NEET Electrostatics and Electric Potential Notes by PW

Electrostatics deals with charges at rest and the electric forces and fields associated with them. Electric potential connects these ideas with work and energy, making concepts such as potential difference, equipotential surfaces and electrostatic potential energy important for solving numerical problems. Understanding the difference between scalar and vector quantities is also essential while revising this chapter.

A clear understanding of Electrostatics and Electric Potential is essential for effective Physics revision. Revise Coulomb’s law, electric field, Gauss’s law, electric dipole, electric potential and electrostatic potential energy, with their key formulas and applications. Physics Wallah explains these concepts in detail, helping you strengthen your preparation, recall important points and solve questions accurately. 

Electric Charge and Its Properties

Electric charge is a fundamental property of matter responsible for electrical interactions. Charges are of two types:

• Positive charge

• Negative charge

A body becomes positively charged when it loses electrons and negatively charged when it gains electrons. The magnitude of electronic charge is:

e = 1.6 × 10⁻¹⁹ C

Important properties of electric charge are:

1. Conservation: The total charge of an isolated system remains constant.

2. Quantisation: Charge exists in integral multiples of the elementary charge:
   q = ±ne, where n is an integer.

3. Invariance: The value of charge does not depend on the frame of reference.

4. Additivity: The net charge of a system is the algebraic sum of individual charges.

5. Scalar nature: Electric charge is a scalar quantity.

6. SI unit: The SI unit of charge is the coulomb (C).

Other commonly used units include:

• 1 Faraday ≈ 96500 C

• 1 Franklin ≈ 3.3 × 10⁻¹⁰ C

Methods of Charging

A body can be charged mainly by friction, conduction and induction.

Charging by Rubbing

When two different neutral insulating materials are rubbed together, electrons can transfer from one body to the other. The body that loses electrons becomes positively charged, while the body that gains electrons becomes negatively charged.

Charging by Conduction

A charged conductor is brought into contact with a neutral conductor. Charge is transferred between the bodies until they reach the same electric potential.

Charging by Induction

A charged body is brought close to a neutral conductor without touching it. The charges inside the conductor redistribute, producing charge separation. Earthing followed by removal of the earth connection and the inducing charge can leave the conductor with a net charge.

Coulomb’s Law

Coulomb’s law gives the electrostatic force between two point charges.

For charges q₁ and q₂ separated by distance r:

F = kq₁q₂/r²

For vacuum or air:

k = 1/(4πε₀) = 9 × 10⁹ N m²/C²

where ε₀ is the permittivity of free space:

ε₀ = 8.85 × 10⁻¹² C²/(N m²)

In a medium:

Fmedium = Fvacuum/εᵣ

where εᵣ is the relative permittivity of the medium.

The electrostatic force:

• Acts along the line joining the two charges

• Is attractive for unlike charges

• Is repulsive for like charges

• Follows the inverse-square law

• Is a central and conservative force

• Depends on the medium between the charges

Coulomb’s law is directly applicable to point charges.

Electric Field and Electric Force

The electric field at a point is the force experienced per unit positive test charge placed at that point.

Electric field is a vector quantity.

For a point charge q, the electric field at distance r is:

E = kq/r²

Its direction is:

• Away from a positive charge

• Towards a negative charge

The SI units of electric field are N/C and V/m.

The force experienced by a charge q placed in an electric field E is:

F = qE

The vector form of Coulomb’s law can be written as:

F₂₁ = k(q₁q₂/r₂₁³) r₂₁

where r₂₁ is the displacement vector directed from charge 1 towards charge 2.

For a conductor in electrostatic equilibrium, the electric field inside the conducting material is:

E = 0

Electric Field Lines

Electric field lines, also called electric lines of force, represent the direction and relative strength of an electric field.

Important properties include:

• Field lines emerge radially from positive charges.

• Field lines terminate on negative charges.

• In electrostatics, field lines do not form closed loops.

• Two electric field lines never intersect.

• Field lines meet the surface of a conductor normally.

• The tangent to a field line at any point gives the direction of the net electric field.

• The number or density of field lines is related to the magnitude of charge.

• Closely spaced field lines indicate a stronger electric field.

• Parallel and equally spaced lines represent a uniform electric field.

Standard Electric Field Results

Electric Field Due to a Charged Ring

At the centre of a uniformly charged ring:

E = 0

At an axial point at distance x from the centre:

E = kQx/(R² + x²)³ᐟ²

where R is the radius of the ring.

The field becomes maximum at:

x = R/√2

Electric Field Due to a Charged Arc

For a uniformly charged arc of radius r, linear charge density λ and angular extent θ, the field along the angle bisector is:

E = (2kλ/r) sin(θ/2)

The direction depends on the sign of the charge on the arc.

Electric Field Due to a Finite Charged Rod

For a charged rod with linear charge density λ, the perpendicular component of the field can be written as:

Eperpendicular = (kλ/r)(sin θ₁ + sin θ₂)

The parallel component is:

Eparallel = (kλ/r)(cos θ₂ − cos θ₁)

For an infinitely long line charge:

E = 2kλ/r

Electric Field Due to Charged Sheets

For an infinite non-conducting sheet with surface charge density σ:

E = σ/(2ε₀)

For a conducting surface:

E = σ/ε₀

For two oppositely charged infinite sheets:

• Electric field outside the sheets = 0

• Electric field between the sheets = σ/ε₀

Gauss’s Law and Electric Flux

Electric flux represents the electric field passing through a surface. It is a scalar quantity.

For a uniform electric field through an area A:

Φ = EA cos θ

For a non-uniform field:

Φ = ∫ E · dA

The SI unit of electric flux is:

N m²/C

Gauss’s law applies to a closed surface:

Φ = qenclosed/ε₀

The net electric flux through a closed surface depends only on the total charge enclosed by that surface. Charges outside the surface can affect the electric field at points on the surface, but their net contribution to the total flux through the closed surface is zero.

Important Electric Flux Results

Electric Dipole

An electric dipole consists of two equal and opposite charges separated by a small distance.

If the charges are −q and +q and their separation is l, the dipole moment is:

p = ql

The direction of electric dipole moment is from the negative charge towards the positive charge.

Its SI unit is C m.

For a short dipole:

Axial Field

E = 2kp/r³

Equatorial Field

E = kp/r³

The electric field at the equatorial point is opposite to the direction of the dipole moment.

Potential Due to a Dipole

At a general point:

V = kp cos θ/r²

At an equatorial point, θ = 90°, so:

V = 0

This happens because the scalar potential contributions due to the positive and negative charges cancel each other.

Torque and Potential Energy of an Electric Dipole

When a dipole is placed in a uniform electric field, the net force on it is zero, but it can experience torque.

The torque is:

τ = pE sin θ

The potential energy of the dipole is:

U = −pE cos θ

In a non-uniform electric field, the forces acting on the two charges can have different magnitudes. Therefore, a dipole may experience both a net force and torque.

Electric Potential and Potential Difference

Electric potential difference between two points is related to the work done per unit charge in moving a test charge between them.

It can be expressed as:

V_B − V_A = W/Q

The electric potential due to a point charge q at distance r is:

V = kq/r

Electric potential is a scalar quantity, so the potential due to multiple charges is obtained by algebraically adding the individual potentials.

The relationship between electric field and potential is:

E = −dV/dr

For one-dimensional variation:

E = −dV/dx

The negative sign indicates that the electric field points in the direction of decreasing potential.

For a potential-versus-distance graph, the electric field is related to the negative slope of the graph. Similarly, the potential difference can be obtained from the area under an electric field-versus-distance graph, with the appropriate sign.

For three-dimensional potential:

E = −[(∂V/∂x)i + (∂V/∂y)j + (∂V/∂z)k]

Equipotential Surfaces and Conductors

An equipotential surface is a surface on which the electric potential remains the same at every point.

Therefore:

ΔV = 0

The work done in moving a charge along an equipotential surface is:

W = qΔV = 0

Electric field lines are perpendicular to equipotential surfaces and point towards decreasing potential.

For a conductor in electrostatic equilibrium:

• The electric field inside the conductor is zero.

• Excess charge resides on the surface.

• The entire conductor is at the same potential.

• Electric field lines meet the conducting surface normally.

• Surface charge density is greater near sharp regions.

• The electric field just outside the surface is:

E = σ/ε₀

Electric Potential of Spheres

Conducting Shell or Hollow Sphere

For a charged conducting shell of radius R and charge q:

Outside the shell:

V = kq/r

At the surface:

V = kq/R

Inside the shell:

V = kq/R

Thus, the potential remains constant throughout the interior of a conducting spherical shell.

Uniformly Charged Solid Sphere

For a uniformly charged solid sphere:

Outside:

V = kq/r

At the surface:

V = kq/R

Inside:

V = kq(3 − r²/R²)/(2R)

The potential has its maximum value at the centre and decreases towards the surface.

At the centre:

Vcentre = 3Vs/2

where Vs is the potential at the surface.

Electrostatic Potential Energy

For two point charges Q₁ and Q₂ separated by distance r, the electrostatic potential energy is:

U = kQ₁Q₂/r

For like charges, the potential energy is positive, while for unlike charges, it is negative.

For a system of three charges:

Utotal = kq₁q₂/r₁₂ + kq₂q₃/r₂₃ + kq₃q₁/r₃₁

The work done by an external agent in bringing charges into a configuration is related to the change in electrostatic potential energy:

Wexternal = ΔU

Key Takeaways for NEET 2026

• Electric charge is conserved, quantised, additive and invariant.

• Coulomb’s law gives the electrostatic force between point charges.

• Electric field is a vector quantity, while electric potential and electric flux are scalar quantities.

• Gauss’s law relates the net electric flux through a closed surface to the enclosed charge.

• The electric field inside a conductor in electrostatic equilibrium is zero.

• Electric dipole moment is directed from negative charge to positive charge.

• A dipole in a uniform electric field experiences torque but no net force.

• Electric field is related to the negative gradient of electric potential.

• No work is done in moving a charge along an equipotential surface.

• The potential inside a charged conducting shell remains constant.

• Electrostatic potential energy depends on the arrangement and interaction of charges.

Electrostatics and Electric Potential covers important concepts such as electric charge, Coulomb’s law, electric field, Gauss’s law, electric dipoles, potential, equipotential surfaces and electrostatic potential energy. Revising the key formulas, standard results and relationships between electric field and potential can strengthen your understanding and help you solve NEET 2026 questions effectively. Physics Wallah covers these concepts through concept-based learning, revision and question practice to support your preparation. 

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