capacitor and capacitance

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Capacitor:

Capacitor, a device whose principal electric property is capacitance, the ability to store an electric charge. They are important components in many kinds of electrical equipment, including radio and television transmitters and receivers, some automobile ignition systems, and some types of motors. An early form of capacitor, the Leyden jar, was used by 18th-century scientists in studying the nature of electricity and is used today in physics laboratory demonstrations.

The ability of a capacitor to store an electric charge is useful in controlling the flow of an electric current. In some automobile ignition systems, for example, a capacitor (called a condenser) temporarily stores a charge when the breaker points of the distributor open. If there were no condenser, the charge would jump the gap and damage the points.

Another use of capacitors is in circuits that filter electrical signals. A capacitor whose capacitance can be varied is used in the tuning circuit of radio and television receivers. Varying the capacitance changes the resonant frequency of the tuner circuit so that it matches the frequency of the desired station or channel, filtering out signals of all other frequencies.

The typical commercial capacitor consists of two plates (conductors such as metal plates or foils) separated from one another by an insulator, or dielectric, with each plate connected to a terminal. There are two principal types of capacitors, those with continuously variable capacitance and those with a fixed capacitance.

When voltage is placed across the terminals of an uncharged capacitor, charge flows up to the plates but not across the insulator; one plate receives a positive charge, the other a negative charge. As the plates become charged, a voltage is produced across them that opposes the externally applied voltage. When these two voltages have the same magnitude, the current ceases and the capacitor is said to be charged. A charged capacitor is discharged by reducing the external voltage; when this occurs, charge flows off the plates, producing a current and decreasing the voltage across the plates until the external voltage and the plate voltage are equal.

Common Commercial Capacitors

Capacitors. Three types are shown (front to back): mica, ceramic cylinder, and dielectric. The penny gives an indication of their size.

Air Capacitors

use air as the insulator. Most variable capacitors are of this type. Variable capacitors are most often made of two sets of parallel aluminum plates that are interleaved. One set of plates is fixed, while the other can be rotated. Rotation changes the effective area of the plates, thereby varying the capacitance.

Oil or Liquid Dielectric Capacitors

consist of rigid metal plates immersed in oil or some other liquid insulator. The entire unit is sealed in a leakproof container.

Mica Capacitors

A mica capacitor consists of alternate layers of mica and aluminum foil in a plastic case. Such capacitors are compact, durable, and stable; they are used in precision work.

Ceramic Capacitors

One type is a hollow cylinder of a ceramic material, forming the insulator; the plates are thin films of metal deposited on the cylinder's inner and outer surface. Another type is a block containing many plates interleaved with a ceramic material. Both types are sealed in plastic to protect them from damage and moisture. Ceramic capacitors are much used in situations involving very high frequencies, as in television sets.

Paper Capacitors

Two metal foils are separated by a layer of paper or polyester film. Another layer of paper or film is placed on the outside of one of the pieces of foil. This sandwich is rolled up, impregnated with oil, and sealed in a moisture-tight container. Paper capacitors are widely used.

Electrolytic Capacitors

One of the conductors consists of a metal—usually tantalum or aluminum—covered by a thin oxide film. The oxide film serves as the insulator separating the metal from an electrolyte or some other nonmetallic conductor. The most common type of electrolytic capacitor is a block of tantalum with numerous interconnected oxide-lined pores containing the nonmetallic conductor. Electrolytic capacitors provide a relatively large amount of capacitance for their size.

Capacitance:

Capacitance, the ability of an object or a system of objects to store an electric charge. All objects have this property to varying degrees. A familiar example of capacitance is the ability of a storm cloud to store electricity and then give it up in a bolt of lightning.

Quantitatively, the capacitance (C) of a body is defined as the ratio of the electric charge (Q) on the body to the voltage (V) of the body; that is, C = Q/V

The capacitance of a system depends upon the size, shape, and composition of the bodies in the system and their orientations with respect to one another. For example, a parallel plate capacitor—a system consisting of two identical flat, parallel metal plates separated by an insulator—has a capacitance given by the formula C = eA/d, where E is a constant that depends upon the insulator, A is the area of one of the plates, and d is the distance between the plates.

Capacitance is measured in farads (f). A parallel plate capacitor has a capacitance of 1 farad if a charge of 1 coulomb on each plate is needed to produce a voltage of 1 volt between the plates. The farad is too large a unit for practical purposes. Hence capacitance is commonly measured in millionths of a farad, or microfarads (mf).

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