Capacitance

Capacitor — two conducting plates separated by an insulator, called the dielectric. Connected to a supply, charge builds on the plates: positive on one, an equal negative charge on the other.

Capacitance (C) — the charge stored per unit potential difference:

C = Q / V

measured in farads (F), where one farad is one coulomb per volt.

The farad is enormous — practical capacitors are measured in microfarads (10⁻⁶ F), nanofarads (10⁻⁹ F) or picofarads (10⁻¹² F). A one-farad capacitor is a large component.

What is actually stored — the net charge on a capacitor is zero. One plate carries +Q and the other −Q, and Q in the equation means the charge on one plate. What is stored is charge separation, and with it energy in the electric field between the plates.

Example: this is why a capacitor "blocks DC but passes AC". No charge crosses the dielectric — the gap is an insulator. But charge flowing onto one plate pushes an equal charge off the other, so a current appears to flow through the component while the voltage is changing. Once it is fully charged in a DC circuit, that stops.

Capacitance depends on construction, not on charge — for parallel plates it increases with plate area and decreases with separation, and is increased by a factor of the relative permittivity of the dielectric. Charging a capacitor more does not change its capacitance; it simply raises Q and V together in proportion.

Q against V is a straight line through the origin for a fixed capacitor, with gradient equal to the capacitance.

Uses — smoothing rectified supplies, timing circuits, camera flashes, and backup power for memory. The common thread is that a capacitor can release its energy very quickly, far faster than a chemical cell, though it stores far less of it.