Capacitor and capacitance

A capacitor is a device of two conductors (plates) separated by a layer of dielectric (insulator); it is used to store electric charge. The plates receive equal charges of opposite sign: +q on one and −q on the other. The ratio of the charge q to the voltage U between the plates is called the capacitance of the capacitor: C = q/U. The unit of capacitance is the farad (F): 1 F = 1 C/1 V. In practice small units are used: 1 µF = 10⁻⁶ F, 1 nF = 10⁻⁹ F, 1 pF = 10⁻¹² F. Capacitance depends on the capacitor itself (its size, shape, dielectric), not on q and U. A charged capacitor holds its charge, so do not touch capacitors in old devices — only a specialist does that.