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'''Elektriese stroom''' is die vloei (beweging) van [[elektriese lading]]s. Die [[SI|SI-eenheid]] van elektriese stroom is die [[Ampère|ampère]] (A), wat gelykstaande aan 'n vloei van een [[Charles Coulomb|coulomb]] se lading per sekonde.
== Definisie ==
Die hoeveelheid elektriese stroom (gemeet in ampere) deur 'n sekere oppervlak, bv. die deursnee van 'n kopergeleier, word gedefinieer as die hoeveelheid elektriese lading (gemeet in coulomb) wat deur sodanige oppervlak per eenheidstyd vloei. As Q die hoeveelheid lading is wat deur die oppervlak vloei in 'n tyd T, dan is die gemiddelde stroom I gelyk aan:
:<math>i(t) = \frac{dQ}{dt}.</math>
Die [[Ampère|ampère]], die mate van elektriese stroom, is 'n [[SI|SI-basiseenheid]] wat beteken dat die mate van elektriese lading afgelei word vanaf die definisie van die ampère.
== Stroom in 'n metaaldraad ==
In 'n soliede [[elektriese geleier|geleidende]] metaal is daar 'n groot aantal mobiele [[vrye elektron|vrye elektrone]]e. Hierdie elektrone is aan die metaalraamwerk verbind maar nie tot enige indiwiduele atoom nie. Selfs sonder die toepassing van 'n eksterne elektriese veld, beweeg hierdie elektrone willekeurig rond as gevolg van [[termiese energie]]. Daar is egter geen netto stroom in die metaal nie, omdat die aantal elektrone wat vloei van een kant van 'n denkbeeldige vlak waardeur die draad loop na 'n ander kant daarvan in enige tydsinterval gelyk is aan die aantal elektrone wat in die teenoorgestelde rigting beweeg.
[[Beeld:Stranded lamp wire.jpg|thumbduimnael|'n Tipiese metaaldraad wat gebruik word vir elektriese geleiding is die gevlegdegevlegte [[koper|koperdraad]]draad.]]
Wanneer 'n metaaldraad verbind word aan die twee terminale van 'n [[gelykstroom]] [[spanningsbron]] soos 'n [[battery]], veroorsaak die bron 'n elektriese veld oor die geleier. Die oomblik wanneer kontak gemaak word, word die [[vrye elektron]]e gedwing om na die positiewe terminaal te beweeg onder die invloed van hierdie veld. Die vrye elektrone kan daarom beskou word as die [[stroomdraer]] in 'n tipiese vastestofgeleier. Vir 'n elektriese stroom van 1 [[Ampère|ampère]], beweeg 1 [[Charles Coulomb|coulomb]] se [[elektriese lading]] elke [[sekonde]] deur die denkbeeldige vlak waardeur die geleier loop.
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The current ''I'' in [[ampere]]s can be calculated with the following equation:
:<math>I = {Q \over t}</math>
where
:<math>Q \!\ </math> is the [[electric charge]] in [[coulomb]]s (ampere seconds)
:<math>t \!\ </math> is the [[time]] in [[second]]s
It follows that:
:<math>Q=It \!\ </math> and <math>t = {Q \over I}</math>
== Current density ==
{{main|Current density}}
'''Current density''' is a measure of the [[density]] of electrical current. It is defined as a [[vector (spatial)|vector]] whose magnitude is the electric current per cross-sectional area. In [[SI|SI units]], the current density is measured in [[ampere]]s per [[square meter]].
== The drift speed of electric charges ==
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 [[metal]]s and they follow an erratic path, bouncing from atom to atom, but generally drifting in the direction of the [[electric field]]. The speed at which they drift can be calculated from the equation:
:<math>I=nAvQ \!\ </math>
where
:<math>I \!\ </math> is the electric current
:<math>n \!\ </math> is number of charged particles '''per unit volume'''
:<math>A \!\ </math> is the cross-sectional area of the conductor
:<math>v \!\ </math> is the drift velocity, and
:<math>Q \!\ </math> is the charge on each particle.
Electric currents in solid matter are typically very slow flows. For example, in a [[copper]] [[wire]] of cross-section 0.5 mm², carrying a current of 5 A, the ''[[drift velocity]]'' of the electrons is of the order of a millimetre per second. To take a different example, in the near-vacuum inside a [[cathode ray tube]], the electrons travel in near-straight lines ("ballistically") at about a tenth of the [[speed of light]].
However, we know that electrical [[Signal (information theory)|signals]] are [[electromagnetic]] waves which propagate at very high speed outside the surface of the conductor (moving at the speed of light, as can be deduced from [[Maxwell's Equations]]). For example, in [[electric power transmission|AC power lines]], the waves of electromagnetic energy propagate rapidly through the space between the wires, moving from a source to a distant [[external electric load|load]], 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 nature of these three velocities can be clarified by analogy with the three similar velocities associated with gases. The low drift velocity of charge carriers is analogous to air motions; to wind. The large signal velocity is roughly analogous to the rapid propagation of sound waves, while the large random motion of charges is analogous to heat; to the high thermal velocity of randomly vibrating gas particles.
== Ohm's law ==
[[Ohm's law]] predicts the current in an (ideal) [[resistor]] (or other [[ohmic device]]) to be applied [[voltage]] divided by [[electrical resistance|resistance]]:
:<math>
I = \frac {V}{R}
</math>
where
:''I'' is the current, measured in [[ampere]]s
:''V'' is the [[potential difference]] measured in [[volt]]s
:''R'' is the [[electrical resistance|resistance]] measured in [[Ohm (unit)|ohm]]s
== Conventional current ==
[[Image:Galvanic cell.png|350px|thumb|Scheme of a discharging [[galvanic cell]]: The electric current is carried by electrons outside the cell (electric current going the opposite way of the electrons), and is carried by positively charged [[cation]]s inside the cell (electric current going in the same way as the [[anion]]s)]]
Conventional current was defined early in the history of electrical science as '''a flow of positive charge.''' In solid metals, like wires, the positive charge carriers are immobile, and only the negatively charged [[electron]]s flow. Because the electron carries negative charge, the ''electron'' current is in the direction opposite that of the conventional (or ''electric'') current.
[[Image:Conven current.PNG|left|230px|thumb|Diagram showing conventional current notation. Electric charge moves from the positive side of the power source to the negative.]]
In other conductive materials, the ''electric'' current is due to the flow of charged particles in both directions at the same time. Electric currents in [[electrolytes]] are flows of electrically charged atoms ([[ion]]s), which exist in both positive and negative varieties. For example, an [[electrochemistry|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 chloride ions, so a net current results. Electric currents in [[Plasma physics|plasma]] are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, flowing [[proton]]s constitute the electric current. To simplify this situation, the original definition of conventional current still stands.
There are also materials where the electric current is due to the flow of electrons and yet it is conceptually easier to think of the current as due to the flow of positive "[[electron hole|holes]]" (the spots that should have an electron to make the conductor neutral). This is the case in a p-type [[semiconductor]].
An electric current is a flow of microscopic particles called ELECTRONS flowing through wires
and electronic components.
It can be likened to the flow of water through pipes and radiators, etc.
As water is pushed through pipes by a pump, electric current is pushed through wires by a battery.Hot water does work by heating radiators.
Electric current does work by heating fires, lighting lamps, ringing bells, electroplating, etc. A basic law of the universe is that like charges repel and unlike attract. Two negatives will repel each other. A negative and a positive will attract each other.An electron has a negative charge. The negative (-ve) terminal of a battery will push negative electrons along a wire. The positive (+ve) terminal of a battery will attract negative electrons along a wire.Electric current will therefore flow from the -ve terminal of a battery, through the lamp, to the positive terminal.'''
== Examples ==
Natural examples include [[lightning]] and the [[solar wind]], the source of the [[polar aurora]]s (the [[aurora borealis]] and [[aurora australis]]). The most familiar artificial form of electric current is the flow of [[electrical conduction|conduction]] [[electron]]s in metal [[wire]]s, such as the overhead power lines that deliver [[electric power transmission|electrical energy]] across long distances and the smaller wires within electrical and electronic equipment. In [[electronics]], other forms of electric current include the flow of electrons through [[resistor]]s or through the vacuum in a [[vacuum tube]], the flow of [[ion]]s inside a [[Battery (electricity)|battery]], and the flow of [[Electron hole|holes]] within a [[semiconductor]].
[[Image:Electromagnetism.svg|175px|thumb|According to [[Ampère's law]], an electric current produces a [[magnetic field]].]]
== Electromagnetism ==
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]] associated with the current. Devices used for this include [[Hall effect]] [[sensor]]s, [[current clamp]]s, [[current transformer]]s, and [[Rogowski coil]]s.
== Reference direction ==
When solving electrical circuits, the actual direction of current through a specific circuit element is usually unknown. Consequently, each circuit element is assigned a current variable with an arbitrarily chosen ''reference direction''. When the circuit is solved, the circuit element currents may have positive or negative values. A negative value means that the actual direction of current through that circuit element is opposite that of the chosen reference direction.
== Electrical safety ==
The most obvious hazard is electrical shock, where a current passing through part of the body can cause a slight tingle, to [[cardiac arrest]], or severe [[Burn (injury)|burns]]. It is the amount of current passing through the body 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 [[electric shock]].)
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 [[mains electricity|mains]] power is [[List of countries with mains power plugs, voltages and frequencies|distributed in much of the Americas, and 4 other countries, mostly in Asia]]) can certainly be lethal.
[[Electric arc|Electric arcs]], which can occur with supplies of any voltage (for example, a typical [[arc welding]] machine has a voltage between the [[electrode]]s of just a few tens of volts), are very hot and emit [[ultra-violet]] (UV) and [[infra-red radiation]] (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 [[power cable]] is a frequent cause of fire. A battery as small as an [[AA cell]] placed in a pocket with metal coins can lead to a short circuit heating the battery and the coins which may inflict burns. [[Nickel-cadmium battery|NiCad]], [[Nickel metal hydride battery|NiMh cells]], and [[Lithium battery|Lithium batteries]] are particularly risky because they can deliver a very high current due to their low [[internal resistance]].
== See also ==
{{portalpar|Electronics|Nuvola_apps_ksim.png}}
*[[Alternating current]]
*[[Current density]]
*[[Direct current]]
*[[Electrical conduction]] for more information on the physical mechanism of current flow in materials
*[[Four-current]]
*[[Hydraulic analogy]]
*[[SI electromagnetism units]]
== External links ==
*[http://amasci.com/amateur/elecdir.html Which direction does electricity ''really'' flow?]
*[http://www.allaboutcircuits.com All about circuits - a useful site introducing electricity and electronics]
*[http://www.sengpielaudio.com/calculator-ohmslaw.htm Electric current and Ohm's law]
*[http://www.sengpielaudio.com/calculator-ohm.htm Electric current and power]
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[[Kategorie:Elektrisiteit]]
[[Kategorie:Elektronika]]
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