Editing Alternating current (section)

== Effects at high frequencies ==
{{Main|Skin effect}}

A direct current flows uniformly throughout the cross-section of a homogeneous [[electrical conductivity|electrically conducting]] wire. An alternating current of any frequency is forced away from the wire's center, toward its outer surface. This is because an alternating current (which is the result of the acceleration of [[electric charge]]) creates [[electromagnetic waves]] (a phenomenon known as [[electromagnetic radiation]]). Electric conductors are not conducive to electromagnetic waves (a [[Perfect conductor|perfect electric conductor]] prohibits all electromagnetic waves within its boundary), so a wire that is made of a non-perfect conductor (a conductor with finite, rather than infinite, electrical conductivity) pushes the alternating current, along with their associated electromagnetic fields, away from the wire's center. The phenomenon of alternating current being pushed away from the center of the conductor is called [[skin effect]], and a direct current does not exhibit this effect, since a direct current does not create electromagnetic waves. 

At very high frequencies, the current no longer flows ''in'' the wire, but effectively flows ''on'' the surface of the wire, within a thickness of a few [[skin depth]]s. The skin depth is the thickness at which the current density is reduced by 63%. Even at relatively low frequencies used for power transmission (50&nbsp;Hz – 60&nbsp;Hz), non-uniform distribution of current still occurs in sufficiently thick [[Electrical conductor|conductors]]. For example, the skin depth of a copper conductor is approximately 8.57&nbsp;mm at 60&nbsp;Hz, so high-current conductors are usually hollow to reduce their mass and cost. This tendency of alternating current to flow predominantly in the periphery of conductors reduces the effective cross-section of the conductor. This increases the effective AC [[Electrical resistance|resistance]] of the conductor since resistance is inversely proportional to the cross-sectional area. A conductor's AC resistance is higher than its DC resistance, causing a higher energy loss due to [[Ohmic heating]] (also called I<sup>2</sup>R loss).

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=== Techniques for reducing AC resistance ===
For low to medium frequencies, conductors can be divided into stranded wires, each insulated from the others, with the relative positions of individual strands specially arranged within the conductor bundle. Wire constructed using this technique is called [[Litz wire]]. This measure helps to partially mitigate skin effect by forcing more equal current throughout the total cross section of the stranded conductors. Litz wire is used for making [[Quality factor|high-Q]] [[inductor]]s, reducing losses in flexible conductors carrying very high currents at lower frequencies, and in the windings of devices carrying higher [[radio frequency]] current (up to hundreds of kilohertz), such as switch-mode [[power supply|power supplies]] and [[radio frequency]] [[transformer]]s.

=== Techniques for reducing radiation loss ===
As written above, an alternating current is made of [[electric charge]] under periodic [[acceleration]], which causes [[electromagnetic radiation|radiation]] of [[electromagnetic waves]]. Energy that is radiated is lost. Depending on the frequency, different techniques are used to minimize the loss due to radiation.

==== Twisted pairs ====
At frequencies up to about 1&nbsp;GHz, pairs of wires are twisted together in a cable, forming a [[twisted pair]]. This reduces losses from [[electromagnetic radiation]] and [[inductive coupling]]. A twisted pair must be used with a [[Balanced line|balanced]] signaling system so that the two wires carry equal but opposite currents. Each wire in a twisted pair radiates a signal, but it is effectively canceled by radiation from the other wire, resulting in almost no radiation loss.

==== Coaxial cables ====
[[Coaxial cable]]s are commonly used at [[Audio frequency|audio frequencies]] and above for convenience. A coaxial cable has a conductive wire inside a conductive tube, separated by a [[dielectric]] layer. The current flowing on the surface of the inner conductor is equal and opposite to the current flowing on the inner surface of the outer tube. The electromagnetic field is thus completely contained within the tube, and (ideally) no energy is lost to radiation or coupling outside the tube. Coaxial cables have acceptably small losses for frequencies up to about 5&nbsp;GHz. For [[microwave]] frequencies greater than 5&nbsp;GHz, the losses (due mainly to the dielectric separating the inner and outer tubes being a non-ideal insulator) become too large, making [[Waveguide (electromagnetism)|waveguides]] a more efficient medium for transmitting energy. Coaxial cables often use a perforated dielectric layer to separate the inner and outer conductors in order to minimize the power dissipated by the dielectric.

==== Waveguides ====
[[Waveguide (electromagnetism)|Waveguides]] are similar to coaxial cables, as both consist of tubes, with the biggest difference being that waveguides have no inner conductor. Waveguides can have any arbitrary cross section, but rectangular cross sections are the most common. Because waveguides do not have an inner conductor to carry a return current, waveguides cannot deliver energy by means of an [[electric current]], but rather by means of a ''guided'' [[electromagnetic field]]. Although [[Current density|surface currents]] do flow on the inner walls of the waveguides, those surface currents do not carry power. Power is carried by the guided electromagnetic fields. The surface currents are set up by the guided electromagnetic fields and have the effect of keeping the fields inside the waveguide and preventing leakage of the fields to the space outside the waveguide. Waveguides have dimensions comparable to the [[wavelength]] of the alternating current to be transmitted, so they are feasible only at microwave frequencies. In addition to this mechanical feasibility, [[electrical resistance]] of the non-ideal metals forming the walls of the waveguide causes [[dissipation]] of power (surface currents flowing on lossy [[electrical conductor|conductors]] dissipate power). At higher frequencies, the power lost to this dissipation becomes unacceptably large.

==== Fiber optics ====
At frequencies greater than 200&nbsp;GHz, waveguide dimensions become impractically small, and the [[ohmic heating|ohmic losses]] in the waveguide walls become large. Instead, [[fibre optics|fiber optics]], which are a form of dielectric waveguides, can be used. For such frequencies, the concepts of voltages and currents are no longer used.{{citation needed|date=October 2024}}