What is a Capacitor?

A capacitor is a energy storing device made up of two parallel conductive plates separated by an insulating (dielectric) material.  When a voltage is applied across the plates, the electric field in the dielectric displaces electric charges, and thus stores energy.  It is assumed that there are no free charges in the dielectric (at least in the ideal case), and that while they are displaced, they are not free to move around as in a conductor.  The closest analogy in the mechanical world is probably energy stored by a spring.  Dielectrics come in two types, "polar" and "nonpolar".  Molecules where the "center-of-gravity" (as it were) of the negative and positive charges are at the same point are nonpolar.  If this isn´t the case, the molecule is polar.  H 2O, for example, is polar, but H2 is nonpolar.

The plates may be actual metal plates of various shapes but are most often in the form of metal foil or a metal film deposited on the insulating material.  Since the first capacitor was invented, the Leyden jar, almost every conceivable dielectric material and form has been tried by someone.  Wax, paper, plastics, ceramics, glass, oils, minerals, electrochemical films, air, etc., either alone or in combination such as paper/wax, paper/epoxy, plastic/plastic, paper/oil, plastic/oil have been used, to name just a few.  Some people seperate capacitors types into "electrostatic" and electrolytic".  This is really an artificial distinction, more of a construction detail than a fundamental difference.

Tags: Capacitor