Capacitance is a fundamental electrical property describing a system’s ability to store electrical energy. This property is associated with a device called a capacitor, which typically consists of two conductive plates separated by an insulating material called a dielectric. When voltage is applied, an electric field forms, allowing electrical charge to accumulate and be held within the device. Measuring this capacity is a prerequisite for designing and understanding electronic circuits.
The Standard Unit: The Farad
The standard unit of measure for capacitance within the International System of Units (SI) is the Farad, symbolized by the letter F. Named after physicist Michael Faraday, the Farad formally defines the relationship between stored electrical charge and the voltage across the device. One Farad (1 F) is defined as the capacitance required to store one Coulomb (C) of electrical charge when one Volt (V) is applied across its terminals. This means the Farad can also be expressed as an equivalent unit of one Coulomb per Volt (F = C/V).
Practical Measurement Scales
The Farad represents an extremely large quantity of capacitance rarely encountered in standard electronic components, which typically have values that are fractions of a single Farad. Therefore, practical measurements rely almost entirely on SI sub-units indicated by metric prefixes. The most common subunit is the microfarad (\(\mu\)F), which is one-millionth of a Farad (\(10^{-6}\) F), frequently used for power supply filtering and energy storage. Smaller units include the nanofarad (nF, \(10^{-9}\) F) and the picofarad (pF, \(10^{-12}\) F), which are common in high-frequency circuits, radio tuning, and signal processing applications.
Capacitance and Circuit Function
The measured capacitance value provides direct insight into how a capacitor will perform its function within a circuit.
Energy Storage
One primary function is energy storage, where the capacitor acts like a temporary reservoir of electrical energy that can be quickly discharged to supply sudden surges of current. This is often necessary when a digital chip switches from a low-power state to a high-power state.
Filtering and Decoupling
Another pervasive application is filtering and smoothing electrical signals. In power supplies, large capacitors smooth out ripple—small, unwanted voltage fluctuations remaining after converting alternating current (AC) to direct current (DC). Decoupling capacitors are placed near integrated circuits to provide a stable, local source of charge, shunting away high-frequency noise and maintaining voltage stability.
Signal Conditioning
Capacitors also allow alternating current signals to pass between stages of a circuit while simultaneously blocking direct current, a process important for signal conditioning.