Phys2212_28.1+to+28.5

Phys2212_28.1+to+28.5 - Physics 2212 Electricity and...

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Physics 2212 Electricity and Magnetism Lecture 9 (Knight: 28.1 to 28.5) Current and Resistance
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10/06/09 Physics 2212 - Lecture 9 2 The Electron Current We start with some “thought experiments” on a simple system. We have a parallel plate capacitor that has been charged, e.g. with glass and plastic rods. Now we connect the plates with a wire. What happens? The plates quickly become neutral, and we say that the capacitor has been discharged. Further study shows that while the discharge is taking place, the wire gets warm, a light bulb can be made to glow, and a compass needle can be deflected. These are indicators of current flow in the wire.
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10/06/09 Physics 2212 - Lecture 9 3 Charge Carriers and Inertia We have asserted that the current in metals is caused by the flow of negative electrons . The first direct evidence that this was the case was provided in 1916 by the Tolman-Stewart experiment , which showed that negative charges “go to the bottom” of an accelerated conductor. We model a metallic conductor as a rigid lattice of positive charges pervaded by a “sea” of conduction electrons, 1 per atom, that move freely in the material. Does the discharge occur because positive charges are moving to the negative plate , or because negative charges are moving to the positive plate ? + + + + + + + + + + + + - - - - - - - - - - - -
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Physics 2212 - Lecture 9 4 The Electron Current In a metallic conductor in electrostatic equilibrium, the conduction electrons move around quite rapidly, but there is no net movement of charge. This can be changed by “pushing” on the sea of electrons with an electric field, thereby causing the entire sea of electrons to move in a particular direction, like a gas or liquid flowing through a pipe. The net motion, the “drift speed” v d , is superposed on the random thermal motions of the individual electrons, and it is very slow , typically around 10 -4 m/s. We define the electron current i as the number of electrons N e that pass through a cross section of wire or other conductor in a time interval t . In other words:
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Phys2212_28.1+to+28.5 - Physics 2212 Electricity and...

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