What is a capacitor?

Role of the capacitor

A capacitor is an electronic component capable of storing energy in electrical form, by accumulating electric charges on two conductive surfaces called plates, separated by an insulator called a dielectric (air, plastic, mica...).

Structure and symbol

Concretely, a parallel-plate capacitor is made up of two parallel metal plates. When it is connected to a generator, one plate charges positively (+q) and the other negatively (-q), the two charges always being opposite. Its standard symbol consists of two parallel lines connected to the circuit terminals.

Capacitance C

The quantity that characterizes a capacitor is its capacitance, denoted C, expressed in farads (F). It relates the charge q stored (in coulombs, C) to the voltage U across its terminals (in volts, V):

q = C * U

In practice, the farad is a very large unit: instead, we use the microfarad (1 uF = 10^-6 F), the nanofarad (1 nF = 10^-9 F), or the picofarad (1 pF = 10^-12 F).

Example

A capacitor with capacitance C = 100 uF charged under a voltage U = 12 V carries a charge:

q = C * U = 100x10^-6 * 12 = 1.2x10^-3 C = 1.2 mC

Common pitfall

Be careful not to confuse capacitance (a fixed property of the component, in farads) with charge (a variable quantity, in coulombs, which depends on the applied voltage). The capacitance C does not change if U changes; it is q that changes proportionally.