Conceptual Questions

ssm Solution is in the Student Solutions Manual.

| 1 | Suppose the coil and the magnet in Figure 22-1a were each moving with the same velocity relative to the earth. Would there be an induced current in the coil? Explain.

| 2 | In the discussion concerning Figure 22-5, we saw that a force of 0.086 N from an external agent was required to keep the rod moving at a constant speed. Suppose the light bulb in the figure is unscrewed from its socket. How much force would now be needed to keep the rod moving at a constant speed? Justify your answer.

| 3 | Eddy currents are electric currents that can arise in a piece of metal when it moves through a region where the magnetic field is not the same everywhere. The picture shows, for example, a metal sheet moving to the right at a velocity and a magnetic field that is directed perpendicular to the sheet. At the instant represented, the magnetic field only extends over the left half of the sheet. An emf is induced that leads to the eddy current shown. Explain why this current causes the metal sheet to slow down. This action of eddy currents is used in various devices as a brake to damp out unwanted motion.

| 4 | ssm A magnetic field is necessary if there is to be a magnetic flux passing through a coil of wire. Yet, just because there is a magnetic field does not mean that a magnetic flux will pass through a coil. Account for this observation.

| 5 | Suppose the magnetic flux through a 1-m2 flat surface is known to be 2 Wb. From these data, it is possible to determine certain information about the average magnetic field at the surface, but not the magnitude and direction of the total field. What exactly can be as certained about the field? Explain.

| 6 | A conducting rod is free to slide along a pair of conducting rails, in a region where a uniform and constant (in time) magnetic field is directed into the plane of the paper, as the drawing illustrates. Initially the rod is at rest. Describe the rod’s motion after the switch is closed. Be sure to account for the effect of any motional emf that develops.

| 7 | ssm Explain how a bolt of lightning can produce a current in the circuit of an electrical appliance, even when the lightning does not directly strike the appliance.

| 8 | Review Conceptual Example 7 before answering this question. A solenoid is connected to an ac source. A copper ring is placed inside the solenoid, with the normal to the ring being parallel to the axis of the solenoid. The copper ring gets hot, yet nothing touches it. Why?

| 9 | A robot is designed to move parallel to a cable hidden under the floor. The cable carries a steady direct current I. A sensor mounted on the robot consists of a coil of wire. The coil is near the floor and parallel to it. As long as the robot moves parallel to the cable, with the coil directly over it, no emf is induced in the coil, since the magnetic flux through the coil does not change. But when the robot deviates from the parallel path, an induced emf appears in the coil. The emf is sent to electronic circuits that bring the robot back to the path. Explain why an emf would be induced in the sensor coil.

| 10 | In a car, the generator-like action of the alternator occurs while the engine is running and keeps the battery fully charged. The headlights would discharge an old and failing battery quickly if it were not for the alternator. Explain why the engine of a parked car runs more quietly with the headlights off than with them on when the battery is in bad shape.

| 11 | In Figure 22-3 a coil of wire is being stretched. (a) Using Lenz’s law, verify that the induced current in the coil has the direction shown in the drawing. (b) Deduce the direction of the induced current if the direction of the external magnetic field in the figure were reversed. Explain.

| 12 | (a) When the switch in the circuit in the drawing is closed, a current is established in the coil and the metal ring jumps upward. Explain this behavior. (b) Describe what would happen to the ring if the battery polarity were reversed.

| 13 | The string of an electric guitar vibrates in a standing wave pattern that consists of nodes and antinodes. (Section 17.5 discusses standing waves.) Where should an electromagnetic pickup be located in the standing wave pattern to produce a maximum emf, at a node or an antinode? Why?

| 14 | ssm An electric motor in a hair dryer is running at normal speed and, thus, is drawing a relatively small current, as in part (b) of Example 12. What happens to the current drawn by the motor if the shaft is prevented from turning, so the back emf is suddenly reduced to zero? Remembering that the wire in the coil of the motor has some resistance, what happens to the temperature of the coil? Justify your answers.

| 15 | One transformer is a step-up device, while another is step-down. These two units have the same voltage across and the same current in their primary coils. Does either one deliver more power to the circuit attached to the secondary coil? If so, which one? Ignore any heat loss within the transformers and account for your answer.

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