Capacitors

A capacitor is two conductors separated by an insulator (dielectric):

$Q = C V \qquad C = \varepsilon \frac{A}{d}$

Current flows only while the voltage changes:

$i = C \frac{dv}{dt}$

So the voltage across a capacitor cannot change instantly. Stored energy: $E = \tfrac{1}{2} C V^2$.

Series capacitors add like parallel resistors; parallel capacitors simply add. Uses: decoupling, filtering, timing, coupling AC signals and energy storage.