Electromagnet

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

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. Its main disadvantages are that it consumes electrical power, and is usually heavier than a permanent magnet of the same strength.

The operation of an electromagnet is one aspect of electromagnetism.

Applications

Degaussing

An AC electromagnet can be used to undo permanent magnetization of a ferrous material such as iron. The AC electromagnet's magnetic field, whose polarity reverses rapidly and whose amplitude is slowly reduced from maximum strength to zero, leaves the magnetic domains of the material oriented in random directions, thus making them cancel out. This procedure is known as degaussing.

The process is used to demagnetize cathode ray tubes, whose metallic components can acquire a permanent magnetic field from nearby permanent magnets, or from the Earth. This field bends the path of the electrons and distorts the image. (Most modern televisions and monitors have internal circuitry to do this automatically, typically when power is applied.) Magnetic tape heads also acquire a permanent magnetic field over time, and need to be degaussed.

Actuators

These are devices that use an electromagnet to exert a force on a ferrous metal object. The electromagnet is usually in the form of a cylindrical coil called a solenoid, in which case the ferrous metal object is an iron bar placed inside the coil. Typical uses are relays, electromagnetic door locks and solenoid valves.

Particle beam deflection

Any magnetic field, such as that generated by an electromagnet, deflects the path of any electrically charged particle passing through it. This principle is used in the cathode ray tube, where the charged particles are electrons; and in the mass spectrometer, where the particles are ions.

Electric motors

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.