capacitor
A capacitor is a device built to store electric charge and energy, usually two conductors held close but not touching, with an insulator between them. The simplest is a pair of parallel metal plates. Connect it to a battery and one plate gathers positive charge while the other gathers an equal negative charge; disconnect it and the charge stays, banked for later use.
When a capacitor holds charge Q, the two plates sit at a potential difference V, and the two are linked by its capacitance, Q = C V. Between the plates the stored charges make a nearly uniform electric field, and the energy is held in that field: U = (1/2) Q V = (1/2) C V^2. A capacitor does not let steady current pass through the gap; instead it charges up and then blocks direct current, while readily passing changing (alternating) signals.
Capacitors are one of the three basic circuit components, alongside resistors and inductors. They smooth out bumpy voltages in power supplies, set the timing in flashers and oscillators together with resistors (an RC circuit), store the punch for a camera flash or a defibrillator, and tune radios. Slipping an insulating slab called a dielectric between the plates increases the capacitance and lets the capacitor hold more charge at the same voltage.
Two plates each 0.01 m^2 in area, separated by 1 mm (0.001 m) of air, have C = epsilon_0 A / d = (8.85 x 10^-12)(0.01) / 0.001 = about 9 x 10^-11 F, roughly 90 picofarads.
A parallel-plate capacitor stores charge Q at voltage V with C = Q/V set by its geometry.
A capacitor stores energy in the electric field between its plates, not as charge sitting still doing nothing. It blocks steady direct current but passes changing signals, the opposite of a plain wire.