A **capacitor** (formerly known as a "condenser") is a device that stores electric charge, or, more accurately, consists of two plates which each store an opposite charge. These two plates are conductive and separated by an insulator or *dielectric.* The charge is stored on the inside of the plates, at the boundary with the dielectric.

The capacitor's *capacitance* (C) is a measure of how much voltage (V) appears across the plates for a given charge (Q) stored in it:

A capacitor has a capacitance of one farad when one coulomb of charge causes a potential difference of one volt across the plates. Since the farad is a very large unit, values of capacitors are usually expressed in microfarads (μF), nanofarads (nF) or picofarads (pF).

When the voltage across a capacitor changes, the capacitor will be charged or discharged. The associated current is given by

The energy (in joules) stored in a capacitor is given by:

The capacitance of a parallel-plate capacitor is approximately equal to the following:

In a tuned circuit such as a radio receiver, the frequency selected is a function of the inductance (L) and the capacitance (C) in series, and is given by

Electrons cannot pass from one plate of the capacitor to the other. When a voltage is applied to a capacitor, current flows to one plate, charging it, while flowing away from the other plate, charging it oppositely. In the case of a constant voltage (DC) soon an equilibrium is reached, where the charge of the plates corresponds with the applied voltage, and no further current will flow in the circuit. Therefore direct current cannot pass. However, effectively alternating current (AC) can: every change of the voltage gives rise to a further charging or a discharging of the plates and therefore a current. The amount of "resistance" of a capacitor to AC is known as **capacitive reactance**, and varies depending on the AC frequency. Capacitive reactance is given by this formula:

- X
_{C}= capacitive reactance, measured in ohms - f = frequency of AC in hertz
- C = capacitance in farads

Thus the reactance is inversely proportional to the frequency. Since DC has a frequency of zero, the formula confirms that capacitors completely block direct current. For high-frequency alternating currents the reactance is small enough to be considered as zero in approximate analyses.

The impedance of a capacitor is given by:

Hence, capacitive reactance is the negative imaginary component of impedance.

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2 Variable capacitors 3 History |

- ceramic (low values up to about 1μF)
- polystyrene (usually in the picofarad range)
- polyester (from about 1nF to 1μF)
- polypropylene (low-loss, high voltage, resistant to breakdown)
- tantalum (compact, low-voltage devices up to about 100μF)
- electrolytic (high-power, compact but lossy, in the 1μF-1000μF range)
- air-gap

Since capacitors have such a low resistance, they have the capacity to deliver huge currents into short circuits, which can be dangerous. For safety purposes, all large capacitors should be discharged before handling. This is done by placing a small 1 to 10 ohm resistor across the terminals, i.e. shorting through a resistance.

Capacitors can be fabricated in semiconductor integrated circuit devices using metal lines and insulators on a substrate. Such capacitors are used to store analogue signals in switched-capacitor filters, and to store digital data in dynamic random-access memory (DRAM).

- Those that use a mechanical construction to change the distance between the plates, or the surface of the area of the overlapping plates. These devices are called tuning capacitors or simply "variable capacitors", and are used in telecommunication equipment for tuning and frequency control.
- Those that use the fact that the thickness of the depletion layer of a diode varies with the DC voltage across the diode. These diodes are called variable capacitance diodes, varactors or varicaps. Any diode exhibits this effect, but devices specifically sold as varactors have a large junction area and a doping profile specifically designed to maximize capacitance.

See also: electricity, electronics, inductor.