Farad

The farad (symbol F) is the SI unit of capacitance: 1 F = 1 C/V. It measures how much charge a capacitor accumulates per volt applied across it. A capacitor of 1 F charged to 1 V stores 1 C of charge, which is a colossal amount by electronic standards.

Practical capacitors almost never reach a farad. Typical values range from picofarads (pF, 10⁻¹² F) in radio-frequency tuning circuits, through nanofarads (nF) in timing circuits, to microfarads (μF) and millifarads (mF) in power-supply filters. Only modern electrochemical "supercapacitors" — which exploit double-layer charge storage at atom-scale interfaces — break the one-farad barrier, and they reach hundreds to thousands of farads in devices the size of a soup can.

The unit was named in 1881, after Michael Faraday, at the International Electrical Congress in Paris. Faraday never saw a capacitor labelled in farads during his lifetime; the naming was a posthumous tribute from a profession that was still busy laying down the units he had helped make necessary.