nelectric currentis a flow ofelectric charge. Electric charge flows 3 33 when there isvoltagepresent across a conductor.In electric circuits this charge is often carried by movingelectronsinawire. It can also be carried byionsin anelectrolyte, or by both ions and electrons such as in aplasma.[1]TheSIunit for measuring an electriccurrent is theampere, which is the flow of electric charges through a surface at the rate of onecoulombper second. Electric current can be measured using anammeter.[2]Electric currents cause many effects, notably heating, but also induce magnetic fields, which are widely used for motors, inductors and generators.SymbolThe conventional symbol for current is, which originates from the French phraseintensité de courant, or in Englishcurrent intensity.[3][4]This phrase is frequently used when discussing the value of an electric current, but modern practice often shortens thisto simplycurrent. Thesymbol wasused byAndré-Marie Ampère, afterwhom the unit of electric current is named, in formulating the eponymousAmpères force lawwhich he discovered in 1820.[5]The notation travelled from France to Britain, where it became standard, although at least one journal did not change from usingtountil 1896.[6]ConventionsTheelectrons, thecharge carriersin an electrical circuit, flow in the opposite direction of theconventionalelectric current.Thesymbolfor a battery in acircuitdiagram.A flow of positive charges gives the sameelectriccurrent, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of either positive or negative charges, or both, a convention for the direction of current which is independent of thetype ofcharge carriersis needed. The direction ofconventionalh currentis arbitrarily defined to be the same as the direction of the flow of positive charges.In metals, which make up the wires and other conductors in mostelectrical circuits, the positive charges are immobile, and the charge carriers areelectrons. Because the electrons carry negative charge, their motion in a metal conductor is in the direction opposite to that of conventional (orelectric) current.Reference directionWhen analyzing electrical circuits, the actual direction of current through a specific circuit element isusually unknown. Consequently, each circuit element is assigned a current variable with an arbitrarily chosenreference direction. This is usually indicated on the circuit diagram with an arrow next to the current variable. When the circuit issolved, 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. In electronic circuits, the reference current directions are often chosen so that all currents are toward ground. Thisoften matches conventional currentdirection, because in many circuits thepower supplyvoltage is positive with respect to ground.Ohms lawMain article:Ohms lawOhms law states that the current through a conductor between two points is directlyproportionalto thepotential differenceacross the two points. Introducing the constant of proportionality, theresistance,[7]one arrives at the usual mathematical equation that describes this relationship:[8]whereIis the current through the conductor in units ofamperes,Vi
Posted on: Tue, 05 Nov 2013 11:03:38 +0000
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