Farad (F): the SI unit of electrical capacitance

Definition and common sizes

Formally: 1 F = 1 C/V. Capacitance in a simple parallel-plate capacitor depends on the plate area, the separation between plates and the permittivity of the intervening material (dielectric). Common prefixes used in electronics include:

• millifarad (mF) = 10⁻³ F
• microfarad (μF) = 10⁻⁶ F
• nanofarad (nF) = 10⁻⁹ F
• picofarad (pF) = 10⁻¹² F

Typical values and examples

Discrete capacitors used in electronic circuits typically range from picofarads to microfarads: small ceramic capacitors used for filtering and timing are often in the pF–nF range, electrolytic and polymer capacitors for power supply decoupling are usually in the μF range. Special energy-storage devices called supercapacitors can have capacitances exceeding one farad and are used where rapid charge/discharge and high energy density at low voltages are required. Measurement of capacitance in labs and repair shops is commonly performed with an LCR meter or a dedicated capacitance meter.

Applications and importance

Capacitors and their farad ratings are fundamental to many electronic functions: smoothing and filtering in power supplies, timing and frequency selection in oscillators and radios, pulse shaping in digital circuits, coupling and decoupling signals, and energy storage for backup or power conditioning. The choice of capacitance, voltage rating and dielectric material determines a component's suitability for a given task.

History and terminology

The unit is named after the English scientist Michael Faraday, whose experimental work in the early 19th century laid groundwork for understanding electric fields and capacitance. Historical texts and modern references explain the farad and related concepts; for background on the SI unit system see unit references and for general electronics context see electronics resources. For the concept of stored charge and how capacitance is used in circuits, technical introductions to capacitance and electric charge are useful.

Notable facts and further reading

Because a single farad is large for most circuit components, manufacturers label parts in μF, nF or pF. Supercapacitors and ultracapacitors exploit high-surface-area electrodes and special dielectrics to reach capacitances of more than one farad; see resources on supercapacitors. Historical biographies and scientific summaries about the unit's namesake are available via biographical links such as biographical sources and other technical overviews are indexed by community and educational portals like charge and field primers.