PRACTICAL CAPACITORS

It is out of the question to make a capacitor of the preceding dimensions. However, two sheets or strips of foil can be placed one atop the other, separated by a thin, nonconducting sheet such as paper, and then the whole assembly can be rolled up to get a large effective surface area. When this is done, the electrical flux becomes great enough that the device exhibits significant capacitance. Two sets of several plates can be meshed together, with air in between them, and the resulting capacitance is significant at high ac frequencies.
In a capacitor, the electrical flux concentration is multiplied when a dielectric of a certain type is placed between the plates. Some plastics work well for this purpose. The dielectric increases the effective surface area of the plates so that a physically small component can be made to have a large capacitance. Capacitance is directly proportional to the surface area of the conducting plates or sheets. Capacitance is inversely proportional to the separation between conducting sheets; the closer the sheets are to each other, the greater is the capacitance. The capacitance also depends on the dielectric constant of the material between the plates. This is the electrostatic equivalent of magnetic permeability. A vacuum has a dielectric constant of 1. Dry air is about the same as a vacuum. Some substances have high dielectric constants that multiply the effective capacitance many times.
In theory, if the dielectric constant of a material is x, then placing that material between the plates of a capacitor will increase the capacitance by a factor of x compared with the capacitance when there is only dry air or a vacuum between the plates. In practice, this is true only if the dielectric is 100 percent efficient—if it does not turn any of the energy contained in the electrical field into heat. It is also true only if all the electrical lines of flux between the plates are forced to pass through the dielectric material. These are ideal scenarios, and while they can never be attained absolutely, many manufactured capacitors come close.