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13.1
Figure 1
Lodestone consists mainly of iron oxide, a
mineral that was first found near Magnesia,
in Greece, hence the term “magnetism.”
poles: areas of concentrated magnetic force
N-pole: the end of a magnet that seeks the
northerly direction
S-pole: the end of a magnet that seeks the
southerly direction
Magnetic Force and Fields
As early as 600 B.C., the Greeks discovered that a certain type of iron ore, later
known as lodestone, or magnetite, was able to attract other small pieces of iron
(Figure 1). Also, when pivoted and allowed to rotate freely, a piece of lodestone
would come to rest in a north-south position. Because of this property, lodestone
was widely used in navigation.
Today, lodestone is hardly ever used for its magnetic property. Artificial magnets are made from various alloys of iron, nickel, and cobalt.
When a magnet is dipped in iron filings, the filings are attracted to the
magnet and accumulate most noticeably at the opposite ends of the magnet. We
call these areas of concentrated magnetic force poles. When a magnet is allowed
to rotate freely, one of the poles tends to “seek” the northerly direction on Earth
and it is called the north-seeking pole, or N-pole; the other pole is called the
south-seeking, or S-pole.
When the N-pole of one magnet is brought near the N-pole of another freely
swinging magnet, the magnets repel each other, as shown in Figure 2. Similarly, two
S-poles repel each other. On the other hand, N-poles and S-poles always attract
each other. These observations lead to what is called the law of magnetic poles.
Law of Magnetic Poles
• Opposite magnetic poles attract.
• Similar magnetic poles repel.
Figure 2
Testing the laws of magnetic poles
law of magnetic poles: Opposite
magnetic poles attract. Similar magnetic
poles repel.
magnetic field of force: the space
around a magnet in which magnetic forces
are exerted
472 Chapter 13
When an N-pole and an S-pole are brought close to each other, they begin
to attract even before they touch. This “action-at-a-distance” type of force is
already familiar from your examination of gravitational force and electric force.
The effect of those forces was described in terms of a “field of force” in the surrounding space. Similarly, we will consider the space around a magnet in which
magnetic forces are exerted as a magnetic field of force.
To detect the presence of a magnetic field, we need a delicate instrument
that is affected by magnetic forces. Small filings of iron respond to magnetic
forces, but because their poles are not marked, we cannot determine which way
they are pointing.
A magnetic field is represented by a series of lines around a magnet, showing
the path the N-pole of a small test compass would take if it were allowed to move
freely in the direction of the magnetic force. Then, at any point in the field, a
magnetic field line indicates the direction in which the N-pole of the test compass would point.
13.1
Activity 13.1.1
Magnetic Fields
To observe the magnetic force at a given point in a magnetic field we can use a
small test compass with clearly marked poles. To demonstrate the magnetic field
around a magnet, iron filings are sprinkled around the magnet.
Materials
Be careful not to get iron
filings in your eyes. Wash
your hands after handling
iron filings.
two bar magnets
iron filings
sheet of acetate (or paper)
small compass
Procedure
1. Cover the bar magnet with the sheet of acetate (or paper). Carefully sprinkle
the iron filings onto the sheet of acetate around the bar magnet and look for
any patterns formed by the iron filings, especially near the poles of the
magnet. Sketch any patterns that you find.
2. Use the compass to indicate the direction of the field around the space above
the bar magnet. Draw a three-dimensional picture of the magnetic field
around the magnet.
3. Repeat the procedure for the following cases:
(i) like poles close to each other
(ii) opposite poles close to each other
(iii) magnets side-by-side
(a)
(b)
Observations
(a) From what area of the magnets do field lines seem to originate? To what
region do they seem to return? Which field lines, if any, leave the magnets
but seem not to return?
(b) Do magnetic field lines ever cross each other? Could there be any magnetic
lines of force in the regions of space between the field lines you have
drawn? Check to see.
(c) What do you notice about the spacing of the field lines as you move away
from the poles? What does this spacing indicate about the strength of the
magnetic field?
(d) There is a theory of magnetism that states that every magnetic field line is
a closed curve. However, the field lines we have drawn seem to start at the
N-pole and return at the S-pole. For the magnetic field you have drawn for
the bar magnet, choose several field lines and sketch the portion of each
line as you believe it would exist within the magnet.
Characteristics of Magnetic Field Lines
Figure 3 shows the magnetic fields around a single bar magnet, and around pairs
of bar magnets close together.
(c)
Figure 3
The nature of the magnetic fields
(a) Around a bar magnet
(b) Between a pair of opposite poles
(c) Between similar poles
Electromagnetism 473
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The characteristics of magnetic field lines are summarized below.
1. The spacing of the lines indicates the relative strength of the force. The closer
together the lines are, the greater the force.
2. Outside a magnet, the lines are concentrated at the poles. They are closest
within the magnet itself.
3. By convention, the lines proceed from S to N inside a magnet and from N to
S outside a magnet, forming closed loops. (A plotting compass indicates
these directions.)
4. The lines do not cross one another.
S
Figure 4
The three-dimensional magnetic field around
a bar magnet. It is often easier to draw the
fields as they appear in the horizontal; experiments are done that way, which may lead
you to think that the magnetic fields are twodimensional.
Note that the magnetic field around a bar magnet is three-dimensional in nature
(Figure 4); it does not exist just in the horizontal plane.
Practice
Understanding Concepts
1. What name is given to the region in which a magnet influences other
magnetic materials?
N
S
2. Is the magnetic pole area in the northern hemisphere an N-pole or an
S-pole? Explain.
Applying Inquiry Skills
3. Draw a diagram showing the magnetic field lines around a U-shaped
magnet. Include the directions of the field lines.
4. (a) Describe two methods that could be used to detect the presence
of a magnetic field.
(b) Is there a magnetic field present between the lines indicated by
iron filings? Explain.
Figure 5
For question 5
5. Two iron nails are held to a magnet, as shown in Figure 5. Predict
what will happen when the nails are released. If possible, verify your
prediction experimentally.
6. You are given two bars of steel: one is a perfectly good magnet and
the other is unmagnetized. Using only these two bars, how can you
determine which one is the magnet?
DID YOU KNOW ?
Auroras
Earth’s strong magnetic field helps to cause
auroras, the dancing lights that spread across
the sky above the north and south magnetic
poles. Charged particles, such as electrons
and protons, streaming from the Sun spiral in
toward these poles, and collide with atoms in
the upper atmosphere. These energized
atoms then give off energy in the form of
visible light with a variety of colours.
Sometimes large solar storms cause the
auroras to increase and result in the disruption of our communications systems for hours
or even days. In the Northern Hemisphere,
the auroras are called aurora borealis, or the
Northern Lights.
474 Chapter 13
SUMMARY
Magnetic Force and Fields
• The law of magnetic fields states that opposite magnetic poles attract, and
similar magnetic poles repel.
• The space around a magnet in which magnetic forces are exerted is known
as the magnetic field of force.
• The magnetic field around a bar magnet is three-dimensional in nature.
Section 13.1 Questions
Understanding Concepts
1. Describe what would happen to a magnetic compass placed at
(a) the magnetic north pole
(b) the equator
13.2
(a)
2. In the diagrams in Figure 6, each circle represents a compass.
Copy the diagrams into your notebook and show the direction of
the needle in each compass.
compass
3. Compare the law of magnetic poles with the law of electric
charges.
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Applying Inquiry Skills
4. Copy the diagrams in Figure 7 into your notebook and draw the
magnetic field lines between the ends of three bar magnets if
(a) all the S-poles are close together
(b) one N-pole and two S-poles are close together
(a)
(b)
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(b)
N
S
S
S
S
S
N
N
S
N
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Figure 7
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Making Connections
5. Biomagnetism is the study of the relationship between magnetism and living organisms. For example, research has found
that pigeons have a magnetic sense that can detect magnetic
fields. Research more about biomagnetism, including the magnetic sense of pigeons and other animals. Write a brief story
summarizing what you discover. Follow the links for Nelson
Physics 11, 13.1.
GO TO
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www.science.nelson.com
Reflecting
6. Often students are left with the impression that magnetic field
lines exist around a bar magnet only in the horizontal; actually,
the field is three-dimensional.
(a) Why do you think many students misunderstand the threedimensional nature of magnetic field lines?
(b) How would you demonstrate that the magnetic field around
a bar magnet is three-dimensional?
13.2
(c)
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Figure 6
For question 2
Magnetic Materials
Small pieces of iron rubbed in one direction with lodestone become magnetized.
Even bringing a piece of iron near a magnet causes the iron to be magnetized.
Nickel and cobalt, and any alloy containing nickel, cobalt, or iron, behave in the
same way. These substances are called ferromagnetic, and you can induce them
to become magnetized by placing them in a magnetic field.
ferromagnetic: a substance that can
become magnetized
Electromagnetism 475