coil

A coil is a wound conductor or a helical structure used in a range of technologies. In electronics it typically refers to an inductor, whereas in mechanics a coil may describe a coil spring used to store energy or provide deflection force.

Electrical coils operate by passing current through a wire wound into turns. The resulting magnetic field enables inductors to oppose changes in current and to store energy, with the energy given by E = 1/2 L I^2, where L is the inductance. Inductors come in air-core forms or with ferromagnetic or ferrite cores to increase inductance. Coil geometry, including the number of turns N, cross-sectional area A, length l, and the core material, determines inductance. Toroidal, solenoidal, and U-shaped coils are common configurations.

Construction and materials commonly involve enamel-insulated copper or aluminum wire wound on a form or core made of ferrite, powdered iron, laminated steel, or left air-filled. Insulation, winding tightness, and thermal management are important for performance and reliability, especially in high-power or high-frequency applications. Coils are integral to transformers, inductors, chokes, electromagnets, relay coils, and the windings of motors and generators.

Mechanical coils include coil springs, which store elastic energy and provide force or displacement in mechanical systems. Springs vary by material, wire diameter, coil pitch, and diameter, shaping their stiffness and load capacity.

See also: Inductor, transformer, electromagnet, coil spring, solenoid.