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Current (electricity)

In electricity, current is any flow of charge, usually through a metal wire or some other electrical conductor. Conventional current was defined early in the history of electrical science as a flow of positive charge, although we now know that, in the case of metallic conduction, current is caused by a flow of negatively charged electrons in the opposite direction. Despite this understanding, the original definition of conventional current still stands. The symbol typically used for the amount of current (the amount of charge flowing per unit of time) is I, and the SI unit of electrical current is the ampere. Electric current is therefore also informally referred to as amperage, by analogy with the term voltage.

Current density is the current per unit (cross-sectional) area.

In metallic conductors, such as wires, currents are caused by a flow of electrons (negatively charged particles), but this is not case in most non-metallic conductors. Electric currents in electrolytes are flows of electrically charged atoms (ions), which exist in both positive and negative varieties. For example, an electrochemical cell may be constructed with salt water (a solution of sodium chloride) on one side of a membrane and pure water on the other. The membrane lets the positive sodium ions pass, but not the negative chlorine ions, so a net current results. Electric currents in plasma are flows of electrons as well as positive and negative ions. In water ice and in certain solid electrolytes, flowing protons constitute the electric current.

There are also instances where the electrons are the charge that is physically moving, but where it makes more sense to think of the current as the positive "holes" (the spots that should have an electron to make the conductor neutral) as being what moves. This is the case in a p-type semiconductor.

See electrical conduction for more information on the physical mechanism of current flow in materials.

Mathematically, current is defined as the net flux through an area. Thus:

φ = j·A

where A is the area through which the current is flowing, φ is the current, and j is called the "current density". The current density is defined as:

where n is the particle density (number of particles per unit volume), u is the average velocity of the particles in each volume, and x can be mass, charge, or any other characteristic whose flow one would like to measure.

Every electric current produces a magnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire.

Electric current can be directly measured with a galvanometer, but this method involves breaking the circuit, which is sometimes inconvenient. Current can also be measured without breaking the circuit by detecting the magnetic field it creates. Devices used for this include Hall effect sensors, current clamps and Rogowski coils.

Ohm's Law predicts the current in an (ideal) resistor to be the quotient of applied voltage over electrical resistance.

See also: Alternating current, Direct current