Attofarads

An attofarad (symbol aF) is the SI unit of electrical capacitance. It equals 10^-18 farads, since the prefix atto- denotes 10^-18. Therefore, 1 aF = 10^-18 F and 1 F = 10^18 aF. The farad is defined as one coulomb per volt, so an attofarad stores Q = C · V; at a voltage of 1 volt, a 1 aF capacitor holds 1 × 10^-18 coulombs of charge.

Attofarads are extremely small and are not common in everyday electronics. They appear in specialized research contexts involving nanoscale devices, quantum circuits, or measurements of quantum capacitance, where intrinsic or parasitic capacitances can reach the attofarad range. In typical circuits, capacitances are in the picofarad to nanofarad range, making values in the attofarad scale far below standard components. Measuring such small capacitances requires sensitive techniques and careful control of parasitics, often involving high-frequency impedance methods and cryogenic or low-noise instrumentation.

C = Q / V relates charge, voltage, and capacitance, so the energy stored in a capacitor is E = 1/2 C V^2. For an attofarad capacitor, energies remain extremely small at modest voltages, illustrating the vast range of scales encompassed by the farad and its SI prefixes. While the attofarad is not common in routine electronics, it remains a relevant concept in advanced physics, nanotechnology, and quantum-device literature.