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27.3 A Model for Electrical Conduction In this section we describe a classical model of electrical conduction in metals that was Consider a conductor as a regular array of atoms plus a collection of free electrons, which are sometimes called conduction electrons. The conduction electrons, although 6 m/s. The situation is similar to the motion of gas molecules confined in a vessel. In fact, some scientists refer to conduction electrons in a metal as an electron gas. This situation changes when an electric field is applied. Now, in addition to under- going the random motion just described, the free electrons drift slowly in a direction d that is much smaller (typically 10 $ 4 m/s) than their average speed between collisions (typically 10 6 m/s). Figure 27.9 provides a crude description of the motion of free electrons in a conductor. In the absence of an electric field, there is no net displacement after many E modifies the random motion and causes the electrons to drift in a direction opposite that of E (Fig. 27.9b). In our model, we assume that the motion of an electron after a collision is indepen- dent of its motion before the collision. We also assume that the excess energy acquired We are now in a position to derive an expression for the drift velocity. When a free electron of mass m e and charge q (" $ e) is subjected to an electric field E, it experiences a force F " qE. Because this force is related to the acceleration of the electron through Newton’s second law, F " m e a, we conclude that the acceleration of the electron is (27.12) a " q
E m e SECTION 27.3 • A Model for Electrical Conduction 841 At the Active Figures link at http://www.pse6.com, you can adjust the electric field to see the resulting effect on the motion of an electron. Active Figure 27.9 (a) A schematic diagram of the random motion of two charge carriers in a conductor in the absence of an electric field. The drift velocity is zero. (b) The motion of the charge carriers in a conductor in the presence of an electric field. Note that the random motion is modified by the field, and the charge carriers have a drift velocity. (a) – – – – – – – – E (b) |