (N)
- = charges (C)
- = distance (m)
- = permittivity of free space
Important Points
✅ Force acts along the line joining the charges
✅ Like charges → repulsive force
✅ Unlike charges → attractive force
✅ Coulomb force is a central force
5. Permittivity and Dielectric Constant
In a medium, Coulomb’s law becomes:
F = \frac{1}{4\pi\epsilon}\frac{q_1q_2}{r^2}
Where:
\epsilon = K\epsilon_0
Here is the dielectric constant.
So,
F_{medium} = \frac{F_{vacuum}}{K}
✅ Force decreases in a medium.
6. Electric Field
What is an Electric Field?
An electric field is the region around a charge where another charge experiences a force.
Electric Field Intensity
Electric field intensity at a point is defined as the force experienced by a unit positive test charge placed at that point.
E = \frac{F}{q}
Where:
- = electric field (N/C)
- = force (N)
- = test charge (C)
Unit of Electric Field
N/C \quad \text{or} \quad V/m
7. Electric Field due to a Point Charge
The electric field at a distance from a point charge is:
E = \frac{1}{4\pi\epsilon_0}\frac{Q}{r^2}
Direction
- For : field is outward
- For : field is inward
8. Electric Field Lines
Electric field lines are imaginary lines used to represent the electric field.
Properties of Electric Field Lines
✅ Start from positive charge and end on negative charge
✅ Never intersect each other
✅ More dense lines = stronger field
✅ In a uniform field, lines are parallel
✅ Field lines are perpendicular to conductor’s surface
9. Electric Dipole
What is an Electric Dipole?
An electric dipole consists of two equal and opposite charges separated by a small distance.
Distance between charges =
Electric Dipole Moment
p = q(2a)
Unit:
C \cdot m
Direction: from negative charge to positive charge.
10. Electric Field due to a Dipole (Important Results)
(A) Electric Field on Axial Line
E_{axial} = \frac{1}{4\pi\epsilon_0}\frac{2p}{r^3}
(B) Electric Field on Equatorial Line
E_{equatorial} = \frac{1}{4\pi\epsilon_0}\frac{p}{r^3}
⭐ Note: Dipole field decreases as .
11. Torque on a Dipole in Uniform Electric Field
When a dipole is placed in a uniform electric field, it experiences torque:
\tau = pE\sin\theta
Where:
- = torque
- = dipole moment
- = electric field
- = angle between and
Special Cases
- → torque = 0 (stable equilibrium)
- → torque = 0 (unstable equilibrium)
- → torque maximum
12. Electric Flux
Electric flux measures the total electric field lines passing through a surface.
\phi_E = \vec{E}\cdot \vec{A}
\phi_E = EA\cos\theta
Unit:
Nm^2/C
13. Gauss’s Law (Most Important)
Gauss’s law states that the total electric flux through a closed surface is equal to the charge enclosed divided by .
\oint \vec{E}\cdot d\vec{A} = \frac{q_{enclosed}}{\epsilon_0}
Gauss’s law is very useful when there is symmetry:
- spherical symmetry
- cylindrical symmetry
- planar symmetry
14. Applications of Gauss’s Law
(A) Electric Field due to Infinite Line Charge
E = \frac{\lambda}{2\pi\epsilon_0 r}
(B) Electric Field due to Infinite Plane Sheet
E = \frac{\sigma}{2\epsilon_0}
(C) Electric Field due to Charged Conducting Sphere
Outside the sphere:
E = \frac{1}{4\pi\epsilon_0}\frac{Q}{r^2}
Inside the conductor:
E = 0
15. Conductors and Insulators
Conductors
Materials in which charges can move freely.
Example: Copper, Aluminium
Insulators
Materials in which charges cannot move freely.
Example: Rubber, Glass, Plastic
16. Electrostatic Shielding
Electrostatic shielding means the electric field inside a conductor is zero.
E_{inside} = 0
Example:
- Faraday cage
- Lightning protection
- Shielding of sensitive instruments
✅ Important Formulas (Quick Revision)
| Topic | Formula |
|---|---|
| Quantisation | |
| Coulomb’s Law | |
| Electric Field | |
| Field due to point charge | |
| Dipole moment | |
| Axial field | |
| Equatorial field | |
| Torque on dipole | |
| Electric flux | |
| Gauss’s law |
✅ Important Questions for Practice
1 Mark Questions
- What is the SI unit of charge?
- State the quantisation of charge.
- Write the SI unit of electric field.
- Define electric flux.
2–3 Marks Questions
- State Coulomb’s law and write its formula.
- Write any four properties of electric field lines.
- Define electric dipole and dipole moment.
- Explain charging by induction.
5 Marks Questions (Derivation Type)
- Derive the expression for electric field due to a point charge.
- Derive the expression for electric field on the axial line of a dipole.
- Derive the expression for torque on an electric dipole in a uniform field.
- State Gauss’s law and explain one application.
✅ Conclusion-Electric Charges and Fields Class 12 Notes PDF
This chapter Electric Charges and Fields is one of the most scoring chapters of Class 12 Physics. If you learn the concepts of Coulomb’s law, electric field, dipole, flux and Gauss’s law, you will easily solve many questions in CBSE boards and competitive exams.