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Encyclopedia > Electrical current

In electricity, current is the rate of flow of charges, 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 misunderstanding, the original definition of conventional current still stands (notably, this is not the case in Canada). The symbol typically used for the amount of current (the amount of charge flowing per unit of time) is I, from the German word Intensit酹, 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 the 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 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.


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

where


φ is the current, measured in amperes


j is the "current density" measured in amperes per square metre


A is the area through which the current is flowing, measured in square metres


The current density is defined as:

where


n is the particle density (number of particles per unit volume)


x is the mass, charge, or any other characteristic whose flow one would like to measure.


u is the average velocity of the particles in each volume


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:

where


I is the current, measured in amperes


V is the potential difference measured in volts


R is the resistance measured in ohms


Electrical safety

The danger of an electric shock depends on the current (in milliamperes), duration and the current's path in the body:

Currents through the heart and the nervous system are the most dangerous.


SI electricity units

SI electromagnetism units

edit  (http://en.wikipedia.org/w/index.php?title=Template:SI_electromagnetism_units&action=edit)

Name Symbol Dimensions Quantity
ampere (SI base unit) A A Current
coulomb C A新 Electric charge, Quantity of electricity
volt V J/C = kg搶2−3−1 Potential difference
ohm Ω V/A = kg搶2−3−2 Resistance, Impedance, Reactance
ohm metre Ω搶 kg搶3−3−2 Resistivity
farad F C/V = kg−1−224 Capacitance
farad per metre F/m kg−1−324 Permittivity
reciprocal farad F−1 kg12−2−4 Elastance
siemens S Ω−1 = kg−1−232 Conductance, Admittance, Susceptance
siemens per metre S/m kg−1−332 Conductivity
weber Wb V新 = kg搶2−2−1 Magnetic flux
tesla T Wb/m2 = kg新−2−1 Magnetic flux density
ampere per metre A/m m−1 magnetic induction
ampere-turns per weber A/Wb kg−1−222 Reluctance
henry H V新/A = kg搶2−2−2 Inductance
henry per metre H/m kg搶新−2−2 Permeability
(dimensionless) - - Magnetic susceptibility

See also


  Results from FactBites:
 
Electric current - Wikipedia, the free encyclopedia (1321 words)
Electric currents in electrolytes are flows of electrically charged atoms (ions), which exist in both positive and negative varieties.
The SI unit of electric current is the ampere (A), which is equal to a flow of one coulomb of charge per second.
It is the current that passes that determines the effect, and this depends on the nature of the contact, the condition of the body part, the current path through the body and the voltage of the source.
  More results at FactBites »

 
 

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