Download 1.All iron materials are not magnetized because the tiny magnetic

CHAPTER 11: 1, 2, 4, 8, 9, 10, 12, 20, 22, 23, 25, 37, 38, 43, 44, 46.
1.All iron materials are not magnetized because the tiny magnetic domains are most often
oriented in random directions and cancel one another’s effects.
2. Attraction will occur because the magnet induces opposite polarity in a nearby piece of iron.
North will induce south, and south will induce north. This is similar to charge induction,
where a balloon will stick to a wall whether the balloon is negative or positive.
4. An electric field surrounds a stationary electric charge. An electric field and a magnetic field
surround a moving electric charge. (And a gravitational field also surrounds both.)
8. The needle is not pulled toward the north side of the bowl because the south pole of the
magnet is equally attracted southward. The net force on the needle is zero. (The net torque,
on the other hand, will be zero only when the needle is aligned with the Earth’s magnetic
field.)
9. The net force on a compass needle is zero because its north and south poles are pulled in
opposite directions with equal forces in the Earth’s magnetic field. When the needle is not
aligned with the magnetic field of the Earth, then a pair of torques (relative to the center of
the compass) is produced (Figure 11.4). This pair of equal torques, called a “couple,” rotates
the needle into alignment with the Earth’s magnetic field.
10. Cans contain iron. Domains in the can tend to line up with the Earth’s magnetic field. When
the cans are left stationary for several days, the cans become magnetized by induction,
aligning with the Earth’s magnetic field.
12. The wire is aligned with the magnetic field. For a force to act on a current-carrying wire in a magnetic field,
the wire must be at a nonzero angle to the field. Maximum force occurs when the wire is at 90 degrees to the
field.
20. When we write work = force  distance, we really mean the component of force in the
direction of motion multiplied by the distance moved (Chapter 5). Since the magnetic force
that acts on a beam of electrons is always perpendicular to the beam, there is no component
of magnetic force along the instantaneous direction of motion. Therefore a magnetic field
can do no work on a charged particle. (Indirectly, however, a time-varying magnetic field
can induce an electric field that can do work on a charged particle.)
22. Charged particles moving through a magnetic field are deflected most when they move at
right angles to the field lines, and least when they move parallel to the field lines. If we
consider cosmic rays heading toward the Earth from all directions and from great distance,
those descending toward northern Canada will be moving nearly parallel to the magnetic
field lines of the Earth. They will not be deflected very much, and secondary particles they
create high in the atmosphere will also stream downward with little deflection. Over regions
closer to the equator like Mexico, the incoming cosmic rays move more nearly at right
angles to Earth’s magnetic field, and many of them are deflected back out into space before
they reach the atmosphere. The secondary particles they create are less intense at the Earth’s
surface. (This “latitude effect” provided the first evidence that cosmic rays from outer space
consist of charged particles—mostly protons, as we now know.)
23. Cosmic ray intensity at the Earth’s surface would be greater when the Earth’s magnetic field
passed through a zero phase. Fossil evidence suggests the periods of no protective magnetic
field may have been as effective in changing life forms as X-rays have been in the famous
heredity studies of fruit flies.
25. Magnetic levitation will reduce surface friction to near zero. Then only air friction will
remain. It can be made relatively small by aerodynamic design, but there is no way to
eliminate it (short of sending vehicles through evacuated tunnels). Air friction gets rapidly
larger as speed increases.
37. If the lightbulb is connected to a wire loop that intercepts changing magnetic field lines
from an electromagnet, voltage will be induced which can illuminate the bulb. Change is the
key, so the electromagnet should be powered with ac.
38. Induction occurs only for a change in the intercepted magnetic field. The galvanometer will display
a pulse when the switch in the first circuit is closed and current in the coil increases from zero. When
the current in the first coil is steady, no current is induced in the secondary and the galvanometer
reads zero. The galvanometer needle will swing in the opposite direction when the switch is opened
and current falls to zero.
43. The voltage impressed across the lamp is 120 V and the current through it is 0.1 A. We see that the first
transformer steps the voltage down to 12 V and the second one steps it back up to 120 V. The current in the
secondary of the second transformer, which is the same as the current in the bulb, is one-tenth of the current in
the primary, or 0.1 A.
44. Oops! This is a dc circuit. Unless there is a changing current in the primary, no induction takes place. No
voltage and no current are induced in the meter.
46. No, no, no, a thousand times no! No device can step up energy. This principle is at the heart of
physics. Energy cannot be created or destroyed.