Editing Electromagnet (section)

=== Magnetic core ===
'''For definitions of the variables below, see box at end of article.'''

Much stronger magnetic fields can be produced if a [[magnetic core]], made of a [[soft magnetic material|soft]] [[ferromagnetic]] (or [[ferrimagnetic]]) material such as [[iron]], is placed inside the coil.<ref name="Hyperphysics">{{cite web
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 |title       = Electromagnet
 |website     = Hyperphysics
 |publisher   = Dept. of Physics and Astronomy, Georgia State Univ.
 |date        = 2012
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 |archive-date = September 22, 2014
}}</ref><ref name="Merzouki">{{cite book
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 |first1      = Rochdi
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 |first2      = Arun Kumar
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 |first3      = Pushparaj Mani
 |title       = Intelligent Mechatronic Systems: Modeling, Control and Diagnosis
 |publisher   = Springer Science & Business Media
 |date        = 2012
 |pages       = 403–405
 |url         = https://books.google.com/books?id=k81ECeMxyk8C&q=ferromagnetic+electromagnet&pg=PA404
 |isbn        = 978-1447146285
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 |archive-url  = https://web.archive.org/web/20161203041435/https://books.google.com/books?id=k81ECeMxyk8C&pg=PA404&dq=ferromagnetic+electromagnet#v=onepage&q=ferromagnetic%20electromagnet&f=false
 |archive-date = 2016-12-03
}}</ref><ref name="Gates">{{cite book
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 |title       = Introduction to Basic Electricity and Electronics Technology
 |publisher   = Cengage Learning
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 |isbn        = 978-1133948513
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 |archive-date = 2017-01-10
}}</ref><ref name="Shipman">{{cite book
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 |title       = Introduction to Physical Science
 |publisher   = Cengage Learning
 |edition     = 12
 |date        = 2009
 |pages       = 205–206
 |url         = https://books.google.com/books?id=PZs8AAAAQBAJ&q=electromagnet+ferromagnetic+solenoid+coil&pg=PA205
 |isbn        = 978-1111810283
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 |archive-url = https://web.archive.org/web/20170111023648/https://books.google.com/books?id=PZs8AAAAQBAJ&pg=PA205&dq=electromagnet+ferromagnetic+solenoid+coil
 |archive-date = 2017-01-11
}}</ref> A core can increase the magnetic field to thousands of times the strength of the field of the coil alone, due to the high [[Permeability (electromagnetism)|magnetic permeability]] <math>\mu</math> of the material.<ref name="Hyperphysics" /><ref name="Merzouki" /> Not all electromagnets use cores, so this is called a ''ferromagnetic-core'' or ''iron-core'' electromagnet.

This phenomenon occurs because the magnetic core's material (often iron or steel) is composed of small regions called [[magnetic domains]] that act like tiny magnets (see [[ferromagnetism]]). Before the current in the electromagnet is turned on, these domains point in random directions, so their tiny magnetic fields cancel each other out, and the core has no large-scale magnetic field. When a current passes through the wire wrapped around the core, its [[magnetic field]] penetrates the core and turns the domains to align in parallel with the field. As they align, all their tiny magnetic fields add to the wire's field, which creates a large magnetic field that extends into the space around the magnet. The core concentrates the field, and the magnetic field passes through the core in lower [[reluctance]] than it would when passing through air.

The larger the current passed through the wire coil, the more the domains align, and the stronger the magnetic field is. Once all the domains are aligned, any additional current only causes a slight increase in the strength of the magnetic field. Eventually, the field strength levels off and becomes nearly constant, regardless of how much current is sent through the windings.<ref name="Merzouki" /> This phenomenon is called [[Saturation (magnetic)|saturation]], and is the main nonlinear feature of ferromagnetic materials.<ref name="Merzouki" /> For most high-permeability core steels, the maximum possible strength of the magnetic field is around 1.6 to 2 [[Tesla (unit)|teslas]] (T).<ref name="Pauley">"''Saturation flux levels of various magnetic materials range up to 24.5 kilogauss''" (2.5 T) p.1  "''Silicon steel saturates at about 17 kilogauss''" (1.7 T) p.3  {{cite journal |last=Pauley |first=Donald E. |date=March 1996 |title=Power Supply Magnetics Part 1: Selecting transformer/inductor core material |url=http://www.arnoldmagnetics.com/WorkArea/DownloadAsset.aspx?id=4396 |url-status=dead |journal=Power Conversion and Intelligent Motion |archive-url=https://web.archive.org/web/20141224075136/http://www.arnoldmagnetics.com/WorkArea/DownloadAsset.aspx?id=4396 |archive-date=December 24, 2014 |access-date=September 19, 2014}}</ref><ref name="MagneticMaterials">The most widely used magnetic core material, 3% silicon steel, has saturation induction of 20 kilogauss (2 T). {{cite web |date=2013 |title=Material Properties, 3% grain-oriented silicon steel |url=http://ludens.cl/Electron/Magnet.html |url-status=live |archive-url=https://web.archive.org/web/20140920230119/http://ludens.cl/Electron/Magnet.html |archive-date=September 20, 2014 |access-date=September 19, 2014 |website=Catalog |publisher=Magnetic Materials Co. |page=16}}</ref><ref name="Short">"''Magnetic steel fully saturates at about 2 T''" {{cite book |last1=Short |first1=Thomas Allen |url=https://books.google.com/books?id=mVW2D_6XB5EC&pg=PA214 |title=Electric Power Distribution Handbook |date=2003 |publisher=CRC Press |isbn=978-0203486504 |pages=214}}</ref> This is why the very strongest electromagnets, such as superconducting and very high current electromagnets, cannot use cores.

When the current in the coil is turned off, most of the domains in the core material lose alignment and return to a random state, and the electromagnetic field disappears. However, some of the alignment persists because the domains resist turning their direction of magnetization, which leaves the core magnetized as a weak permanent magnet. This phenomenon is called [[hysteresis]] and the remaining magnetic field is called [[remanence|remanent magnetism]]. The residual magnetization of the core can be removed by [[degaussing]]. In alternating current electromagnets, such as those used in motors, the core's magnetization is constantly reversed, and the remanence contributes to the motor's losses.