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Unformatted text preview: Physics 4B Lecture Notes 27-1 Chapter 27 - Current and Resistance Problem Set #6 - due: Ch 27 - 1, 8, 13, 19, 20, 35, 47, 54 When an potential difference is applied to a conductor an electric field is created inside. Immediately the free charges begin to flow to cancel the field. It is this flow of charge that we will study. Lecture Outline 1. The Definitions of Current and Current Density 2. Resistivity and Ohm's Rule 3. Energy Transfer in Circuits 1. The Definitions of Current and Current Density Current: The rate at which charge flows. I dQ dt The Definition of Current The units of current are: I [ ] = Q [ ] t [ ] = 1 Coulomb 1 second 1 Ampere =1 Amp =1A Example 1: Find the number of electrons that pass a point in a wire carrying 1.00A during 1.00s. Using the definition of current, I dQ dt dQ = I dt t Ne Ne = It N = (1.00A) (1.00s) 1.60x10- 19 C = 6.25x10 18 electrons Now let's ask the seemingly harmless question, how fast are the electrons going? The speed of the electrons can be written as, = l t . The time can be found using the definition of current as in example 1, I dQ dt dQ = I dt t Ne Ne = It t = Ne I . Now the speed becomes, = l I Ne = l A N I A 1 e Define the free electron density and the current density, n N vol The Definition of Free Electron Density j I A The Definition of Current Density In terms of these quantities, = j ne j = ne . This speed is called "the drift velocity"....
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