Conceptual Questions Chap. 20
1.
Wearing a metal bracelet in a region with a strong magnetic field can be
hazardous. Why?
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.
2.
How is electrical energy produced in dams (i.e. how is the energy of motion of
the water converted to AC electricity)?
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.
3.
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?
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.
4.
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.
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.