An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The
magnetic field disappears when the current ceases.
A regular magnet (from Greek μαγνήτης λίθος, "Magnesian stone") is a material or object that produces a magnetic
field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on
other ferromagnetic materials and attracts or repels other magnets.
A permanent magnet is one that stays magnetized, such as a magnet used to hold notes on a refrigerator door.
Materials which can be magnetized, which are also the ones that are strongly attracted to a magnet, are called
ferromagnetic. These include iron, nickel, cobalt, some rare earth metals and some of their alloys, and some naturally
occurring minerals such as lodestone. Permanent magnets are made from "hard" ferromagnetic materials which are
designed to stay magnetized, while "soft" ferromagnetic materials like soft iron are attracted to a magnet but don't tend
to stay magnetized.
An electromagnet is made from a coil of wire which acts as a magnet when an electric current passes through it, but
stops being a magnet when the current stops. Often an electromagnet is wrapped around a core of ferromagnetic
material like steel, which enhances the magnetic field produced by the coil.
Although ferromagnetic materials are the only ones strongly enough attracted to a magnet to be commonly
considered "magnetic", all other substances respond weakly to a magnetic field, by one of several other types of
magnetism. Paramagnetic materials, such as aluminum and oxygen are weakly attracted to a magnet. Diamagnetic
materials, such as carbon and water, which include all substances not having another type of magnetism, are weakly
repelled by a magnet.
The overall strength of a magnet is measured by its magnetic moment, while the local strength of the magnetism in a
material is measured by its magnetization.
In physics, a magnetic field is a vector field that permeates space and which can exert a magnetic force on moving
electric charges and on magnetic dipoles (such as permanent magnets). When placed in a magnetic field, magnetic
dipoles tend to align their axes to be parallel with the magnetic field, as can be seen when iron filings are in the
presence of a magnet (see picture at right). In addition, a changing magnetic field can induce an electric field.
Magnetic fields surround and are created by electric currents, magnetic dipoles, and changing electric fields. Magnetic
fields also have their own energy, with an energy density proportional to the square of the field intensity.
There are some notable specific instances of the magnetic field. For the physics of magnetic materials, see
magnetism and magnet, and more specifically ferromagnetism, paramagnetism, and diamagnetism. For constant
magnetic fields, such as are generated by stationary dipoles and steady currents, see magnetostatics. For magnetic
fields created by changing electric fields, see electromagnetism.
The electric field and the magnetic field are tightly interlinked, in two senses. First, changes in either of these fields
will cause ("induce") changes in the other, according to Maxwell's equations. Second, according to Einstein's theory of
special relativity, a magnetic force in one inertial frame of reference may be an electric force in another, or vice-versa
(see relativistic electromagnetism for examples). Together, these two fields make up the electromagnetic field, which
is best known for underlying light and other electromagnetic waves.
Electric current is the flow of electric charge. The electric charge may be either electrons or ions.
The SI unit of electric current intensity is the ampere. Electric current is measured using an ammeter.
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