Capacitors

A capacitor is a component that stores energy in an electric field between two conducting surfaces. Capacitors are everywhere — from the flash unit in a camera to the smoothing circuits in power supplies and the timing circuits in electronics. Understanding how they store charge and energy is a core part of the Edexcel A-Level specification.

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Capacitance

Capacitance is defined as the charge stored per unit potential difference across the capacitor:

C = Q / V

where:

• C is the capacitance in farads (F)
• Q is the charge stored (C) — this refers to the charge on one plate (the other plate carries −Q)
• V is the potential difference across the capacitor (V)

One farad is an enormous capacitance. In practice, capacitors are measured in microfarads (μF = 10⁻⁶ F), nanofarads (nF = 10⁻⁹ F), or picofarads (pF = 10⁻¹² F).

Rearranging gives:

• Q = CV (charge stored is proportional to voltage)
• V = Q / C

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Parallel Plate Capacitor

The capacitance of a parallel plate capacitor with a vacuum (or air) between the plates is:

C = ε₀A / d