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Unformatted text preview: 27 CHAPTER OUTLINE 27.1 Electric Current 27.2 Resistance 27.3 A Model for Electrical Conduction 27.4 Resistance and Temperature 27.5 Superconductors 27.6 Electric Power Current and Resistance ANSWERS TO QUESTIONS Q27.1 Individual vehiclescars, trucks and motorcycleswould correspond to charge. The number of vehicles that pass a certain point in a given time would correspond to the current. Q27.2 Voltage is a measure of potential difference, not of current. Surge implies a flowand only charge, in coulombs, can flow through a system. It would also be correct to say that the victim carried a certain current, in amperes. Q27.3 Geometry and resistivity. In turn, the resistivity of the material depends on the temperature. Q27.4 Resistance is a physical property of the conductor based on the material of which it is made and its size and shape, including the locations where current is put in and taken out. Resistivity is a physical property only of the material of which the resistor is made. Q27.5 The radius of wire B is 3 times the radius of wire A, to make its crosssectional area 3 times larger. Q27.6 Not all conductors obey Ohms law at all times. For example, consider an experiment in which a variable potential difference is applied across an incandescent light bulb, and the current is measured. At very low voltages, the filament follows Ohms law nicely. But then long before the filament begins to glow, the plot of V I becomes non-linear, because the resistivity is temperature- dependent. Q27.7 A conductor is not in electrostatic equilibrium when it is carrying a current, duh! If charges are placed on an isolated conductor, the electric fields established in the conductor by the charges will cause the charges to move until they are in positions such that there is zero electric field throughout the conductor. A conductor carrying a steady current is not an isolated conductorits ends must be connected to a source of emf, such as a battery. The battery maintains a potential difference across the conductor and, therefore, an electric field in the conductor. The steady current is due to the response of the electrons in the conductor due to this constant electric field. 105 106 Current and Resistance Q27.8 The bottom of the rods on the Jacobs Ladder are close enough so that the supplied voltage is sufficient to produce dielectric breakdown of the air. The initial spark at the bottom includes a tube of ionized air molecules. Since this tube containing ions is warmer than the air around it, it is buoyed up by the surrounding air and begins to rise. The ions themselves significantly decrease the resistivity of the air. They significantly lower the dielectric strength of the air, marking longer sparks possible. Internal resistance in the power supply will typically make its terminal voltage drop, so that it cannot produce a spark across the bottom ends of the rods. A single continuous spark, therefore will rise up, becoming longer and longer, until the potential difference is not large enough...
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