capacitance(redirected from Nominal capacity)
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The ratio of the charge q on one of the plates of a capacitor (there being an equal and opposite charge on the other plate) to the potential difference v between the plates; that is, capacitance (formerly called capacity) is C = q/v.
In general, a capacitor, often called a condenser, consists of two metal plates insulated from each other by a dielectric. The capacitance of a capacitor depends on the geometry of the plates and the kind of dielectric used, since these factors determine the charge which can be put on the plates by a unit potential difference existing between the plates.
In an ideal capacitor, no conduction current flows between the plates. A real capacitor of good quality is the circuit equivalent of an ideal capacitor with a very high resistance in parallel or, in alternating-current (ac) circuits, of an ideal capacitor with a low resistance in series. See Capacitor, Dielectric materials
a quantitative measure of a conductor’s ability to hold an electric charge. In an electrostatic field, all points of a conductor are at the same potential φ. If infinity is taken as the zero point for potential, φ is proportional to the charge q on the conductor; that is, the ratio of q to φ is not a function of q. The capacitance C of an isolated conductor is equal to the ratio of the charge on the conductor to the conductor’s potential: C = q/φ. Thus, the greater the capacitance, the greater the charge that can be stored by the conductor at a given φ.
Capacitance is determined by the size and shape of a conductor and by the electrical properties of the surrounding medium, that is, by the medium’s dielectric constant. Capacitance does not depend on the material of the conductor. Specifically, in the centimeter-gram-second (cgs) electrostatic system, the capacitance of a conducting sphere in vacuo is equal to the radius of the sphere. Since the potential of a conductor also depends on the electric fields created by the charges induced in the surrounding bodies as a result of electrostatic induction, the presence of bodies near a conductor changes its capacitance. Capacitance is measured in centimeters in the cgs system and in farads (1 farad = 9 × 1011 cm) in the International System of Units (SI).
The concept of capacitance applies not only to a single conductor but also to systems of conductors. It applies in particular to a system of two conductors that are separated by a thin dielectric layer. Such a system is called a capacitor. The capacitance of a capacitor is expressed by the equation C = q/(φ1 – φ2). where q is the charge on one plate, –q is the charge on the other plate, and φ1 – φ2 is the potential difference between the plates. The capacitance of a capacitor is virtually independent of the presence of surrounding bodies and can be very high in capacitors with small dimensions.
REFERENCETamm, I. E. Osnovy teorii elektrichestva, 9th ed. Moscow, 1976. Chapter 1.
Kalashnikov, S. G. Elektrichestvo, 4th ed. (Obshchii kurs fiziki). Moscow, 1977.
G. IA. MIAKISHEV