CurrentElectrons in motion make up an electric current. Thiselectric current is usually referred to as “current” or“current flow,” no matter how many electrons aremoving. Current is a measurement of a rate at which acharge flows through some region of space or a conductor.The moving charges are the free electrons found inconductors, such as copper, silver, aluminum, and gold.The term “free electron” describes a condition in someatoms where the outer electrons are loosely bound totheir parent atom. These loosely bound electrons canbe easily motivated to move in a given direction whenan external source, such as a battery, is applied to thecircuit. These electrons are attracted to the positive terminalof the battery, while the negative terminal is thesource of the electrons. The greater amount of chargemoving through the conductor in a given amount oftime translates into a current.

The System International unit for current is the Ampere(A), where

That is, 1 ampere (A) of current is equivalent to 1coulomb (C) of charge passing through a conductor in1 second(s). One coulomb of charge equals 6.28 billionbillion electrons. The symbol used to indicate currentin formulas or on schematics is the capital letter “I.”When current flow is one direction, it is called directcurrent (DC). Later in the text, we will discuss the formof current that periodically oscillates back and forth within the circuit. The present discussion will only beconcerned with the use of direct current.

The velocity of the charge is actually an average velocityand is called drift velocity. To understand the ideaof drift velocity, think of a conductor in which thecharge carriers are free electrons. These electrons arealways in a state of random motion similar to that of

gas molecules. When a voltage is applied across theconductor, an electromotive force creates an electricfield within the conductor and a current is established.The electrons do not move in a straight direction butundergo repeated collisions with other nearby atoms.These collisions usually knock other free electronsfrom their atoms, and these electrons move on towardthe positive end of the conductor with an average velocitycalled the drift velocity, which is relatively a slowspeed. To understand the nearly instantaneous speed ofthe effect of the current, it is helpful to visualize a longtube filled with steel balls.

It can be seen that a ball introduced in one end of thetube, which represents the conductor, will immediatelycause a ball to be emitted at the opposite end of thetube. Thus, electric current can be viewed as instantaneous,even though it is the result of a relatively slowdrift of electrons.