Page 180 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch06_139-176.indd Page 157 9/1/10 9:40 PM user-f465
tiL12214_ch06_139-176.indd Page 157 9/1/10 9:40 PM user-f465 /Users/user-f465/Desktop

domain becomes essentially a tiny magnet with a north pole and
a south pole. In an unmagnetized piece of iron, the domains

Battery N are oriented in all possible directions and effectively cancel any

E overall magnetic eff ect. The net magnetism is therefore zero or
near zero.
When an unmagnetized piece of iron is placed in a mag-
netic field, the orientation of the domain changes to align

with the magnetic field, and the size of aligned domains may

FIGURE 6.23 With the wire oriented along a north-south line, grow at the expense of unaligned domains. This explains why

the compass needle deflects away from this line when there is a a “string” of iron paper clips is picked up by a magnet. Each
current in the wire. paper clip has domains that become temporarily and slightly
aligned by the magnetic field, and each paper clip thus acts

as a temporary magnet while in the field of the magnet. In a

wire to move. When the wire was not connected to a battery, the strong magnetic field, the size of the aligned domains grows
needle of the compass was lined up with the wire and pointed to such an extent that the paper clip becomes a “permanent

north as usual. But when the wire was connected to a battery, magnet.” The same result can be achieved by repeatedly strok-
the compass needle moved perpendicular to the wire ( Figure 6.23). ing a paper clip with the pole of a magnet. Th e magnetic

Oersted had discovered that an electric current produces a mag- effect of a “permanent magnet” can be reduced or destroyed
netic field. An electric current is understood to be the move- by striking, dropping, or heating the magnet to a suffi ciently

ment of electric charges, so Oersted’s discovery suggested that high temperature (770˚C for iron). These actions randomize
magnetism is a property of charges in motion. the direction of the magnetic domains, and the overall mag-
netic fi eld disappears.
Permanent Magnets

The magnetic fields of bar magnets, horseshoe magnets, and Earth’s Magnetic Field
other so-called permanent magnets are explained by the rela-
Earth’s magnetic field is believed to originate deep within the

tionship between magnetism and moving charges. Electrons

earth. Like all other magnetic fields, Earth’s magnetic fi eld is
in atoms are moving around the nucleus, so they produce a
believed to originate with moving charges. Earthquake waves
magnetic field. Electrons also have a magnetic fi eld associated

and other evidence suggest that Earth has a solid inner core
with their spin. In most materials, these magnetic fi elds cancel
with a radius of about 1,200 km (about 750 mi), surrounded by

one another and neutralize the overall magnetic effect. In other
a fluid outer core some 2,200 km (about 1,400 mi) thick. Th is

materials, such as iron, cobalt, and nickel, the electrons are
core is probably composed of iron and nickel, which fl ows as
arranged and oriented in a complicated way that imparts a mag-
Earth rotates, creating electric currents that result in Earth’s

netic property to the atomic structure. These atoms are grouped

magnetic field. How the electric currents are generated is not
in a tiny region called a magnetic domain. A magnetic domain
yet understood.
is roughly 0.01 to 1 mm in length or width and does not have a
Other planets have magnetic fields, and there seems to be

fixed size (Figure 6.24). The atoms in each domain are magneti-

a relationship between the rate of rotation and the strength of
cally aligned, contributing to the polarity of the domain. Each
the planet’s magnetic field. Jupiter and Saturn rotate faster than

Earth and have stronger magnetic fields than Earth. Venus and

Mercury rotate more slowly than Earth and have weaker mag-
netic fi elds. This is indirect evidence that the rotation of a planet

is associated with internal fluid movements, which somehow

A generate electric currents and produce a magnetic fi eld.
In addition to questions about how the electric current
is generated, there are puzzling questions from geologic evi-
dence. Lava contains magnetic minerals that act as tiny com-

passes that are oriented to Earth’s magnetic field when the
North lava is fluid but become frozen in place as the lava cools. Stud-

ies of these rocks by geologic dating and studies of the frozen
magnetic mineral orientation show that Earth’s magnetic fi eld
B
has undergone sudden reversals in polarity: the north mag-
netic pole becomes the south magnetic pole and vice versa.
FIGURE 6.24 (A) In an unmagnetized piece of iron, the mag- This has happened many times over the distant geologic past,

netic domains have a random arrangement that cancels any overall

magnetic effect. (B) When an external magnetic field is applied to and the most recent shift occurred about 780,000 years ago.

the iron, the magnetic domains are realigned, and those parallel to The cause of such magnetic field reversals is unknown, but it

the field grow in size at the expense of the other domains, and the must be related to changes in the flow patterns of Earth’s fl uid
iron is magnetized. outer core of iron and nickel.
6-19 CHAPTER 6 Electricity 157