Editing Wireless power transfer (section)

=== Inductive coupling ===
{{main|Inductive charging}}
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| caption2 = Prototype inductive electric car charging system at 2011 Tokyo Auto Show
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| image3 = Power 2.0 Ecosystem illustration - Powermat charging spots on counter in a coffee shop.jpg
| caption3 = [[Powermat Technologies|Powermat]] inductive charging spots in a coffee shop. Customers can set their phones and computers on them to recharge.
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| image4 = U. S. Marine Forces Reserve scored a 100 percent on physical facility security during a Command Cyber Readiness Inspection conducted by the Defense Information Security Agency at Marine Corps Support Facility 130521-M-IU921-961.jpg
| caption4 = Wireless powered access card.
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| image5 = Magne Chargers.jpg
| caption5 = GM EV1 and Toyota RAV4 EV inductively charging at a now-obsolete [[Magne Charge]] station
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| image1   = 2010-12-08-Sonicare-4.jpg
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| image2   = Lamp powered by induction 1910.jpg
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| footer   = Left: modern inductive power transfer, an electric toothbrush charger.  A coil in the stand produces a magnetic field, inducing an alternating current in a coil in the toothbrush, which is rectified to charge the batteries. Right: a light bulb powered wirelessly by induction, in 1910
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In [[inductive coupling]] (''[[electromagnetic induction]]''<ref name="Valtchev" /><ref name="Davis">{{cite journal |last1=Davis |first1=Sam |title=Wireless power minimizes interconnection problems |journal=Power Electronics Technology |pages=10–14 |date=July 2011 |url=https://www.powerelectronics.com/technologies/power-electronics-systems/article/21861344/wireless-power-minimizes-interconnection-problems |access-date=16 January 2015}}</ref> or ''inductive power transfer'', IPT), power is transferred between [[electromagnetic coil|coils of wire]] by a [[magnetic field]].<ref name="Sazonov" /> The transmitter and receiver coils together form a [[transformer]].<ref name="Sazonov" /><ref name="Valtchev" /> An [[alternating current]] (AC) through the transmitter coil ''(L1)'' creates an oscillating magnetic field ''(B)'' by [[Ampere's circuital law|Ampere's law]].  The magnetic field passes through the receiving coil ''(L2)'', where it induces an alternating [[electromotive force|EMF]] ([[voltage]]) by [[Faraday's law of induction]], which creates an alternating current in the receiver.<ref name="Gopinath" /><ref name="Davis" /> The induced alternating current may either drive the load directly, or be [[rectifier|rectified]] to [[direct current]] (DC) by a rectifier in the receiver, which drives the load. A few systems, such as electric toothbrush charging stands, work at 50/60&nbsp;Hz so AC [[Mains power|mains current]] is applied directly to the transmitter coil, but in most systems an [[electronic oscillator]] generates a higher frequency AC current which drives the coil, because transmission efficiency improves with [[frequency]].<ref name="Davis" />

Inductive coupling is the oldest and most widely used wireless power technology, and virtually the only one so far which is used in commercial products. It is used in [[inductive charging]] stands for [[cordless]] appliances used in wet environments such as [[electric toothbrush]]es<ref name="Valtchev" /> and shavers, to reduce the risk of electric shock.<ref name="Wilson">{{cite web |last=Wilson |first=Tracy V. |title=How Wireless Power Works |website=How Stuff Works |publisher=InfoSpace LLC |year=2014 |url=http://electronics.howstuffworks.com/everyday-tech/wireless-power.htm |access-date=15 December 2014}}</ref> Another application area is "transcutaneous" recharging of biomedical [[prosthetic devices]] [[implant (medicine)|implanted]] in the human body, such as [[Artificial cardiac pacemaker|cardiac pacemakers]], to avoid having wires passing through the skin.<ref name="Puers" /><ref name="Sun2">{{cite book |last1=Sun |first1=Tianjia |last2=Xie |first2=Xiang |last3=Zhihua |first3=Wang |title=Wireless Power Transfer for Medical Microsystems |publisher=Springer Science & Business Media |date=2013 |url=https://books.google.com/books?id=kTA_AAAAQBAJ&q=%22wireless+power%22&pg=PA6 |isbn=978-1461477020}}</ref> It is also used to charge [[electric vehicle]]s such as cars and to either charge or power transit vehicles like buses and trains.<ref name="Valtchev" />

However the fastest growing use is wireless charging pads to recharge mobile and handheld wireless devices such as [[laptop computer|laptop]] and [[tablet computer]]s, [[computer mouse]], [[cellphones]], [[digital media player]]s, and [[video game controller]]s.{{citation needed|date=April 2021}}  In the United States, the Federal Communications Commission (FCC) provided its first certification for a wireless transmission charging system in December 2017.<ref>{{Cite news |url=https://www.engadget.com/2017/12/26/fcc-approves-first-wireless-power-at-a-distance-charging-syste/ |title=FCC approves first wireless 'power-at-a-distance' charging system |work=Engadget |access-date=2018-03-27}}</ref>

The power transferred increases with frequency<ref name="Davis" /> and the [[mutual inductance]] <math>M</math> between the coils,<ref name="Gopinath" /> which depends on their geometry and the distance <math>D_\text{range}</math> between them. A widely used figure of merit is the [[Mutual inductance|coupling coefficient]] <math>k\; =\; M/\sqrt{L_1 L_2}</math>.<ref name="Davis" /><ref name="Agbinya4">{{cite book |url=https://books.google.com/books?id=zDPqqBJ76ZAC&q=%22coupling+coefficient%22&pg=PA140 |last=Agbinya |year=2012 |title=Wireless Power Transfer |page=140 |publisher=River Publishers |isbn=9788792329233}}</ref> This dimensionless parameter is equal to the fraction of [[magnetic flux]] through the transmitter coil <math>L1</math> that passes through the receiver coil <math>L2</math> when L2 is open circuited. If the two coils are on the same axis and close together so all the magnetic flux from <math>L1</math> passes through <math>L2</math>, <math>k = 1</math> and the link efficiency approaches 100%. The greater the separation between the coils, the more of the magnetic field from the first coil misses the second, and the lower <math>k</math> and the link efficiency are, approaching zero at large separations.<ref name="Davis" /> The link efficiency and power transferred is roughly proportional to <math>k^2</math>.<ref name="Davis" /> In order to achieve high efficiency, the coils must be very close together, a fraction of the coil diameter <math>D_\text{ant}</math>,<ref name="Davis" /> usually within centimeters,<ref name="Tan" /> with the coils' axes aligned. Wide, flat coil shapes are usually used, to increase coupling.<ref name="Davis" /> [[Allotropes of iron|Ferrite]] "flux confinement" cores can confine the magnetic fields, improving coupling and reducing [[electromagnetic interference|interference]] to nearby electronics,<ref name="Davis" /><ref name="Puers" /> but they are heavy and bulky so small wireless devices often use air-core coils.

Ordinary inductive coupling can only achieve high efficiency when the coils are very close together, usually adjacent. In most modern inductive systems [[resonant inductive coupling]] is used, in which the efficiency is increased by using [[resonant circuit]]s.<ref name="Agbinya" /><ref name="Wong" /><ref name="Davis" /><ref name="Shinohara" /> This can achieve high efficiencies at greater distances than nonresonant inductive coupling.