Conceptual Questions Chap. 20

1. Conceptual Questions Chap. 20 If the magnetic field should change rapidly, a large emf could be induced in the bracelet. This could cause either a high-voltage arc in a gap in the bracelet, or a large current that leads to resistance heating in the case of a continuous band. • Wearing a metal bracelet in a region with a strong magnetic field can be hazardous. Why? • How is electrical energy produced in dams (i.e. how is the energy of motion of the water converted to AC electricity)? • A bar magnet is dropped toward a conducting ring at rest on the floor. As the magnet falls toward the ring, does it move as an object falling freely? • A magneto is used to cause the spark in a spark plug in many lawn mowers today. A magneto consists of a permanent magnet mounted on the flywheel so that it spins past a fixed coil. Explain how this arrangement generates a large enough potential difference to cause the spark. Falling water pushes against the blades of a turbine, causing the rotor of an AC generator to spin. The spinning rotor causes a coil to turn in a magnetic field, which creates an induced emf in the coil. This induced emf is the voltage source for the current in our electric power lines. As the bar magnet falls toward the ring, a magnetic field is produced by the ring that opposes the change in flux through it. The net result is an upward force that prevents the bar magnet from experiencing free fall. As the magnet moves at high speed past the fixed coil, the magnetic flux through the coil changes very rapidly, increasing as the magnet approaches the coil and decreasing as the magnet moves away. The rapid change in flux through the coil induces a large emf, large enough to cause a spark across the gap in the spark plug.