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A '''solenoid''' is a useful device that generates a nearly uniform magnetic field, similar to that of a bar magnet, from an electrical current.
A '''solenoid''' is a useful device that generates a nearly uniform [[magnetic field]], similar to that of a [[bar magnet]], from an [[electrical current]].<ref>[https://www.britannica.com/science/solenoid-electronics Solenoid] from britannica.com</ref> It consists of a tightly-wound coil of wire to generate a magnetic field along the axis of the coil. Their length tends to be much greater than their [[diameter]]. The word solenoid can also refer to a specific [[electromagnetism|electromagnetic]] device that creates motion when an electrical current is applied.<ref name=britannica>[https://www.britannica.com/science/electromagnet#ref244719 Electromagnet] from britannica.com</ref> Often this motion is in the form of an [[iron]] plunger that moves in and out.
A '''solenoid''' consists of a tightly-wound coil of wire to generate a magnetic field perpendicular to the ends of the coil.
Solenoids have many uses. They are often used in relays to open or close low [[electrical current|current]] circuits or as circuit breakers/contactors in high current circuits.<ref name=britannica/> [[Car]]s have solenoids (known as starter solenoids or starter relays), which are used to switch on a large current to the starter motor, allowing the engine to start.
==Physics==
A solenoid can be very useful as the [[magnetic field]] it produces along its axis is uniform.<ref name=hyperphysics>[http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html Solenoid] from hyperphysics.phy-astr.gsu.edu</ref> Its overall magnetic if is very similar to that of a [[bar magnet]]. Its field can be strengthened by placing a ferromagnetic core composed of [[iron]] or [[steel]]. These contain magnetic [[domains], which act like tiny magnets and all align when put in an external magnetic field, increasing the overall field.
The field within the solenoid tends to be very uniform, while the surrounding field is normally extremely weak.<ref name=hyperphysics/> The field, ''B'', at the center of a solenoid is given by:<ref name=hyperphysics/>
<math>
B = \mu n I
</math>
where ''I'' is the current flowing through the solenoid and ''μ'' is the permeability of the core. ''n'' is the number of coils per unit length of the solenoid which is the number of coils divided by the length of the solenoid. Strictly speaking, this is the field for an idealized, [[infinity|infinity]] long coil. In practice however, it is a good application so long as the solenoid is much longer than its width and the relative permeability of its core is not much larger than that of the surrounding medium.<ref>Lerner, L. (2011) Magnetic field of a finite solenoid with a linear permeable core, ''American Journal of Physics'', Vol 79. (1030), Available on [http://aapt.scitation.org/doi/abs/10.1119/1.3602096 appt.scitation.org]</ref>
==See also==
*[[Electromagnet]]
*[[Magnetic field]]
*[[Inductor]]
==References==
{{reflist}}
[[Category:Physics]]
[[Category:Physics]]
[[Category:Engineering]]
[[Category:Engineering]]
[[Category:Electromagnetism]]