Capacitor

Capacitor, pronounced kuh PAS uh tuhr, is a device that stores electric energy in the form of an electric charge. Capacitors have many uses in electric and electronic circuits. For example, they can smooth the flow of an alternating current. They also can block the flow of a direct current. In combination with other devices, capacitors can be used to tune radios and television sets to different stations. The capacitors used in today's integrated electronic circuits are so tiny they can be seen only under a microscope. These circuits are used in calculators, digital watches, microcomputers, and many other kinds of devices.

How a capacitor works. A capacitor consists of two metal plates with an insulating material called a dielectric between them. Wires may connect the plates to a source of electric current, such as a battery. When an electric charge flows through the wires from one plate of the capacitor to the other, both plates become charged--one with a positive charge and the other with a negative charge. The two plates then have a potential difference in energy, called a voltage, between them. The plates will release their charge if their wires are disconnected from the source and touched together.

The ability of a capacitor to store electric energy is called its capacitance. A capacitor with large plates and a thin dielectric has a higher capacitance than one with small plates and a thick dielectric. Capacitance also depends on how well the dielectric acts as an insulator (see Capacitance).

Kinds of capacitors. Capacitors are made from many materials and in a variety of shapes and sizes. Capacitors are generally classified by the insulating material they use as a dielectric. The materials commonly used include ceramics, glass, mica, plastics, and paper soaked in oil. For example, one type of capacitor consists of a thin layer of plastic sandwiched between two long strips of aluminum foil. The plastic is the dielectric. The plastic and foil are rolled up to save space.

Some capacitors are designed for special purposes. Electrolytic capacitors are often used in industry, especially by electric utilities. Between the plates of these capacitors is a sheet of thin paper that has been dipped in an electrolyte (solution that conducts electricity). The action of the electrolyte causes a dielectric barrier to form on the surface of one of the plates.

Oil-filled capacitors are chiefly used in devices that require high voltage. Most of them have paper or plastic soaked with mineral oil as the dielectric between rows of plates.

Tuning capacitors are used to tune radios and televisions to the desired station. A type of tuning capacitor found in radios uses air as a dielectric between two sets of plates. One set of plates can slide between the plates of the other set, changing the air space between the plates and thus the capacitance.

Contributor: Raymond D. Findlay, Ph.D., Director, Engineering and Management, McMaster Univ.

Capacitor

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