Download Magnets and Magnetic Fields

Magnets and Magnetic Fields
SECTION OBJECTIVES
•
For given situations, predict
whether magnets will repel
or attract each other.
•
Describe the magnetic field
around a permanent magnet.
•
Describe the orientation of
Earth's magnetic field.
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4
MAGNETS
Most people have had experience with different kinds of magnets, such as
those shown in Figure 1. You have probably seen a variety of magnet shapes,
such as horseshoe magnets, bar magnets, and the flat magnets frequently used
to attach items to a refrigerator. All types of magnets attract iron-containing
objects such as paper clips and nails. In the following discussion, we will
assume that the magnet has the shape of a bar. Iron objects are most strongly
attracted to the ends of such a magnet. These ends are called poles; one is
called the north pole, and the other is called the south pole. The names derive
from the behavior of a magnet on Earth. If a bar magnet is suspended from its
midpoint so that it can swing freely in a horizontal plane, it will rotate until its
north pole points north and its south pole points south. In fact, a compass is
just a magnetic needle that swings freely on a pivot.
The list of important technological applications of magnetism is very
long. For instance, large electromagnets are used to pick up heavy loads.
Magnets are also used in meters, motors, and loudspeakers. Magnetic tapes
are routinely used in sound- and video-recording equipment, and magnetic
recording material is used on computer disks. Superconducting magnets are
currently being used to contain extremely high-temperature
plasmas that are used in controlled nuclear fusion research.
Superconducting magnets are also used to levitate modern
trains. These maglev trains are faster and provide a smoother
ride than the ordinary track system because of the absence of
friction between the train and the track.
Like poles repel each other, and unlike poles attract
each other
Figure 1
Magnets come in a variety of shapes and sizes, but like
poles of two magnets always repel one another.
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Chapter 19
The magnetic force between two magnets can be likened to
the electric force between charged objects in that unlike poles
of two magnets attract one another and like poles repel one
another. Thus, the north pole of a magnet is attracted to the
south pole of another magnet, and two north poles (or two
south poles) brought close together repel each other. Electric
charges differ from magnetic poles in that they can be isolated,
whereas magnetic poles cannot. In fact, no matter how many
times a permanent magnet is cut, each piece always has a
north pole and a south pole. Thus, magnetic poles always
occur in pairs.
MAGNETIC DOMAINS
f he magnetic properties of many materials are explained in terms of a model
in which an electron is said to spin on its axis much like a top does. (This classical description should not be taken literally. The property of electron spin
can be understood only with the methods of quantum mechanics.) The spinning electron represents a charge that is in motion. As you will learn in the
next section of this chapter, moving charges create magnetic fields.
In atoms containing many electrons, the electrons usually pair up with
their spins opposite each other causing their fields to cancel each other. For
this reason, most substances, such as wood and plastic, are not magnetic.
However, in materials such as iron, cobalt, and nickel, the magnetic fields produced by the electron spins do not cancel completely. Such materials are said
lobe ferromagnetic.
In ferromagnetic materials, strong coupling occurs between neighboring
atoms to form large groups of atoms whose net spins are aligned; these groups
are called magnetic domains. Domains typically range in size from about
10"4 cm to 10""1 cm. In an unmagnetized substance, the domains are randomly oriented, as shown in Figure 2. When an external magnetic field is applied,
the orientation of the magnetic fields of each domain may change slightly to
more closely align with the external magnetic field, or the domains that are
already aligned with the external field may grow at the expense of the other
domains. This alignment enhances the applied magnetic field.
Figure 2
When a substance is unmagnetized,
its domains are randomly oriented.
magnetic domain
a region composed of a group of
atoms whose magnetic fields are
aligned in the same direction
Some materials can be made into permanent magnets
Just as two materials, such as rubber and wool, can become charged after they
Integrating Chemistry
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are rubbed together, an unmagnetized piece of iron can become a permanent
"Molecular Magnetism."
magnet by being stroked with a permanent magnet. Magnetism can be
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induced by other means as well. For example, if a piece of unmagnetized iron
is placed near a strong permanent magnet, the piece of iron will eventually
become magnetized. The process can be reversed either by heating and cooling the iron or by hammering the iron, because these actions cause the magnetic domains to jiggle and lose their alignment.
A magnetic piece of material is classified as magnetically hard or soft,
5 depending on the extent to which it retains its magnetism. Soft magnetic
I materials, such as iron, are easily magnetized but also tend to lose their magj netism easily. In hard magnetic materials, domain alignment persists after the
:; external magnetic field is removed; the result is a permanent magnet. In con| trast, hard magnetic materials, such as cobalt and nickel, are difficult to mag^. netize, but once they are magnetized, they tend to retain their magnetism. In
I soft magnetic materials, once the external field is removed, the random
Motion of the particles in the material changes the orientation of the domains
a
nd the material returns to an unmagnetized state.
Magnetism
679
MAGNETIC FIELDS
magnetic field
a region in which a magnetic
force can be detected
You know that the interaction between charged objects can be described
the concept of an electric field. A similar approach can be used to describe the
magnetic field that surrounds any magnetized material. As with an electric
field, a magnetic field, B, is a vector quantity that has both magnitude and
direction.
Magnetic field lines can be drawn with the aid of a compass
Table 1
Conventions for
Representing the
Direction of a
Magnetic Field
In the plane of the page
Into the page
ITT"
Out of the page
The magnetic field of a bar magnet can be explored using a compass, as illustrated in Figure 3. If a small, freely suspended bar magnet, such as the needle
of a compass, is brought near a magnetic field, the compass needle will align
with the magnetic field lines. The direction of the magnetic field, B, at any
location is defined as the direction that the north pole of a compass needle
points to at that location.
Magnetic field lines appear to begin at the north pole of a magnet and to
end at the south pole of a magnet. However, magnetic field lines have no
beginning or end. Rather, they always form a closed loop. In a permanent
magnet, the field lines actually continue within the magnet itself to form a
closed loop. (These lines are not shown in the illustration.)
This text will follow a simple convention to indicate the direction of B. An
arrow will be used to show a magnetic field that is in the same plane as the page, as
shown in Table 1. When the field is directed into the page, we will use a series of
blue crosses to represent the tails of arrows. If the field is directed out of the page,
we will use a series of blue dots to represent the tips of arrows.
Magnetic flux relates to the strength of a magnetic field
One useful way to model magnetic field strength is to define a
quantity called magnetic flux, <I>M. It is defined as the number of
field lines that cross a certain area at right angles to that area.
Magnetic flux can be calculated by the following equation.
MAGNETIC FLUX
= ABcos#
magnetic flux = (surface area) x (magnetic field
component normal to the plane of surface)
Figure 3
The magnetic field (a) of a bar magnet can be traced
with a compass (b). . Note
that the north poles
ote
poes of
o„ the
.. tat.. teoff nor
„e
.,
,, j ,.lines
( from
compasses point in the
the direction
direction
the rrfield
the
fiel
the magnet's north pole to its south pole.
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Chapter 1 9
Now look again at Figure 3. Imagine two circles of the same
size that are perpendicular to the axis of the magnet. One circle
is located near one pole of the magnet, and the other circle is
alongside the magnet. More magnetic field lines cross the circle
a.-*is near the
u_ polei of cthe
*.u magnet.* This
TU- greater
fl,,^nux i
that
that the magnetic field is strongest at the magnet's poles.
£arth has a magnetic field similar to that of a bar magnet
Magnetic south pole
The north and south poles of a small bar magnet are correctly
described as the "north-seeking" and "south-seeking" poles. This
description means that if a magnet is used as a compass, the
north pole of the magnet will seek, or point to, a location near
the geographic North Pole of Earth. Because unlike poles attract,
we can deduce that the geographic North Pole of Earth corresponds to the magnetic south pole and the geographic South
Pole of Earth corresponds to the magnetic north pole. Note that
the configuration of Earth's magnetic field, pictured in Figure 4,
resembles the field that would be produced if a bar magnet were
buried within Earth.
Geographic
If a compass needle is allowed to rotate both perpendicular to
and parallel to the surface of Earth, the needle will be exactly parallel with respect to Earth's surface only near the equator. As the compass is
moved northward, the needle will rotate so that it points more toward the surface of Earth. Finally, at a point just north of Hudson Bay, in Canada, the north
pole of the needle will point perpendicular to Earth's surface. This site is considered to be the location of the magnetic south pole of Earth. It is approximately
1500 km from Earth's geographic North Pole. Similarly, the magnetic north
I pole of Earth is roughly the same distance from the geographic South Pole.
The difference between true north, which is defined by the axis of rotation
1 of Earth, and north indicated by a compass, varies from point to point on
Earth. This difference is referred to as magnetic declination. An imaginary line
running roughly north-south near the center of North America currently has
zero declination. Along the line a compass will indicate true north. However,
:; in the state of Washington, a compass aligns about 20° east of true north. To
further complicate matters, geological evidence indicates that Earth's magnetic
;
field has changed—and even reversed—throughout Earth's history.
Although Earth has large deposits of iron ore deep beneath its surface, the
| high temperatures in Earth's liquid core prevent the iron from retaining any
: permanent magnetization. It is considered more likely that the source of
' < Earth's magnetic field is the movement of charges in convection currents in
Quick Lab
Magnetic Field
of a File Cabinet
f compass
jf metal file cabinet
Stand in front of the file cabinet, and
hold the compass face up and parallel to
the ground. Now move the compass from
the top of the file cabinet to the bottom.
Making sure that the compass is parallel to
the ground, check to see if the direction of
the compass needle changes as it moves
from the top of the cabinet to the bottom.
If the compass needle changes direction,
the file cabinet is magnetized. Can you
Geographic North Pole
South Pole
Magnetic north pole
Figure 4
Earth's magnetic field has a configuration similar to a bar magnet's. Note
that the magnetic south pole is near
the geographic North Pole and that
the magnetic north pole is near the
geographic South Pole.
Dftl you Icnoinr?
By convention, the north pole of a
magnet is frequently painted red.
This practice comes from the longstanding use of magnets, in the form
of compasses, as navigational aids.
Long before Global Positioning System (GPS) satellites, the compass
gave humans an easy way to orient
themselves.
explain what might have caused the file
cabinet to become magnetized? Remember that Earth's magnetic field has a vertical component as well as a horizontal
component.
Try tracing the field around some large
metal objects around your house. Can you
find an object that has been magnetized by
the horizontal component of Earth's magnetic field?
Magnetism
681
Earth's core. These currents occur because the temperature in Earth's liquid
core is not evenly distributed. Charged ions or electrons circling in the liquid
interior of Earth could produce a magnetic field. There is also evidence that
the strength of a planet's magnetic field is related to the planet's rate of rotation. For example, Jupiter rotates at a faster rate than Earth does, and recent
space probes indicate that Jupiter's magnetic field is stronger than Earth's is
Conversely, Venus rotates more slowly than Earth does and has been found to
have a weaker magnetic field than Earth does. Investigation into the cause of
Earth's magnetism continues.
Integrating Technology
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"Magnetic Resonance Imaging."
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SECTION REVIEW
1. For each of the cases in the figure below, identify whether the magnets
will attract or repel one another.
a.
b.
S
N
N
S
s
N
S
N
2. When you break a bar magnet in half, how many poles does each piece have?
3. Interpreting Graphics Which of the compass-needle orientations
in the figure below might correctly describe the magnet's field at that
point?
Cb)
s~*^
(c)
(a)
N
S
col
He)
4. Critical Thinking Satellite ground operators use the feedback from
a device called a magnetometer, which senses the direction of Earth's
magnetic field, to decide which torque coil to activate. What direction
will the magnetometer read for Earth's magnetic field when the satellite
passes over Earth's equator?
5. Critical Thinking In order to protect other equipment, the body of
a satellite must remain unmagnetized, even when the torque coils have
been activated. Would hard or soft magnetic materials be best for building the rest of the satellite?
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Chapter 19