Editing Electric current (section)

==Current density and Ohm's law==
{{Main|Current density}}
Current density is the rate at which charge passes through a chosen unit area.<ref name="Zangwill2013">{{cite book|first=Andrew |last=Zangwill|title=Modern Electrodynamics|year=2013|publisher=Cambridge University Press|isbn=978-0-521-89697-9}}</ref>{{rp|31}} It is defined as a [[Vector (geometric)|vector]] whose magnitude is the current per unit cross-sectional area.<ref name=Walker/>{{rp|749}} As discussed in [[#Reference direction|Reference direction]], the direction is arbitrary. Conventionally, if the moving charges are positive, then the current density has the same sign as the velocity of the charges.  For negative charges, the sign of the current density is opposite to the velocity of the charges.<ref name=Walker/>{{rp|749}} In [[SI|SI units]], current density (symbol: j) is expressed in the SI base units of amperes per square metre.<ref name="SI"/>{{rp|22}}

In linear materials such as metals, and under low frequencies, the current density across the conductor surface is uniform. In such conditions, [[Ohm's law]] states that the current is directly proportional to the potential difference between two ends (across) of that metal (ideal) [[resistor]] (or other [[ohmic device]]):
<math display=block>I = {V \over R} \, ,</math>

where <math>I</math> is the current, measured in amperes; <math>V</math> is the [[potential difference]], measured in [[volt]]s; and <math>R</math> is the [[electrical resistance|resistance]], measured in [[ohm]]s. For [[alternating current]]s, especially at higher frequencies, [[skin effect]] causes the current to spread unevenly across the conductor cross-section, with higher density near the surface, thus increasing the apparent resistance.