A basic, parallel-plate capacitor is easy to picture. It is
two sheets of conductive foil with an insulating film in between. The
two sheets and the film are rolled up like
a jelly roll to save space.
When a capacitor is put in the path of an
electrons flow onto its upstream plate,
making it negatively charged (see figure).
Because like charges repel, electrons are driven off the other plate,
leaving positively charged atoms called ions. Eventually,
the capacitor fills up with charge, rebuffing additional current.
But charge is now trapped in the capacitor. Positive ions
beckon adjacent electrons
that are blocked by the insulation. The charges are held in
place, creating an electric field. The capacitor is now storing
electromagnetic energy and a
voltage appears across its plates.
Large capacitors have lots of storage space or "capacitance".
As a result, the charge spreads out, weakening its concentration.
A larger capacitor needs more charge to create a potential difference
of one volt across it.
The unit of capacitance is the farad. A one farad capacitor needs one coulomb of charge, 6.24
quintillion electrons, to produce one volt of voltage.
Mathematically, capacitance = charge per volt.
Farad is abbreviated capital F, for Michael Faraday;
coulomb is capital C, for Charles-Augustin de Coulomb; and
volt is capital V, for Alessandro Volta. So, F = C/V
The farad is an impractically large unit.
Most capacitors are measured in one of the following subunits, starting with the smallest:
picofarad (pF) = a trillionth (10-12) of a farad
nanofarad (nF) = a billionth (10-9) of a farad, equal to 1,000pF
microfarad (μF or MF) = a millionth (10-6) of a farad,
equal to 1,000nF