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

Parallel plate: C = $epsilon_0$A/d (A = area, d = separation). With dielectric K: C = K$epsilon_0$*A/d. This is the most common capacitor in JEE problems.

Spherical (inner a, outer b): C = 4pi$epsilon_0$$\frac{ab}{b-a}$. Isolated sphere: C = 4pi*$epsilon_0$*R (second conductor at infinity). Earth: C ~ 711 uF, showing 1 F is enormously large.

Cylindrical (radii a, b, length L): C = 2pi$epsilon_0$*L/ln$\frac{b}{a}$. 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_0$$\frac{A}{d-t}$ (effective gap reduced). Dielectric slab of thickness t — C = $epsilon_0$$\frac{A}{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.