Capacitors

A capacitor is a passive electronic component that stores electrical energy in an electric field. In its simplest form it consists of two conducting plates separated by an insulating material called the dielectric. When a voltage is applied across the plates, charge accumulates on them — positive on one plate and negative on the other — and the resulting field holds energy until the capacitor is discharged. Unlike a resistor, an ideal capacitor dissipates no power; it stores and returns energy.

The quantity that describes how much charge a capacitor holds for a given voltage is its capacitance, measured in farads (F). One farad represents one coulomb of stored charge per volt. Because the farad is a large unit, practical components are usually rated in microfarads, nanofarads or picofarads. The capacitance of a parallel-plate capacitor increases with larger plate area, smaller plate spacing, and a dielectric material with a higher permittivity. Many small capacitors carry printed value codes or color bands rather than a directly readable figure.

Capacitors are frequently combined to obtain a desired value. Connecting them in parallel adds their capacitances together, giving a larger equivalent value, while connecting them in series reduces the equivalent capacitance in a manner similar to resistors in parallel. In circuits, capacitors block direct current while passing alternating current, which makes them central to filtering, coupling, decoupling, timing and energy-storage tasks. Their behavior depends partly on the dielectric used, which sets the working voltage and stability.

Frequently asked questions

What does a capacitor actually store?

It stores electrical energy in the electric field between its plates, held in place by separated positive and negative charge.

How do series and parallel connections affect capacitance?

Capacitors in parallel add directly for a larger total, while capacitors in series combine reciprocally for a smaller total.

What is the unit of capacitance?

The farad (F). Practical parts are usually rated in microfarads, nanofarads or picofarads because the farad is very large.