Conceptual Questions Chap. 20

1
Conceptual Questions Chap. 20

• 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.

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.
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.