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Capacitance C = Q/V measures a capacitor's ability to store charge per unit voltage. SI unit: farad (F) = C/V. Dimensions: [M^(-1) L^(-2) T^4 A^2]. Capacitance depends only on geometry and dielectric, not on Q or V.

Parallel plate: C = epsilon_0A/d (A = area, d = separation). With dielectric K: C = Kepsilon_0*A/d. This is the most common capacitor in JEE problems.

Spherical (inner a, outer b): C = 4piepsilon_0ab/(b-a). Isolated sphere: C = 4piepsilon_0R (second conductor at infinity). Earth: C ~ 711 uF, showing 1 F is enormously large.

Cylindrical (radii a, b, length L): C = 2piepsilon_0*L/ln(b/a). Used in coaxial cable calculations.

Combinations: Series — 1/C_eq = 1/C1 + 1/C2 + ... (same charge, voltage adds, C_eq < smallest C). Parallel — C_eq = C1 + C2 + ... (same voltage, charge adds, C_eq > largest C). Note: capacitor combinations are "opposite" to resistor combinations.

Special cases: conducting slab of thickness t inserted — C = epsilon_0A/(d-t) (effective gap reduced). Dielectric slab of thickness t — C = epsilon_0A/(d-t+t/K) (series model). Multiple dielectrics stacked (perpendicular to E) — series. Multiple dielectrics side by side (parallel to E) — parallel.

For n identical capacitors C: series gives C/n, parallel gives nC. The Wheatstone bridge analog: if two nodes of a capacitor are at equal potential (by symmetry), it carries no charge and can be removed.