Capacitance

Capacitance measures how much charge a conductor — or pair of conductors — can hold per volt of potential difference. Put charge Q on an isolated conductor and it rises to some potential V; the ratio C = Q/V is the capacitance, a purely geometric property of the conductor's shape and size. For a pair of conductors ("a capacitor") the voltage is the potential difference between them, and the capacitance depends on their shapes, separation, and the insulating material between them.

The units are farads (F), defined as one coulomb per volt. A farad is an enormous capacitance; real capacitors range from picofarads (10⁻¹² F) in radio circuits to millifarads in power-supply smoothing and, with modern electrochemical "supercapacitors," tens or hundreds of farads in a fist-sized package. A basic parallel-plate capacitor with plates one metre square and a one-millimetre gap has a capacitance of about 9 nF.

Capacitance is ubiquitous in electronics because it lets circuits store charge, smooth voltage, and time events. A capacitor across a DC line steadies the voltage; a capacitor in series with a resistor sets a time constant τ = RC controlling how fast voltages rise and fall. Every pair of conductors in any circuit has some stray capacitance whether you want it or not — at high frequencies, that parasitic capacitance is often what actually limits performance.