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Current (electricity)

In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor.

The symbol typically used for the amount of current (the amount of charge flowing per unit of time) is I, from the German word Intensitšt, 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. Though this is a valid term, some engineers frown on it.

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Conventional current

Conventional current was defined early in the history of electrical science as a flow of positive charge. In solid metals, like wires, the positive charges are immobile, and only the negatively charged electrons flow in the direction opposite conventional current, but this is not the case in most non-metallic conductors. In other materials, charged particles flow in both directions at the same time. 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. To simplify this situation, the original definition of conventional current still stands.

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.

Current density

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

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

\phi = j \cdot A

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:

j=\int_i n_i \cdot x_i \cdot \mathbf{u_i}

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

Electromagnetism

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

Ohm's law predicts the current in an (ideal) resistor (or other ohmic device) to be the quotient of applied voltage over electrical resistance:

I = \frac{V}{R}

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. As most dangerous sources are voltage sources, the current present depends on the resistance of the body between the points of contact and any current limiting built into the source.

SI electricity units

See also

External links

Last updated: 07-30-2005 03:49:28
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