farad
Capacitanceexact by definition
The farad is the SI unit of capacitance, defined as the capacitance that holds one coulomb of charge at a potential difference of one volt. As a coherent derived unit it equals one coulomb per volt exactly, so its relation to the SI base units is exact by definition. One farad is a very large capacitance, which is why practical components are almost always labelled in picofarads through microfarads.
| 1 F | 1,000,000 μF |
Named for Michael Faraday, whose 1830s work on electrostatic induction and dielectrics established that the charge a conductor holds depends on the insulating material around it.
About the farad
Capacitance is the constant of proportionality in \(Q = CV\), so a farad is a coulomb per volt. For a parallel plate capacitor it works out to \(C = \ arepsilon A / d\): more plate area, less separation, and a dielectric with a higher permittivity all raise it. That relationship explains the whole product catalogue. Ceramic capacitors get small values from small area, electrolytics get large values from an extremely thin oxide layer grown on an etched foil, and supercapacitors reach farads by using an electrochemical double layer that is fractions of a nanometre thick across an activated carbon surface of enormous area.
The energy stored is \(E = \ frac{1}{2}CV^2\), which is worth holding next to the battery units. A 1 F supercapacitor at 2.7 V holds about 3.6 joules, which is one millionth of a watt-hour and a thousandth of what a small AAA cell carries. Capacitors win on power rather than energy: they can take and give that charge in milliseconds without chemical wear, and they survive hundreds of thousands of cycles. Batteries win on energy density by two or three orders of magnitude and always will while the mechanism is a surface effect rather than a bulk chemical one.