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Unformatted text preview: Chapter 20 Induced Voltages and Inductance Quick Quizzes 1. b , c , a . At each instant, the magnitude of the induced emf is proportional to the rate of change of the magnetic field (hence, proportional to the slope of the curve shown on the graph). 2. (a). All charged particles within the metal bar move straight downward with the bar. According to right-hand rule #1, positive changes moving downward through a magnetic field that is directed northward will experience magnetic forces toward the east. This means that the free electrons (negative charges) within the metal will experience westward forces and will drift toward the west end of the bar, leaving the east end with a net positive charge. 3. (b). According to Equation 20.3, because B and v are constant, the emf depends only on the length of the wire moving in the magnetic field. Thus, you want the long dimension moving through the magnetic field lines so that it is perpendicular to the velocity vector. In this case, the short dimension is parallel to the velocity vector. From a more conceptual point of view, you want the rate of change of area in the magnetic field to be the largest, which you do by thrusting the long dimension into the field. 4. (c). In order to oppose the approach of the north pole, the magnetic field generated by the induced current must be directed upward. An induced current directed counterclockwise around the loop will produce a field with this orientation along the axis of the loop. 5. (b). When the iron rod is inserted into the solenoid, the inductance of the coil increases. As a result, more potential difference appears across the coil than before. Consequently, less potential difference appears across the bulb, and its brightness decreases. 175 176 CHAPTER 20 Answers to Conceptual Questions 2. Consider the copper tube to be a large set of rings stacked one on top of the other. As the magnet falls toward or falls away from each ring, a current is induced in the ring. Thus, there is a current in the copper tube around its circumference. 4. The flux is calculated as cos B BA B A = = . The flux is therefore maximum when the magnetic field vector is perpendicular to the plane of the loop. We may also deduce that the flux is zero when there is no component of the magnetic field that is perpendicular to the loop. 6. No. Once the bar is in motion and the charges are separated, no external force is necessary to maintain the motion. An applied force in the x direction will cause the bar to accelerate in that direction. 8. As water falls, it gains velocity and kinetic energy. It then pushes against the blades of a turbine transferring this energy to the rotor or coil of a large alternating current generator....
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