Capacitance

Capacitance is a measure of the quantity of electric charge stored for a given electric potential. The most general form of charge storage device is a two-plate capacitor. If the charges on the plates are +Q and -Q, and V gives the voltage difference between the plates, then the capacitance is given by c=Q/v. The capacitance of the majority of capacitors used in electronic circuits is several orders of size smaller than the farad. The most ordinary units of capacitance in use today are the millifarad (mF), microfarad (µF), the nanofarad (nF) and the picofarad (pF).
The dielectric constant for a number of very useful dielectrics changes as a function of the applied electrical field, e.g. ferroelectric materials, so the capacitance for these devices is no longer purely a purpose of device geometry. If a capacitor is driven with a sinusoidal voltage, the dielectric constant, or more exactly referred to as the dielectric permittivity, is a function of frequency. A changing dielectric constant with frequency is referred to as a dielectric dispersion, and is governed by dielectric recreation processes, such as Debye relaxation.