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Unformatted text preview: Introduce the following new concepts: Electric current ( symbol i ) Electric current density vector ( symbol ) Drift speed ( symbol v d ) Resistance ( symbol R ) and resistivity ( symbol ) Chapter 26 Current and Resistance J r 1 of a conductor Ohmic and nonOhmic conductors Cover the following topics: Ohms law Power in electric circuits 1 Electric Current We have discussed so far electrostatics the physics of stationary charges. Lets now study the physics of electric currents , i.e. charges in motion Electric current is a stream of moving charges . To be precise, if there is to be an electric current through a given surface, there must be a net flow of charge through that surface wo examples: 2 Two examples: The free conduction electrons in an isolated copper wire are in random motion at speeds of 10 6 m/s . These electrons pass through a (cross sectional) plane in the wire in both directions. Thus no net transport of charge, and thus no current through the wire . Lets connect ends of the wire to a battery, i.e. create an electric field inside. Conduction electrons will now move in the field constituting an electric current through the wire The flow of water through a hose represents the flow of positive charge ( protons of water molecules ). However, there is no net transport of charge because there is a parallel flow of negative charge ( electrons of water molecules ) of the same amount moving in exactly the same direction Consider an isolated conducting loop as shown in Fig. a. The conductor has the same potential at each point no electric field exists within the conductor or along its surface no net electric force acts on conduction electrons thus there is no current Electric Current (contd) 3 A B Insert a battery in the loop ( Fig. b ), i.e. the points A and B are at different potentials there is an electric field inside the loop electric force acts on conduction electrons causing them to move and thus establish a current . After a very short time, the electron flow reaches a constant value and the current is in its steady state The figure shows a section of conductor in which current has been established. If charge dq passes through a hypothetical plane ( aa or bb or cc ) in time dt , then current i through that plane is defined as q Electric Current (contd) 4 dt dq i = The SI unit for current is ampere: 1 A = 1 C/s Ampere is a base unit in SI system. We will learn formal definition of ampere in Chapter 29 Under a steadystate conditions, the current is the same for all planes that pass through the conductor (e.g. planes aa, bb and cc )....
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 Fall '08
 IASHVILI
 Current, Resistance

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