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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 ρ ) Chapter 26 Current and Resistance J r 1 of a conductor • Ohmic and nonOhmic conductors Cover the following topics: • Ohm’s 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 (cont’d) 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 (cont’d) 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, Electric charge, current density

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