CBSE Class 12 Physics Revision Notes Chapter 1 explain Electric Charges and Fields through charge, Coulomb’s law, electric field, dipole, flux and Gauss’s law. For CBSE 2026 Physics, Electric Charges and Fields builds the electrostatics base for force, field, flux and charge distributions.
Why does a synthetic sweater crackle in dry weather and why does lightning appear during a thunderstorm? The NCERT Class 12 Physics chapter Electric Charges and Fields begins with these familiar discharge effects and connects them to static electricity. Electrostatics studies forces, fields and flux produced by charges at rest.
Chapter 1 moves from charge transfer by rubbing to the mathematical laws of electrostatics. Students learn why charge is conserved, why charge appears in multiples of e, how Coulomb’s law gives force between point charges and how electric field describes force per unit test charge. These class 12 physics chapter 1 electric charges and fields notes also cover electric field lines, electric flux, electric dipole, continuous charge distribution and Gauss’s law applications.
Key Takeaways
- Elementary charge: The basic unit of charge is e = 1.602192 × 10⁻¹⁹ C.
- Coulomb constant: In SI units, k ≈ 9 × 10⁹ N m² C⁻².
- Free space permittivity: ε0 = 8.854 × 10⁻¹² C² N⁻¹ m⁻².
- Spherical shell result: Electric field inside a uniformly charged thin spherical shell is zero.
CBSE Class 12 Physics Revision Notes Chapter 1 Structure 2026
| Question Type |
What to Focus On |
Answer Angle |
| Force-based |
Coulomb’s law and superposition |
Use vector direction with charge signs |
| Field-based |
Electric field, field lines and dipole field |
State source charge, test charge and direction |
| Flux-based |
Electric flux and Gauss’s law |
Choose a symmetric Gaussian surface |
Electric Charge and Static Electricity
Electric charge is the physical property responsible for electric force between bodies. In electric charges and fields class 12 notes, charge appears through rubbing, attraction, repulsion and discharge.
The NCERT Class 12 Physics chapter explains electrostatics as the study of forces, fields and potentials due to static charges. Chapter 1 focuses on charge and field, while potential continues in the next chapter.
Charging by rubbing
A body gets charged when electrons are transferred from one body to another. No new charge is created during rubbing.
When a glass rod is rubbed with silk, electrons move from glass to silk. The glass rod becomes positively charged and silk becomes negatively charged.
Positive and negative charges
There are two kinds of electric charge: positive and negative. Like charges repel each other and unlike charges attract each other.
By convention, the charge on a glass rod rubbed with silk is positive. The charge on a plastic rod rubbed with fur is negative.
Gold-leaf electroscope
A gold-leaf electroscope detects electric charge on a body. When a charged body touches its metal knob, charge flows to the leaves.
The leaves diverge because they acquire similar charges. Greater divergence indicates a larger amount of charge.
Conductors, Insulators and Charge Transfer
Conductors allow electric charges to move easily through them. Insulators resist the movement of electric charges.
This distinction explains why a plastic comb gets charged after rubbing, while a metal spoon usually loses charge through the hand. Metals, human bodies and earth act as conductors.
Conductors
Conductors contain charges that are comparatively free to move. Metals, human bodies, animal bodies and earth are conductors.
When charge is given to a conductor, it spreads over the surface. This happens because mobile charges repel each other and redistribute.
Insulators
Insulators do not allow charges to move freely. Glass, plastic, nylon, porcelain and wood are common insulators.
When charge is placed on an insulator, it remains near the same region. This is why a plastic comb can stay charged after rubbing dry hair.
Why metal articles do not stay charged easily
A metal object held in hand loses charge through the body to the earth. Both the human body and earth conduct electricity.
A metal rod can show charging if it is held with a plastic or wooden handle. The handle prevents charge leakage.
Basic Properties of Electric Charge
Electric charge has three basic properties: additivity, conservation and quantisation. These properties make class 12 physics chapter 1 notes useful for conceptual questions and small numerical problems.
In CBSE Class 12 Physics Chapter 1 Electric Charges and Fields, these properties connect everyday charging with mathematical charge calculation.
Additivity of charge
Charges add algebraically like real numbers. The total charge of a system is the sum of all individual charges.
Example:
(+1) + (+2) + (−3) + (+4) + (−5) = −1
Charge has magnitude but no direction. It is a scalar quantity.
Conservation of charge
Total charge of an isolated system remains conserved. Charges may get transferred or redistributed, but net charge remains the same.
When two bodies are rubbed, one body gains electrons and the other loses electrons. The total charge of both bodies remains unchanged.
Quantisation of charge
Charge exists in integral multiples of the elementary charge.
Formula:
q = ne
Where:
q = charge on a body
n = integer
e = 1.602192 × 10⁻¹⁹ C
At macroscopic levels, charge appears continuous because e is extremely small. At microscopic levels, charge appears in discrete packets.
Coulomb’s Law for Point Charges
Coulomb’s law gives the electrostatic force between two point charges. It applies when the size of charged bodies is much smaller than their separation.
Coulomb’s law class 12 questions usually test magnitude, direction and change in force when charge or distance changes. The force acts along the line joining the charges.
Scalar form of Coulomb’s law
For two point charges q1 and q2 separated by distance r in vacuum:
F = kq1q2/r²
In SI units:
k = 1/4πε0
So,
F = q1q2/4πε0r²
Where:
ε0 = 8.854 × 10⁻¹² C² N⁻¹ m⁻²
k ≈ 9 × 10⁹ N m² C⁻²
Direction of Coulomb force
Like charges repel each other. Unlike charges attract each other.
If q1 and q2 have the same sign, the force is repulsive. If q1 and q2 have opposite signs, the force is attractive.
Why one coulomb is large
One coulomb is a very large unit in electrostatics. A charge of −1 C contains about 6 × 10¹⁸ electrons.
If 10⁹ electrons leave a body every second, nearly 198 years are needed to collect 1 C. This explains why microcoulomb and millicoulomb are common in electrostatics.
Forces Between Multiple Charges
The force on a charge due to several charges is the vector sum of separate Coulomb forces. This is called the principle of superposition.
Each pairwise force remains unaffected by the presence of other charges. The total force is found by adding all individual force vectors.
Superposition principle
For charges q1, q2, q3 and so on, the force on q1 is:
F1 = F12 + F13 + F14 + ...
Each term is calculated using Coulomb’s law. The final answer must include vector direction.
Symmetry in charge systems
Symmetry can reduce force calculations. Equal forces placed at equal angles may cancel each other.
Example:
Three equal charges at the vertices of an equilateral triangle produce zero net force on a same-sign charge placed at the centroid. The three force vectors cancel by symmetry.
Electric Field Due to Charges
Electric field at a point is the force experienced by a unit positive test charge placed at that point. It describes the electrical effect of a source charge in surrounding space.
Electric field class 12 physics questions usually ask for field due to point charges, systems of charges or dipoles. Electric field is a vector quantity.
Definition of electric field
Electric field is defined as:
E = F/q
Where:
E = electric field
F = force on test charge
q = small positive test charge
SI unit:
N C⁻¹
For a point charge Q:
E = Q/4πε0r²
The field is radially outward for positive Q and radially inward for negative Q.
Electric field due to system of charges
Electric field due to a system of charges follows superposition.
Formula:
E = E1 + E2 + E3 + ...
The net field is the vector sum of fields due to individual source charges. The test charge does not affect the source charge distribution.
Physical significance of electric field
Electric field describes the electrical environment created by charges. It tells the force per unit positive test charge at each point.
The field concept becomes essential in time-dependent electromagnetic effects. Electromagnetic fields can propagate through space and transport energy.
Electric Field Lines
Electric field lines represent the direction and relative strength of electric field. The tangent to a field line gives the direction of electric field at that point.
Field lines are closer where the field is strong. They are farther apart where the field is weak.
Field lines around point charges
Field lines start from a positive charge and move outward. They end on a negative charge and point inward.
For an isolated positive charge, field lines go to infinity. For an isolated negative charge, field lines come from infinity.
Properties of electric field lines
Important properties:
- Field lines start from positive charges and end at negative charges.
- Field lines are continuous in charge-free regions.
- Two electric field lines never cross each other.
- Electrostatic field lines do not form closed loops.
- Uniform electric field lines are parallel and equally spaced.
What field line density shows
The density of field lines shows field strength. More lines per unit area mean stronger electric field.
For a point charge, field strength decreases as 1/r². This is why field lines spread farther apart with distance.
Electric Flux
Electric flux measures how much electric field passes through a surface. It depends on field strength, area and angle between field and area vector.
Electric flux is not a physical flow of charge. It is a field measure used to build Gauss’s law.
Area vector
An area vector is perpendicular to the surface. For a closed surface, the area vector points along the outward normal.
For a small area element:
ΔS = ΔS n̂
Where n̂ is the unit normal to the surface.
Electric flux formula
Electric flux through an area element is:
Δϕ = E · ΔS
So,
Δϕ = E ΔS cos θ
Where:
θ = angle between electric field and area vector
SI unit:
N C⁻¹ m²
When flux is maximum or zero
Flux is maximum when electric field is perpendicular to the surface. This means θ = 0° between E and area vector.
Flux is zero when electric field is parallel to the surface. This means θ = 90° between E and area vector.
Electric Dipole
An electric dipole consists of two equal and opposite charges separated by a small distance. Its total charge is zero, but its electric field is not zero.
Electric dipole class 12 questions often test dipole moment, axial field, equatorial field and torque in a uniform electric field.
Dipole moment
If charges +q and −q are separated by distance 2a, dipole moment is:
p = q × 2a
Direction:
From −q to +q
SI unit:
C m
Dipole moment is a vector quantity.
Electric field on axial line
At a far point on the axial line of a dipole:
E = 2p/4πε0r³
Direction is along the dipole moment for a point on the side of +q. The field varies as 1/r³ for large r.
Electric field on equatorial line
At a far point on the equatorial line of a dipole:
E = −p/4πε0r³
Direction is opposite to dipole moment. The field again varies as 1/r³ for large r.
Physical significance of dipoles
Some molecules have zero permanent dipole moment because positive and negative charge centres coincide. CO2 and CH4 are examples.
Some molecules have permanent dipole moment because charge centres do not coincide. H2O is an example of a polar molecule.
Dipole in a Uniform Electric Field
A dipole in a uniform electric field experiences torque. It has no net force because the forces on +q and −q are equal and opposite.
This result is important in CBSE Class 12 Physics Chapter 1 Electric Charges and Fields. It connects dipole moment with rotational effect.
Torque on electric dipole
Torque on a dipole in a uniform electric field is:
τ = p × E
Magnitude:
τ = pE sin θ
Where:
p = dipole moment
E = electric field
θ = angle between p and E
When torque is maximum or zero
Torque is maximum when θ = 90°.
τmax = pE
Torque is zero when θ = 0° or 180°. The dipole is aligned or anti-aligned with the field.
Potential energy of dipole
The potential energy of a dipole in a uniform electric field is:
U = −p · E
So,
U = −pE cos θ
The dipole has minimum potential energy when it aligns with the electric field.
Continuous Charge Distribution
Continuous charge distribution is used when charge is spread over a line, surface or volume. At the microscopic level, charge is discrete, but at the macroscopic level it can be treated as continuous.
This idea helps apply Coulomb’s law and Gauss’s law to wires, sheets and spherical shells. It also reduces long sums into charge density expressions.
Linear charge density
Linear charge density is charge per unit length.
Formula:
λ = ΔQ/Δl
SI unit:
C m⁻¹
It is used for charged wires or rods.
Surface charge density
Surface charge density is charge per unit area.
Formula:
σ = ΔQ/ΔS
SI unit:
C m⁻²
It is used for charged sheets and shells.
Volume charge density
Volume charge density is charge per unit volume.
Formula:
ρ = ΔQ/ΔV
SI unit:
C m⁻³
It is used for charges spread through a volume.
Gauss’s Law
Gauss’s law relates total electric flux through a closed surface to the charge enclosed by that surface. It is especially useful for symmetric charge distributions.
Gauss law class 12 physics questions usually need a suitable Gaussian surface. Symmetry decides whether the field can be taken outside the flux integral.
Statement of Gauss’s law
Gauss’s law states:
ϕ = qenc/ε0
Where:
ϕ = total electric flux through a closed surface
qenc = total charge enclosed
ε0 = permittivity of free space
The law uses only enclosed charge. Charges outside the closed surface do not change net flux through it.
Choosing a Gaussian surface
A Gaussian surface is an imaginary closed surface used to apply Gauss’s law. It should match the symmetry of the charge distribution.
Useful choices:
- Cylindrical surface for long charged wire
- Pillbox surface for infinite plane sheet
- Spherical surface for spherical shell
Why Gauss’s law is useful
Gauss’s law simplifies electric field calculation for symmetric charge distributions. It avoids direct vector integration.
It works best when the electric field has constant magnitude on the chosen surface. It also helps show that field inside a charged spherical shell is zero.
Applications of Gauss’s Law
Gauss’s law gives simple electric field results for infinite line charge, infinite plane sheet and uniformly charged spherical shell. These are standard CBSE Class 12 Physics Chapter 1 results.
Each application depends on symmetry. The field direction and Gaussian surface must match the charge distribution.
Field due to infinitely long straight wire
For a thin infinitely long straight wire with uniform linear charge density λ:
E = λ/2πε0r
Direction:
Radially outward for positive λ
Radially inward for negative λ
Here r is the perpendicular distance from the wire.
Field due to infinite plane sheet
For an infinite plane sheet with uniform surface charge density σ:
E = σ/2ε0
Direction:
Normal to the sheet, outward on both sides for positive charge
The field is independent of distance from the sheet.
Field due to uniformly charged spherical shell
For a thin spherical shell of radius R and total charge q:
Outside the shell, r ≥ R:
E = q/4πε0r²
Direction is radially outward for positive q.
Inside the shell, r < R:
E = 0
The outside field is the same as if the whole charge were concentrated at the centre.
Important Formulas in Electric Charges and Fields
The main formulas in CBSE Class 12 Physics Chapter 1 Electric Charges and Fields connect charge, force, field, flux, dipole and Gauss’s law. These formulas make electric charges and fields class 12 revision notes useful for quick numerical revision before CBSE 2026 exams.
| Formula |
Use |
Key Quantity |
| F = q1q2/4πε0r² |
Force between two point charges |
F |
| E = F/q |
Electric field from force on test charge |
E |
| ϕ = qenc/ε0 |
Flux through a closed surface |
ϕ |
More formulas:
- q = ne
- e = 1.602192 × 10⁻¹⁹ C
- k = 1/4πε0
- E = Q/4πε0r²
- Δϕ = E ΔS cos θ
- p = q × 2a
- Eaxial = 2p/4πε0r³
- Eequatorial = −p/4πε0r³
- τ = pE sin θ
- U = −pE cos θ
- Ewire = λ/2πε0r
- Esheet = σ/2ε0
- Eshell,outside = q/4πε0r²
- Eshell,inside = 0
Important Terms in Electric Charges and Fields
Electric Charges and Fields uses fixed terms in definitions, formulas and numerical questions. These terms help students answer one-mark, reasoning and derivation-based questions.
Electrostatics
Electrostatics is the study of forces, fields and potentials due to static charges.
Electric charge
Electric charge is the property that causes electric attraction or repulsion between bodies.
Point charge
A point charge is a charged body whose size is negligible compared with its distance from other charges.
Coulomb’s law
Coulomb’s law gives the force between two point charges separated by a distance.
Electric field
Electric field is force per unit positive test charge at a point.
Electric field line
An electric field line is a curve whose tangent gives the direction of electric field.
Electric flux
Electric flux is the dot product of electric field and area vector.
Electric dipole
An electric dipole is a pair of equal and opposite charges separated by a small distance.
Gaussian surface
A Gaussian surface is an imaginary closed surface used to apply Gauss’s law.
NCERT-Style Questions from Electric Charges and Fields
In CBSE Class 12 Physics Chapter 1 Electric Charges and Fields, NCERT-style questions usually test charge quantisation, Coulomb’s law, field direction, flux, dipole torque and Gauss’s law. Strong answers use formulas with direction and units.
Q1. How many electrons make a charge of −1 C?
A charge of −1 C contains about 6.25 × 10¹⁸ electrons.
Step 1:
q = ne
Step 2:
n = q/e
Step 3:
n = 1 / 1.6 × 10⁻¹⁹
n = 6.25 × 10¹⁸ electrons
Q2. Why is charge conserved during rubbing?
Charge is conserved during rubbing because electrons are transferred, not created or destroyed.
Explanation:
One body gains electrons and becomes negatively charged. The other loses electrons and becomes positively charged.
Fact:
The total charge of the isolated two-body system remains constant.
Q3. State Coulomb’s law in electrostatics.
Coulomb’s law states that force between two point charges is directly proportional to the product of charges and inversely proportional to the square of distance.
Formula:
F = q1q2/4πε0r²
Fact:
The force acts along the line joining the two charges.
Q4. What is the electric field due to a point charge?
The electric field due to a point charge Q at distance r is Q/4πε0r².
Formula:
E = Q/4πε0r²
Direction:
Outward for positive Q
Inward for negative Q
Q5. What does Gauss’s law say?
Gauss’s law says total electric flux through a closed surface equals enclosed charge divided by ε0.
Formula:
ϕ = qenc/ε0
Fact:
Only enclosed charge affects net flux through a closed surface.
Useful Links for Class 12 Physics