Electromagnet

An electromagnet is a type of magnet in which the magnetic field is induced by a flow of an electric current through a coil of wire. The magnetic field disappears when the current ceases.

The simplest type of electromagnet is a coil of wire. A coil forming the shape of a straight tube is called a solenoid; a solenoid bent so that the ends meet is a toroid. Much stronger magnetic fields can be produced if a "core" of paramagnetic or ferromagnetic material (commonly iron) is placed inside the coil. The field due to the coil causes the iron to magnetize and generate a field of its own. This field can be hundreds or thousands of times stronger than that of the coil itself.

The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be switched on and off, reversed, or varied in strength by controlling the electric current. It is disadvantageous, though, because it consumes electrical power, and usually generates a weaker field than the iron core would produce if magnetized permanently. Also, if an electromagnet with a ferromagnetic core is turned on and off again, the field does not disappear entirely. The core retains some residual magnetization due to hysteresis.

Applications

Electromagnets are used in many situations where a rapidly or easily variable magnetic field is desired. Many of these applications involve deflection of charged particle beams; the cathode ray tube and mass spectrometer fall into this category.

Other devices cause electromagnetic fields to interact with fields from permanent magnets and produce forces. Electromagnetic actuators take advantage of the fact that, if the core of a solenoid is pulled toward one end of the coil, a force will occur tending to push the core farther in that direction. Typical uses include relays, electromagnetic door locks, solenoid valves. Doorbells and similar devices are commonly made by causing the core to strike a bell.

Electromagnets are used in a rotary electric motor to produce a rotating magnetic field that turns the rotor, or in a linear motor to produce a travelling magnetic field that propels the projectile.