Editing Electric current (section)

=== Metals ===

In a [[metal]], some of the outer electrons in each atom are not bound to the individual molecules as they are in [[molecular solid]]s, or in full bands as they are in insulating materials, but are free to move within the [[Metal#Definition|metal lattice]]. These [[conduction electron]]s serve as [[charge carrier]]s that can flow through the conductor as an electric current when an electric field is present. Metals are particularly conductive because there are many of these free electrons. With no external [[electric field]] applied, these electrons move about randomly due to [[thermal energy]] but, on average, there is zero net current within the metal. At room temperature, the average speed of these random motions is 10<sup>6</sup> metres per second.<ref>{{cite web|url=http://library.thinkquest.org/C0111709/English/DC-Circuts/mechanism.html|title=The Mechanism Of Conduction In Metals|archive-url=https://web.archive.org/web/20121025004809/http://library.thinkquest.org/C0111709/English/DC-Circuts/mechanism.html|archive-date=2012-10-25|website=Think Quest}}</ref> Given a surface through which a metal wire passes, electrons move in both directions across the surface at an equal rate. As [[George Gamow]] wrote in his [[popular science]] book, ''[[One, Two, Three...Infinity]]'' (1947), "The metallic substances differ from all other materials by the fact that the outer shells of their atoms are bound rather loosely, and often let one of their electrons go free. Thus the interior of a metal is filled up with a large number of unattached electrons that travel aimlessly around like a crowd of displaced persons. When a metal wire is subjected to electric force applied on its opposite ends, these free electrons rush in the direction of the force, thus forming what we call an electric current."

When a metal wire is connected across the two terminals of a [[Direct current|DC]] [[voltage source]] such as a [[battery (electricity)|battery]], the source places an electric field across the conductor. The moment contact is made, the free electrons of the conductor are forced to drift toward the [[Positive (electrical polarity)|positive]] terminal under the influence of this field. The free electrons are therefore the [[charge carrier]] in a typical solid conductor.

For a steady flow of charge through a surface, the current ''I'' (in amperes) can be calculated with the following equation:
<math display=block>I = {Q \over t} \, ,</math>
where ''Q'' is the electric charge transferred through the surface over a [[time]] ''t''.  If ''Q'' and ''t'' are measured in [[coulomb]]s and seconds respectively, ''I'' is in amperes.

More generally, electric current can be represented as the rate at which charge flows through a given surface as:
<math display=block>I = \frac{\mathrm{d}Q}{\mathrm{d}t} \, .</math>