Capacitor

A capacitor consists of two conductors separated by a non-conductive region.[7] The non-conductive substance is called the dielectric medium, although this may also mean a vacuum or a semiconductor depletion region chemically identical to the conductors. A capacitor is assumed to be self-contained and isolated, with no net electric charge and no influence from an external electric field. The conductors thus contain equal and opposite charges on their facing surfaces,[8] and the dielectric contains an electric field. The capacitor is a reasonably general model for electric fields within electric circuits.

An ideal capacitor is wholly characterized by a constant capacitance C, defined as the ratio of charge ±Q on each conductor to the voltage V between them:[7]

C= \frac{Q}{V}

Sometimes charge buildup affects the mechanics of the capacitor, causing the capacitance to vary. In this case, capacitance is defined in terms of incremental changes:

C= \frac{\mathrm{d}q}{\mathrm{d}v}

In SI units, a capacitance of one farad means that one coulomb of charge on each conductor causes a voltage of one volt across the device