Covers NCERT Class 12 Physics Part I, Chapters 1 & 2 — Electric Charges and Fields and Electrostatic Potential and Capacitance. Consistently one of the highest-weightage chapters in NEET Physics.
Electric dipole field (axial & equatorial), Gauss's law applications (infinite sheet, uniformly charged sphere, cylindrical symmetry), electrostatic potential energy of a charge system, equipotential surfaces & their properties, capacitors in series/parallel with dielectric slabs, and energy stored in a capacitor / energy density. These sub-areas account for a disproportionate share of repeat-style NEET questions from this chapter.
Each point is tagged to the NCERT section it comes from (section numbers are stable across editions — check the matching heading in your own copy for the full derivation).
Charge is quantised (q = ne) and conserved in an isolated system — both properties are directly testable as assertion-reason or one-line factual MCQs.
Coulomb's Law: F = kq₁q₂/r², with k = 1/4πε₀ ≈ 9×10⁹ N m² C⁻². Vector form matters for problems with multiple charges at angles — always resolve components before adding.
Electric field E = F/q₀ (test charge → 0). For a point charge, E = kq/r², directed away from positive charge, towards negative.
Dipole moment p = q×2a (from −q to +q). Axial field E ≈ 2kp/r³; equatorial field E ≈ kp/r³ (direction opposite to p). The factor-of-2 difference between axial and equatorial is a classic trap in options.
Torque on a dipole in uniform field: τ = pE sinθ = p×E. Net force is zero in a uniform field but non-zero in a non-uniform field — a frequent assertion-reason pairing.
Gauss's law: Φ = q_enclosed/ε₀. Use symmetry to pick the Gaussian surface — sphere for point/spherical charge, cylinder for line charge, pillbox for infinite sheet. Field due to infinite sheet: E = σ/2ε₀ (independent of distance); for a conductor surface: E = σ/ε₀.
Potential V = kq/r for a point charge; potential is a scalar, so contributions from multiple charges simply add algebraically (unlike field, which is a vector sum).
Equipotential surfaces are always perpendicular to field lines; no work is done moving a charge along one; surfaces are closer where the field is stronger.
Potential energy of a two-charge system: U = kq₁q₂/r. For a system of three or more charges, sum U over every unique pair — a common source of missed terms under time pressure.
Inside a conductor, E = 0 and the entire conductor (including its surface) is an equipotential region; charge resides only on the outer surface.
Capacitance C = Q/V. Parallel plate capacitor: C = ε₀A/d (vacuum). Introducing a dielectric of constant K anywhere between the plates always increases capacitance.
Series: 1/C_eq = Σ(1/Cᵢ) — equivalent capacitance is always less than the smallest individual capacitor. Parallel: C_eq = ΣCᵢ — always greater than the largest.
Energy stored: U = ½CV² = ½QV = Q²/2C. Energy density in the field: u = ½ε₀E². When a battery stays connected, V is constant during a change; when disconnected, Q is constant instead — this single distinction resolves most "before/after inserting dielectric" questions.
Original questions modelled on recurring NEET question types for this chapter — not verbatim reproductions of any official paper. Each includes a worked solution.
Scaled from direct NCERT application (Q1–Q6) to mixed NEET-level difficulty (Q7–Q15). Attempt in 18 minutes, then check the answer key.
| Situation | Battery stays connected | Battery disconnected |
|---|---|---|
| What's constant? | V (voltage) | Q (charge) |
| Insert dielectric K | C↑, Q↑, U↑ | C↑, V↓, U↓ |
| Increase plate separation d | C↓, Q↓ | C↓, V↑ |
Memory hook: "Still connected → V is Still" (Still/Still/V) vs "Isolated → charge is Imprisoned" (Isolated/Imprisoned/Q). Two S-words pair with V, two I-words pair with Q.
"Axial is Active" — axial field is twice as strong (2kp/r³) and points along p; equatorial is the "quiet" one (kp/r³) and points opposite to p.