Electrical Engineering is a free introductory textbook to the basics of electrical engineering. See the editorial for more information....  # Capacitance

Author: E.E. Kimberly

If a device consisting of two plates of a conducting material separated by an air-gap be connected to a source of voltage, as in Fig. 5-14, the excess protons of the positive plate will attract the excess electrons of the negative plate and the plates are said to be charged. The electrons will flow to the upper (-) plate until the attraction from the positive plate is balanced by the mutual repulsion of the electrons on the negative plate, and then the flow will cease. The plates are then said to be fully charged. Such a device is called a condenser, or capacitor. Fig. 5-14. Electron Flow in a Condenser

If the plates of a condenser be moved closer together, the attraction between electrons and protons will increase, and more electrons will flow into the negative plate. If, furthermore, the opposing areas of the two plates be increased, the electrons will redistribute themselves, their number per unit of surface will be less, their mutual repulsion will be less, and more electrons will flow into the negative plate to restore the balance. If the applied voltage be increased, more electrons will flow into the negative plate. For any certain opposing (facing) plate area and distance between plates, the device will have a certain capacity for assuming a charge. This capacity is called capacitance. Its conventional symbol is C, and its unit of measurement is the farad. The farad is inconveniently large for most practical purposes, and hence the microfarad (mf), which is one-millionth of a farad, is commonly used.

A condenser is said to have a capacitance of 1 farad if its capacity is 1 coulomb of electricity per volt of applied voltage. A coulomb is that quantity of electricity which will pass a given point in a conductor in 1 second if the rate of current flow is 1 ampere.

Let Q denote the charge in the condenser. Then, the rate of current flow is But, since Q = CE, (5-14)

where C = capacitance in microfarads.

Not all condensers have only air between their plates. Some have waxed paper, mica, or other suitable material between the plates; and this material, which is called the dielectric, makes the capacitance greater than it would be if air alone were present.

Last Update: 2010-10-05