solenoid

A solenoid is a coil of wire that produces a magnetic field when electric current flows through it. In engineering usage, the term often refers to a device that converts electrical energy into linear motion: a coil with a movable ferromagnetic core, or plunger, that is drawn into the coil when energized.

Solenoids are typically cylindrical windings of insulated conductor around an inner core made of air or ferromagnetic material. When current I flows, the coil generates a magnetic field along its axis. For an ideal long solenoid, B ≈ μ n I, where n is the turns per unit length and μ the magnetic permeability of the core. A soft iron core increases μ and strengthens the field, producing a larger force on the core. The axial force depends on the gradient of the magnetic energy and the geometry.

Variants and devices: An air-core solenoid has little or no magnetic core, giving lower inductance but faster response. A ferromagnetic-core solenoid provides greater force and inductance. When used as actuators, the moving core acts against a spring or load. Solenoids are common in relays, valve actuators, locking devices, and vending or pinball mechanisms. A solenoid valve uses the magnetic force to open or close a fluid flow path.

Electrical characteristics: Inductance L, resistance R, and current I determine performance. The energy stored is 1/2 L I^2. Heating limits duty cycle; design must consider saturation of the core and thermal management.