A solid conductive metal contains mobile, or free, electrons. These electrons are bound to the metal lattice but not to any individual atom. Even 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. Given a plane through which the wire passes, the number of electrons moving from one side to the other in any period of time is on average equal to the number passing in the opposite direction. As George Gamow put in his science popularizing 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.'
The current is the rate of flow of charge. If you take a
cross-section of a wire and count how many Coulombs flow the cross-section in one second, the will be the
current, expressed in C/s, or A (Ampere).
In a wire with uniform cross-sectional area A, the current can be
shown to be equal to Anqv where n is the number of charge carriers (in conductors, these are electrons), q is the
charge carried by a charge carrier, and v is the drift velocity of the charge carriers.
Ohms law: V = I R. The potential difference across a wire is equal
to the resistance R of the wire, times the current in the wire.
For a wire of uniform cross-sectional area A and length L, the
resistance is given by R = (rho) L / A, where rho is the resistivity of the material from which the wire is
Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.