Lasted edited by Andrew Munsey, updated on June 14, 2016 at 9:43 pm.
Electric current is by definition the flow of electric charge. The SI unit of electric current is the ampere (A), which is equal to a flow of one coulomb of charge per second.
In There was an error working with the wiki: Code[13]
metal, with no external forces applied, there exists a random motion of There was an error working with the wiki: Code[42]
s created by the Thermal energy that the electrons gain from the surrounding medium. When an Atom loses a free Electron, it acquires a net There was an error working with the wiki: Code[43]
. The free electron can move amongst these There was an error working with the wiki: Code[44]
Ions, while the positive ions can only oscillate about their mean fixed positions. The free electron is therefore the There was an error working with the wiki: Code[45]
in a typical solid conductor. Given an imaginary plane through which the wire passes, the number of Electrons moving from one side to the other in any period of time is exactly equal to the number passing in the opposite direction.
When a wire is connected across the two terminals of a Direct current There was an error working with the wiki: Code[14]
, the source places an electric field across the conductor. The moment contact is made, the There was an error working with the wiki: Code[46]
s of the conductor will drift toward the There was an error working with the wiki: Code[47]
terminal under the influence of this field. For every ampere of current, 1 There was an error working with the wiki: Code[48]
of Electric charge (which consists of about 6.242 × 1018 electrons) drifts every There was an error working with the wiki: Code[49]
at the same velocity through the imaginary plane through which the conductor passes.
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This SI unit is named after There was an error working with the wiki: Code[50]
. As for all SI units whose names are derived from the proper name of a person, the first letter of its There was an error working with the wiki: Code[51]
is There was an error working with the wiki: Code[52]
(A). But when an SI unit is spelled out, it should always be written in There was an error working with the wiki: Code[53]
(ampere), unless it begins a sentence or is the name "degree Celsius". &mdash Based on The International System of Units, section 5.2.
}
The ampere (symbol: A) is the SI base unit of Electric current. It is named after There was an error working with the wiki: Code[54]
, one of the main discoverers of Electromagnetism.
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular There was an error working with the wiki: Code[55]
, and placed 1 Metre apart in vacuum, would produce between these conductors a force equal to 2There was an error working with the wiki: Code[1]
There was an error working with the wiki: Code[56]
per metre of length.
Electric current is the time rate of change or displacement of Electric charge. One ampere represents the rate of 1 There was an error working with the wiki: Code[57]
of charge per second.
:{1 \,A= 1 \,C/s} \,
The ampere is defined first (it is a SI base unit, along with the Metre, the There was an error working with the wiki: Code[58]
, and the There was an error working with the wiki: Code[59]
), without reference to the quantity of charge. The unit of charge, the coulomb, is defined to be the amount of charge displaced by a one ampere current in the time of one There was an error working with the wiki: Code[58]
.
Because it is a base unit, the definition of the ampere is not tied to any other electrical unit. The definition for the ampere is equivalent to fixing a value of the There was an error working with the wiki: Code[15]
of vacuum to ?0 = 4?There was an error working with the wiki: Code[2]
H/m. Prior to 1948, the socalled "international ampere" was used, defined in terms of the Electrolysis deposition rate of There was an error working with the wiki: Code[61]
. The older unit is equal to 0.999 85 A.
The ampere is most accurately realized using an There was an error working with the wiki: Code[16]
, the There was an error working with the wiki: Code[17]
, since the latter two can be tied to physical phenomena that are relatively easy to reproduce, the There was an error working with the wiki: Code[62]
and the There was an error working with the wiki: Code[63]
, respectively.
The unit of Electric charge, the There was an error working with the wiki: Code[64]
, is defined in terms of the ampere: one coulomb is the amount of electric charge (formerly There was an error working with the wiki: Code[65]
) carried in a current of one ampere flowing for one There was an error working with the wiki: Code[66]
. Current (electricity), then, is the rate at which charge flows through a wire or surface. One ampere of current (I) is equal to a flow of one There was an error working with the wiki: Code[64]
of charge (Q) per second of time (t):
:{I=Q/t} \,
Since a coulomb is approximately equal to 6.24150948There was an error working with the wiki: Code[3]
There was an error working with the wiki: Code[68]
s, one ampere is approximately equivalent to 6.24150948There was an error working with the wiki: Code[3]
elementary charges, such as Electrons, moving past a boundary in one second.
As with other SI base units, there have been proposals to redefine the There was an error working with the wiki: Code[69]
in such a way as to define some presently measured There was an error working with the wiki: Code[70]
to fixed values. One proposed definition of the kilogram is:
: The kilogram is the mass which would be accelerated at precisely 2There was an error working with the wiki: Code[5]
m/s2 if subjected to the per metre force between two straight parallel conductors of infinite length, of negligible circular cross section, placed 1 metre apart in vacuum, through which flow a constant current of exactly 6 241 509 479 607 717 888 elementary charges per second.
This redefinition of the kilogram has the effect of fixing the There was an error working with the wiki: Code[18]
and would result in a functionally equivalent definition for the There was an error working with the wiki: Code[71]
as being the sum of exactly 6 241 509 479 607 717 888 elementary charges and the ampere as being the electrical current of exactly 6 241 509 479 607 717 888 elementary charges per second. This is consistent with the current 2002 CODATA value for the elementary charge which is 1.60217653There was an error working with the wiki: Code[6]
± 0.00000014There was an error working with the wiki: Code[6]
C.
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(CIPM) Recommendation 1 (CI2005):
Preparative steps towards new definitions of the There was an error working with the wiki: Code[73]
, the ampere, the
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in terms of fundamental constants
The International Committee for Weights and Measures (CIPM),
approve in principle the preparation of new definitions and mises en pratique of the kilogram, the ampere and the kelvin so that if the results of experimental measurements over the next few years are indeed acceptable, all having been agreed with the various Consultative Committees and other relevant bodies, the CIPM can prepare proposals to be put to Member States of the Metre Convention in time for possible adoption by the 24th There was an error working with the wiki: Code[20]
in 2011
give consideration to the possibility of redefining, at the same time, the mole in terms of a fixed value of the Avogadro constant
prepare a Draft Resolution that may be put to the 23rd There was an error working with the wiki: Code[21]
in 2007 to alert Member States to these activities
Current density is a measure of the There was an error working with the wiki: Code[22]
whose magnitude is the electric current per crosssectional area. In There was an error working with the wiki: Code[74]
, the current density is measured in Amperes per There was an error working with the wiki: Code[75]
. Electrical current is a coarse, average quantity that tells what is happening in an entire wire. The current density is an important parameter in There was an error working with the wiki: Code[76]
(one of There was an error working with the wiki: Code[77]
), which show the direct link between current density and There was an error working with the wiki: Code[78]
.
Current density is an important consideration in the design of electrical and There was an error working with the wiki: Code[23]
systems. Most electrical conductors have a finite, positive There was an error working with the wiki: Code[24]
excessive current density may generate a strong enough magnetic field to cause spontaneous loss of the superconductive property.
When studying electrical circuits, it is possible that the actual direction of current flow in a specific circuit element is not known at the start. Consequently, we arbitrarily assign each current variable a reference direction. After current values are solved for, some of them might display negative values. Hence, for the negative current variables, the actual current flows in the direction opposite to the reference direction which was originally selected.
The mobile charged particles within a conductor move constantly in random directions. In order for a net flow of charge to exist, the particles must also move together with an average drift rate. Electrons are the charge carriers in There was an error working with the wiki: Code[25]
are There was an error working with the wiki: Code[26]
, the waves of electromagnetic energy propagate rapidly through the space between the wires from a source to a distant There was an error working with the wiki: Code[27]
, even though the electrons in the wires only move back and forth over a tiny distance. Although the velocity of the flowing charges is quite low, the associated electromagnetic energy travels at the speed of light. The ratio of the There was an error working with the wiki: Code[79]
through a medium versus the speed of light in a vacuum is called the There was an error working with the wiki: Code[80]
.
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 There was an error working with the wiki: Code[28]
cell may be constructed with salt water (a solution of There was an error working with the wiki: Code[29]
are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, flowing There was an error working with the wiki: Code[81]
s 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 moving, but where it makes more sense to think of the current as the movement of positive "There was an error working with the wiki: Code[30]
" (the spots that should have an electron to make the conductor neutral). This is the case in a ptype There was an error working with the wiki: Code[82]
.
The magnitude of an electric current is defined as the There was an error working with the wiki: Code[83]
of Electric charge:
:I = {dQ \over dt}
Formally this is written as:
:i(t) = {dq(t) \over dt} or inversely as q(t_0) = \int_{\infty}^{t_0} i(t)\, dt
The amount of charge Q flowing per unit of time t is I, standing for the intensity of the current. The current I in Amperes "flowing" in a wire can be calculated with the following equation:
:I = {Q \over t}
where
:Q \!\ is the Electric charge in There was an error working with the wiki: Code[84]
s (ampere seconds)
:t \!\ is the There was an error working with the wiki: Code[85]
in There was an error working with the wiki: Code[86]
s
It follows that:
:Q=It \!\ and t = {Q \over I}
If we want to describe the distribution of the charge flow, we use the concept of the current density:
:\vec{J}=nq\vec{v_d}=\rho \vec{v_d} \!\
where
:\vec{J} \!\ is the current density vector (SI unit Amperes per There was an error working with the wiki: Code[87]
)
:n \!\ is the particle density in count per volume (SI unit m3)
:q \!\ is the individual particles' charge (SI unit There was an error working with the wiki: Code[88]
s)
:\rho = nq \!\ is the There was an error working with the wiki: Code[89]
(SI unit There was an error working with the wiki: Code[90]
s per There was an error working with the wiki: Code[91]
)
:\vec{v_d} \!\ is the particles' average There was an error working with the wiki: Code[31]
(SI unit Meters per There was an error working with the wiki: Code[92]
)
The current flowing through a surface S can be calculated by the following relation:
:I=\int_S{ \vec{J} \cdot d\vec{S}}
&ndash where the current is in fact the integral of the There was an error working with the wiki: Code[93]
of the current density vector and the differential surface element d \vec{S}, i.e. the net There was an error working with the wiki: Code[94]
of the current density There was an error working with the wiki: Code[95]
flowing through the surface S.
The speed at which electric charges drift can be calculated from the equation:
:I=nAvQ \!\
where
:I \!\ is the electric current
:n \!\ is number of charged particles per unit volume
:A \!\ is the crosssectional area of the conductor
:v \!\ is the drift velocity, and
:Q \!\ is the charge on each particle.
Electric currents in solid matter are typically very slow flows.
There was an error working with the wiki: Code[96]
predicts the current in an (ideal) There was an error working with the wiki: Code[97]
(or other There was an error working with the wiki: Code[98]
) to be applied Voltage divided by Electrical resistance:
:
I = \frac {V}{R}
where
:I is the current, measured in Amperes
:V is the There was an error working with the wiki: Code[99]
measured in Volts
:R is the Electrical resistance measured in There was an error working with the wiki: Code[32]
s
Natural examples include There was an error working with the wiki: Code[33]
There was an error working with the wiki: Code[34]
across long distances and the smaller wires within electrical and electronic equipment. In There was an error working with the wiki: Code[35]
, and the flow of There was an error working with the wiki: Code[36]
within a There was an error working with the wiki: Code[100]
.
The most obvious hazard is electric shock, where a current through part of the body can cause effects from a slight tingle to There was an error working with the wiki: Code[101]
or severe There was an error working with the wiki: Code[102]
. 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. The effect also varies considerably from individual to individual. (For approximate figures see Shock Effects under There was an error working with the wiki: Code[103]
.)
Due to this and the fact that passing current cannot be easily predicted in most practical circumstances, any supply of over 50 volts should be considered a possible source of dangerous electric shock. In particular, note that 110 volts (a minimum voltage at which AC There was an error working with the wiki: Code[37]
power is There was an error working with the wiki: Code[38]
) can certainly be lethal.
Electric arcs, which can occur with supplies of any voltage (for example, a typical There was an error working with the wiki: Code[104]
machine has a voltage between the There was an error working with the wiki: Code[105]
s of just a few tens of volts), are very hot and emit There was an error working with the wiki: Code[106]
(UV) and There was an error working with the wiki: Code[107]
(IR). Proximity to an electric arc can therefore cause severe thermal burns, and UV is damaging to unprotected eyes and skin.
Accidental electric heating can also be dangerous. An overloaded There was an error working with the wiki: Code[39]
, There was an error working with the wiki: Code[40]
, and There was an error working with the wiki: Code[41]
are particularly risky because they can deliver a very high current due to their low There was an error working with the wiki: Code[108]
.
Alternating current and polyphase currents
Direct current and continous currents
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There was an error working with the wiki: Code[110]
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General
Which direction does electricity really flow?
All about circuits  a useful site introducing electricity and electronics, as well as some mathematics involved with circuit calculations.
There was an error working with the wiki: Code[1]
, Wikipedia: The Free Encyclopedia. Wikimedia Foundation.
spencerr node46  A short explanation of the current density
The NIST Reference on Constants, Units, and Uncertainty
A short history of the SI units in electricity
@.ampere : Ampère and history of electricity Correspondence, Main books, Movies, 3D VRML  web site create by the CNRS, edited by the Centre de Recherche en Histoire des Sciences et des Techniques : Mme Christine Blondel  CNRS and M. Stéphane Pouyllau  CNRS
Books
Robert Mullineux Walmsley, The Electric Current, Cassell, 1894
Henry Hutchinson Norris, An Introduction to the Study of Electrical Engineering J. Wiley & Sons, 1909
Franklin Leonard Pope, Modern Practice of the Electric Telegraph. D. Van Nostrand, 1872.
Adolf Thomalen, George William Osborn Howe, A Textbook of Electrical Engineering. Arnold, 1907.
Newsgroups
news://sci.electronics.components Integrated circuits, resistors, capacitors. Activity: Medium. Public  Usenet
news://alt.engineering.electrical Discussing electrical engineering. Activity: High. Public  Usenet.
news://sci.physics.electromag Electromagnetic theory and applications. Activity: Medium. Public  Usenet.