Practical capacitors
It’s out of the question to make a capacitor of the above dimensions. But two sheets, or
strips, of foil can be placed one on top of 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 electric flux becomes great enough so that the
device exhibits significant capacitance. In fact, two sets of several plates each can be
meshed together, with air in between them, and the resulting capacitance will be significant
at high ac frequencies.
In a capacitor, the electric flux concentration is multiplied when a dielectric of a
certain type is placed between the plates. Plastics work very well for this purpose. This
increases the effective surface area of the plates, so that a physically small component
can be made to have a large capacitance.
The voltage that a capacitor can handle depends on the thickness of the metal
sheets or strips, on the spacing between them, and on the type of dielectric used.
In general, capacitance is directly proportional to the surface area of the conducting
plates or sheets. Capacitance is inversely proportional to the separation between
conducting sheets; in other words, the closer the sheets are to each other, the greater
the capacitance. The capacitance also depends on the dielectric constant of the material
between the plates. A vacuum has a dielectric constant of 1; some substances have
dielectric constants that multiply the effective capacitance many times.
