A capacitor stores energy in an electric field between two conductive plates separated by an insulator (dielectric). Capacitors are found throughout aircraft systems — in power supply filtering, radio tuning circuits, timing circuits, and surge protection. Understanding their behaviour in DC and AC circuits is essential.
Capacitance is the ability of a component to store electric charge. The unit is the farad (F). One farad means that 1 coulomb of charge is stored per 1 volt of potential difference.
Capacitance
$$ C = \frac{Q}{V} $$Where: \( C \) = capacitance (F), \( Q \) = charge (C), \( V \) = voltage (V).
In practice, most capacitors are rated in microfarads (μF), nanofarads (nF), or picofarads (pF), since a farad is an enormously large value.
Connect an uncharged capacitor across a source and nothing crosses the dielectric at all — it is an insulator, and that is the whole point of it. What happens instead is that the source pushes electrons onto one plate and pulls an equal number off the other, leaving one plate with a surplus and the other with an identical deficit. The plates therefore hold \( +Q \) and \( -Q \), and the capacitor as a whole stays electrically neutral. The \( Q \) in the definition above is the charge on one plate, not the sum of the two.
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