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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.
Historically, the symbol for current, I, came from the German word Intensität, which means 'intensity'. The
SI unit of electrical current is the ampere. Electric current is therefore sometimes informally referred to as amperage, by
analogy with the term voltage. However, engineers frown on this usage, which
is considered amateurish.
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:
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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:
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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 (or other ohmic device) to be the quotient of
applied voltage over electrical resistance:
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See also: Alternating current, Direct current
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