Editing Skin effect (section)

== Cause ==
[[File:Skineffect reason.svg|thumb|right|Cause of skin effect. A main current '''{{math|I}}''' flowing through a conductor induces a magnetic field '''{{math|H}}'''. If the current increases, as in this figure, the resulting increase in '''{{math|H}}''' induces separate, circulating eddy currents '''{{math|I}}'''{{sub|{{sc|W}}}} which partially cancel the current flow in the center and reinforce it near the skin.]]

Conductors, typically in the form of wires, may be used to transfer electrical energy or signals using an alternating current flowing through that conductor. The charge carriers constituting that current, usually [[electron]]s, are driven by an electric field due to the source of electrical energy. A current in a conductor produces a magnetic field in and around the conductor. When the intensity of current in a conductor changes, the magnetic field also changes. The change in the magnetic field, in turn, creates an electric field that opposes the change in current intensity. This opposing electric field is called [[counter-electromotive force]] (back EMF). The back EMF is strongest / most concentrated at the center of the conductor, allowing current only near the outside skin of the conductor, as shown in the diagram on the right.<ref name="Standard Handbook for Electrical Engineers (14th ed)) p. 2-50">{{cite book |last1=Fink |first1= Donald G. |last2= Beaty |first2= H. Wayne |year= 2000 |title=Standard Handbook for Electrical Engineers |edition=14th |publisher= McGraw-Hill |isbn= 978-0-07-022005-8 |pages=2–50 |quote=These emf's are greater at the center than at the circumference, so the potential difference tends to establish currents that oppose the current at the center and assist it at the circumference. }}</ref><ref name="Black Magic">{{cite book |last1=Johnson |first1=Howard |last2=Graham |first2=Martin |year=2003 |title=High-Speed Signal propagation Advanced Black Magic |edition=3rd |publisher= Prentice Hall |isbn=978-0-13-084408-8 | pages=58–78 |quote= To understand skin effect, you must first understand how eddy currents operate&nbsp;... }}</ref>

Regardless of the driving force, the [[current density]] is found to be greatest at the conductor's surface, with a reduced magnitude deeper in the conductor. That decline in current density is known as the ''skin effect'' and the ''skin depth'' is a measure of the depth at which the current density falls to [[E (mathematical constant)|1/e]] of its value near the surface.
Over 98% of the current will flow within a layer 4&nbsp;times the skin depth from the surface. This behavior is distinct from that of [[direct current]] which usually will be distributed evenly over the cross-section of the wire.

An alternating current may also be ''induced'' in a conductor due to an alternating magnetic field according to the law of [[Electromagnetic induction|induction]]. An [[electromagnetic wave]] impinging on a conductor will therefore generally produce such a current; this explains the attenuation of electromagnetic waves in metals. Although the term ''skin effect'' is most often associated with applications involving transmission of electric currents, skin depth also describes the exponential decay of the electric and magnetic fields, as well as the density of induced currents, inside a bulk material when a plane wave impinges on it at [[normal (geometry)|normal incidence]].