Electrostatic Potential and Capacitance Notes NEET, JEE Main 2027

Electrostatic Potential and Capacitance is Chapter 2 of NCERT Class 12 Physics Part I and part of Unit 11, Electrostatics, in both the NEET UG 2026 and JEE Main 2026 syllabi. These notes cover potential, equipotential surfaces, potential energy, conductors, dielectrics, capacitors, their combinations and stored energy, with a free notes PDF.

Electrostatic Potential and Capacitance Notes for NEET and JEE Main

Section-wise key points from NCERT Chapter 2, formula tables, most-asked facts and common mistakes for 2027 revision.

Electrostatic Potential and Capacitance at a Glance

Topics follow the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025 and the JEE Main 2026 syllabus released by the National Testing Agency (NTA); both stay the reference until the 2027 syllabi are out. Full lists: NEET syllabus and JEE Main syllabus.

What is electrostatic potential?

Electrostatic potential at a point is the work done by an external force, per unit positive charge, in bringing a charge slowly from infinity to that point.

• Electrostatic force is conservative: the work done against it depends only on the initial and final points, not on the path taken.
• Work done by an external force in moving a charge q from R to P is q(V_P − V_R), equal to the change in its potential energy.
• Only the potential difference between two points is physically significant. Potential is fixed only up to an additive constant, so it is usually taken as zero at infinity.
• SI unit: volt (1 V = 1 J C⁻¹). Dimensions: [M L² T⁻³ A⁻¹].
• Point charge Q: V = Q/(4πε₀r). Potential falls as 1/r, while the field falls as 1/r².
• System of charges: V = (1/4πε₀)(q₁/r₁ + q₂/r₂ + … + qₙ/rₙ), an algebraic (scalar) sum by the superposition principle.
• Uniformly charged spherical shell: outside, V = q/(4πε₀r) as if the whole charge were at the centre; inside, the field is zero, so V is constant and equal to its value at the surface, q/(4πε₀R).

Potential Due to an Electric Dipole

A dipole’s potential depends on both distance and direction, and falls off faster than that of a single charge.

• The dipole result is exact for a point dipole and holds for a real dipole (charges ±q separated by 2a) when r >> a.

Equipotential Surfaces and the Field–potential Relation

An equipotential surface has the same potential at every point, and the electric field is always normal to it.

• Point charge: equipotentials are concentric spheres centred on the charge.
• Uniform field along the x-axis: equipotentials are planes normal to the x-axis.
• No work is done in moving a test charge along an equipotential surface.
• E = −dV/dl: the field points in the direction of the steepest decrease of potential, and its magnitude is the change in potential per unit displacement normal to the equipotential surface.
• Where equipotential surfaces are closer together, the field is stronger.

Potential Energy of a System of Charges

The potential energy of a system is the work done by an external agency in assembling the charges at their locations; it does not depend on the order in which they are brought in.

• U is positive for like charges (work is needed to bring them together) and negative for unlike charges.
• 1 eV = 1.6 × 10⁻¹⁹ J is the energy gained by an electron moved through a potential difference of 1 V.
• Dipole: U is minimum (−pE) at θ = 0° (stable equilibrium) and maximum (+pE) at θ = 180° (unstable equilibrium).
• In qV(r), V(r) is the potential due to external charges only, not the charge’s own potential.

Electrostatics of Conductors

In the static situation, free charges in a conductor rearrange until the field inside becomes zero.

• Electrostatic field is zero everywhere inside a conductor.
• At the surface of a charged conductor, the field is normal to the surface at every point.
• The interior has no excess charge; any excess charge resides on the surface.
• Potential is constant throughout the volume of a conductor and has the same value on its surface.
• Field just outside a charged conductor: E = (σ/ε₀) n̂, where σ is the surface charge density.
• Electrostatic shielding: the field inside a charge-free cavity of a conductor is zero, whatever the charges or fields outside. It protects sensitive instruments, but it does not work the other way round: charges placed inside the cavity are not shielded from the outside.
• NCERT examples: aircraft tyres are made slightly conducting and vehicles carrying inflammable material have metallic ropes touching the ground, so that static charge leaks to the earth.

Dielectrics and Polarisation

Dielectrics are non-conductors. An external field polarises them, and the field from the induced surface charges reduces, but does not cancel, the field inside.

• Polarisation P is the dipole moment per unit volume. For a linear isotropic dielectric, P = ε₀χₑE, where χₑ is the electric susceptibility.
• A uniformly polarised dielectric is equivalent to bound surface charge densities +σₚ and −σₚ on its faces; there is no net volume charge.
• Permittivity of a medium: ε = ε₀K. Dielectric constant K = ε/ε₀ is always greater than 1 (K = 1 for vacuum).

Capacitors and Capacitance

A capacitor is two conductors separated by an insulator, carrying charges +Q and −Q; its capacitance is C = Q/V.

• C depends only on the shapes, sizes and separation of the conductors and on the dielectric between them, not on Q or V.
• Unit: farad, 1 F = 1 C V⁻¹. The farad is very large; μF (10⁻⁶ F), nF (10⁻⁹ F) and pF (10⁻¹² F) are used in practice.
• Dielectric strength is the maximum field a medium can withstand without breakdown; for air it is about 3 × 10⁶ V m⁻¹.
• Parallel plate capacitor (vacuum): field between the plates E = σ/ε₀ and C = ε₀A/d. Fringing at the edges is ignored when d² << A.
• With a dielectric filling the gap: C = Kε₀A/d = KC₀.

Energy Stored in a Capacitor

The work done in charging a capacitor is stored as electrostatic energy in the field between its plates.

Inserting a Dielectric Slab (filling the Gap)

Most-asked Facts for NEET and JEE Main: Electrostatic Potential and Capacitance

Revise these one-line facts from the NCERT chapter just before the exam.

• Electrostatic force is conservative, so work done is path-independent.
• V = Q/(4πε₀r) for a point charge; V ∝ 1/r² for a dipole at large distance.
• Dipole potential is zero everywhere on its equatorial plane.
• Field is perpendicular to equipotential surfaces and points towards decreasing potential.
• E = −dV/dl; closer equipotentials mean a stronger field.
• U = q₁q₂/(4πε₀r) for two charges; U = −p·E for a dipole in a uniform field.
• Inside a conductor E = 0, and the potential is constant up to the surface.
• Field just outside a conductor is σ/ε₀, normal to the surface.
• Inside a charged spherical shell, V is constant and equals q/(4πε₀R).
• Non-polar: O₂, H₂; polar: HCl, H₂O.
• Dielectric strength of air is about 3 × 10⁶ V m⁻¹.
• C = ε₀A/d; with a dielectric C = KC₀.
• Series: 1/C = Σ1/Cᵢ (same charge); parallel: C = ΣCᵢ (same voltage).
• U = ½CV² = Q²/2C; energy density ½ε₀E².

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this chapter.

• Adding potentials as vectors. Potential is a scalar: add the values with their signs.
• Assuming zero field means zero potential. Inside a charged shell or conductor, E = 0 but V is constant and non-zero.
• Using the capacitor series formula for resistors (or the reverse). Capacitors in series add as reciprocals; resistors in series add directly.
• Saying a dielectric cancels the field. It only reduces the field, by the factor K.
• Forgetting whether the battery is connected. With Q fixed, energy falls to U₀/K; with V fixed, it rises to KU₀.
• Thinking capacitance depends on the charge or voltage. It depends only on geometry and the dielectric.
• Assuming shielding works both ways. A cavity is shielded from outside charges, but outside is not shielded from charges placed inside the cavity.
• Including the charge’s own potential in qV(r). V must come from external charges only.

Electrostatic Potential and Capacitance Notes PDF and Related Pages

Download the AglaSem Class 12 Physics unit notes (Electrostatics and Current Electricity; key notes and important questions with answers) free: View PDF

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• Source text: NCERT Chapter 2 (NCERT, Class 12 Physics Part I)

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