CPO Science
Foundations of Physics
Unit 7, Chapter 22
Unit 7: Electricity and Magnetism
Chapter 22 Electric Charges and Forces
 22.1 Properties of Magnets
 22.2 Magnetic Properties of Materials
 22.3 The Magnetic Field of the Earth
Chapter 22 Objectives
1. Describe the forces between two permanent
magnets.
2. Sketch the magnetic field of a single permanent
magnet.
3. Predict the direction of the force on a magnet placed
in a given magnetic field.
4. Explain why ferromagnetic materials always attract
magnets of either pole.
5. Describe the theory behind why a compass works.
6. Use a compass to find the direction of true north.
Chapter 22 Vocabulary Terms
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magnet
north pole
south pole
magnetization
demagnetization
magnetic field
compass magnetic
field lines
diamagnetic
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paramagnetic
ferromagnetic
gauss
soft magnet
magnetic declination
magnetic domain
hard magnet
permanent magnet
22.1 Properties of Magnets
Key Question:
How do magnets interact
with each other?
*Students read Section 22.1
AFTER Investigation 22.1
22.1 What is a magnet?
 If a material is magnetic, it has the ability to exert
forces on magnets or other magnetic materials.
 A permanent magnet is a material that keeps its
magnetic properties even when it is NOT close to
other magnets.
22.1 Properties of Magnets
 Magnets have two
opposite poles.
— north
— south
 Magnets exert forces on
each other.
 The forces depend on the
alignment of the poles.
22.1 Properties of Magnets
 Plastics, wood, and most insulating materials are
virtually transparent to magnetic forces.
 Conducting metals, like aluminum, also allow
magnetic forces to pass through, but may change
the forces.
22.1 The force between two magnets
 The strength of the force
between magnets
depends on the distance
between them.
 The magnetic force
decreases with distance
much faster than does
either gravity or the
electric force.
22.1 The force between two magnets
 Two magnets near each other
often feel a twisting force, or
torque.
 This is a result of having two
poles.
 The combination of attractive
and repulsive forces on the
same magnet creates a
torque.
22.1 The magnetic field
 All magnets create a
magnetic field in the
space around them, and
the magnetic field
creates forces on other
magnets.
22.1 The magnetic field
 The number of field lines in a
certain area indicates the relative
strength of the magnetic field in
that area.
 The arrows on the field lines
indicate the direction of the force.
 The closer the lines are together,
the stronger the field.
 Magnetic field lines always point
away from a magnet’s north pole
and toward its south pole.
22.2 Magnetic Properties of Materials
Key Question:
How do magnets interact
with different materials?
*Students read Section 22.2
AFTER Investigation 22.2
22.2 Magnetic Properties of Materials
 The sources of nearly all
magnetic effects in matter
are the electrons in atoms.
 There are two ways in which
electrons create magnetism:
1. Electrons around the
nucleus and their motion
makes the entire atom a
small magnet.
2. Electrons themselves act
as though they were
magnets.
22.2 Magnetic Properties of Materials
 All atoms have electrons, so you might think that all
materials should be magnetic, but there is great
variability in the magnetic properties of materials.
 The electrons in some atoms align to cancel out
one another’s magnetic influence.
 While all materials show some kind of magnetic
effect, the magnetism in most materials is too weak
to detect without highly sensitive instruments.
22.2 Magnetic Properties of Materials
 In diamagnetic materials,
the electrons are oriented
so their individual magnetic
fields cancel each other out.
 Individual atoms in
paramagnetic materials are
magnetic but the atoms
themselves are randomly
arranged so the overall
magnetism of a sample is
zero.
When paramagnetic materials
are placed in a magnetic field,
the atoms align so that the
material is weakly magnetic.
22.2 Magnetic Properties of Materials
 A small group of metals have
very strong magnetic
properties, including iron,
nickel, and cobalt.
 These metals are the best
known examples of
ferromagnetic materials.
 Atoms with similar magnetic
orientations line up with
neighboring atoms in groups
called magnetic domains.
22.2 Magnetic Properties of Materials
 Magnetic domains in a ferromagnetic material will
always orient themselves to attract a permanent
magnet.
— If a north pole approaches, domains grow that have south
poles facing out.
— If a south pole approaches, domains grow that have north
poles facing out.
22.2 Properties of magnets
 Materials that make good
permanent magnets are
called hard magnets.
 Steel, which contains iron
and carbon, is a common
and inexpensive material
used to create hard
magnets.
 Materials that lose their
magnetism quickly are
called soft magnets.
22.3 The Magnetic Field of the Earth
Key Question:
How do we use Earth’s
magnetic field to tell
direction?
*Students read Section 22.3
AFTER Investigation 22.3
22.3 The Magnetic Field of the Earth
 As early as 500 B.C. people
discovered that some naturally
occurring materials— such as
lodestone and magnetite—have
magnetic properties.
 By 1200, explorers from Italy
were using a compass to guide
ocean voyages beyond the sight
of land.
22.3 The Magnetic Field of the Earth
 When you use a compass, the
north-pointing end of the needle
points toward a spot near (but
not exactly at) the Earth’s
geographic north pole.
 The Earth’s magnetic poles are
defined by the planet’s
magnetic field.
 That means the south magnetic
pole of the planet is near the
north geographic pole.
22.3 The Magnetic Field of the Earth
 The gauss is a unit used to measure the strength of a
magnetic field.
 The magnetic field of the Earth is very weak (0.5
gauss) compared with the strength of the field on the
surface of the classroom ceramic magnets (1000
gauss).
 Historical data shows that both the strength of the
Earth’s magnetic field and the location of the north and
south magnetic poles can switch places.
 Today, the Earth’s magnetic field is losing
approximately 7 percent of its strength every 100
years.
22.3 The Magnetic Field of the Earth
 Depending on where you are, a compass will point
slightly east or west of true north.
 The difference between the direction a compass points
and the direction of true north is called magnetic
declination.
After correcting for the
declination, you rotate the
whole compass until the
north-pointing end of the
needle lines up with zero
degrees on the ring.
The large arrow points in the
direction you want to go.
Application: Magnetic Resonance
Imaging