Current and Resistance
Some electrons are free to move about inside a metal. They undergo random collisions with atoms in the metal. If there is an electric field present -- as there would be with an electric potential difference -- then there is a force on these electrons and their "random walk" is no longer random but has a bias to it. They are moved along in the direction opposite to the electric field E. The average velocity of these electrons is the "drift velocity" vd that we encountered earlier.
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(c) Doug Davis, 2002; all rights reserved