Chapter 27 - 27 Current and Resistance CHAPTER OUTLINE 27.1 27.2 27.3 27.4 27.5 27.6 Electric Current Resistance A Model for Electrical Conduction

Chapter 27 - 27 Current and Resistance CHAPTER OUTLINE 27.1...

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27CHAPTER OUTLINE27.1 Electric Current27.2Resistance27.3A Model for ElectricalConduction27.4Resistance and Temperature27.5Superconductors27.6Electric PowerCurrent and ResistanceANSWERS TO QUESTIONSQ27.1Individual vehicles—cars, trucks and motorcycles—wouldcorrespond to charge. The number of vehicles that pass acertain point in a given time would correspond to the current.Q27.2Voltage is a measure of potential difference, not of current.“Surge” implies a flow—and only charge, in coulombs, can flowthrough a system. It would also be correct to say that the victimcarried a certain current, in amperes.Q27.3Geometry and resistivity. In turn, the resistivity of the materialdepends on the temperature.Q27.4Resistance is a physical property of the conductor based on thematerial of which it is made and its size and shape, includingthe locations where current is put in and taken out. Resistivityis a physical property only of the material of which the resistoris made.Q27.5The radius of wire B is 3 times the radius of wire A, to make its cross–sectional area 3 times larger.Q27.6Not all conductors obey Ohm’s law at all times. For example, consider an experiment in which avariable potential difference is applied across an incandescent light bulb, and the current ismeasured. At very low voltages, the filament follows Ohm’s law nicely. But then long before thefilament begins to glow, the plot of VIbecomes non-linear, because the resistivity is temperature-dependent.Q27.7A conductor is not in electrostatic equilibrium when it is carrying a current, duh! If charges areplaced on an isolated conductor, the electric fields established in the conductor by the charges willcause the charges to move until they are in positions such that there is zero electric field throughoutthe conductor. A conductor carrying a steady current is not an isolated conductor—its ends must beconnected to a source of emf, such as a battery. The battery maintains a potential difference acrossthe conductor and, therefore, an electric field in the conductor. The steady current is due to theresponse of the electrons in the conductor due to this constant electric field.105
106 Current and ResistanceQ27.8The bottom of the rods on the Jacob’s Ladderare close enough so that the supplied voltage issufficient to produce dielectric breakdown of the air. The initial spark at the bottom includes a tubeof ionized air molecules. Since this tube containing ions is warmer than the air around it, it is buoyedup by the surrounding air and begins to rise. The ions themselves significantly decrease theresistivity of the air. They significantly lower the dielectric strength of the air, marking longer sparkspossible. Internal resistance in the power supply will typically make its terminal voltage drop, sothat 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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