Electricity and Magnetism
Unit 5: Electricity and Magnetism
Chapter 15: Magnetism
 15.1
Properties of Magnets
 15.2
The Source of Magnetism
 15.3
Earth's Magnetic Field
15.1 Investigation: Magnetism
Key Questions:
How do magnets and compasses
Discuss the properties of permanent magnets.
Measure magnetic force.
Use a compass to detect magnetic force.
Properties of magnets
 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.
Properties of Magnets
 All
magnets have two
opposite magnetic
poles, called the north
pole and south pole.
 If
a magnet is cut in half,
each half will have its
own north and south
Properties of magnets
 Whether
the two magnets attract or repel depends
on which poles face each other.
Magnetic force
 Magnetic
forces can pass through many materials
with no apparent decrease in strength.
Magnetic force
 Magnetic
forces are used in
many applications because
they are relatively easy to
create and can be very
 Large
magnets, such as this
electromagnet, create
forces strong enough to lift
a car or a moving train.
The magnetic field
 All
magnets create a
magnetic field in the
space around them, and
the magnetic field
creates forces on other
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.
Magnetic fields
 Magnets A and
C feel a net
attracting force toward the
source magnet.
 Magnets
B and D feel a
twisting force, or torque,
because one pole is
repelled and the opposite
pole is attracted with
approximately the same
Magnetic fields
 The
force from a magnet
gets weaker as it gets
farther away.
 Separating
a pair of
magnets by twice the
distance reduces the force
by 8 times or more.
Magnetic fields
 You
can actually see the
pattern of the magnetic field
lines by sprinkling magnetic
iron filings on cardboard
with a magnet underneath.
Unit 5: Electricity and Magnetism
Chapter 15: Magnetism
Properties of Magnets
The Source of Magnetism
Earth's Magnetic Field
15.2 Investigation: Electromagnets
Key Questions:
How are electricity and magnetism related?
Use a compass to detect magnetic force.
Build a circuit to control an electromagnet.
Measure the current used by an electromagnet.
The Source of Magnetism
 Electromagnets
are magnets
that are created when there is
electric current flowing in a wire.
 The
simplest electromagnet uses
a coil of wire wrapped around
some iron.
Right hand rule
 To
find the north pole of an
electromagnet, use the
right hand rule.
 When
the fingers of your
right hand curl in the
direction of the wire, your
thumb points toward the
magnet’s north pole.
Electromagnets in toasters
 By
changing the amount of
current, you can easily
change the strength of an
electromagnet or even turn
its magnetism on and off.
 Firedoors
in hospitals and
schools use electromagnets
to release the heavy doors
in emergencies.
A toaster tray is
pulled down by an
electromagnet while
bread is toasting.
Building an electromagnet
 You
can easily build an
electromagnet from wire
and a piece of iron, such as
a nail.
 Wrap
the wire in many turns
around the nail and connect
a battery.
Building an electromagnet
 There
are two ways to
increase the current in a
simple electromagnet:
1. Add more turns of wire
around the nail.
2. Apply more voltage by
adding a second
Why do these two techniques work?
 Some
doorbells contain an
 When
the button of the bell
is pushed, it sends current
through the electromagnet
and the striker hits the bell.
Magnetism in 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.
Magnetism in materials
Atoms act like tiny
magnets with north and
south poles.
When permanent
magnets have their
atoms aligned, we
observe the magnetic
Magnetism in materials
 In
many materials, the magnetic fields of
individual electrons in each atom cancel each
others magnetic effects.
 Lead
and diamond are materials made of these
kinds of atoms and are called diamagnetic.
 It
takes either a very strong magnetic field to
cause the effects or very sensitive instruments
to detect them.
Magnetism in materials
Aluminum is paramagnetic.
In an atom of aluminum, the
magnetism of individual
electrons do not cancel
This makes each aluminum
atom a tiny magnet with a north
and a south pole.
Solid aluminum is
“nonmagnetic” because the
total magnetic field averages to
Nonmagnetic materials
The atoms in nonmagnetic materials, like
plastic, are not free to
move or change their
magnetic orientation.
Ferromagnetic materials
 A small
group of ferromagnetic metals have very
strong magnetic properties.
 Examples
of ferromagnetic materials are iron,
nickel, and cobalt.
 Atoms
in ferromagnetic materials align themselves
with neighboring atoms in groups called magnetic
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 by adding
neighboring atoms that have south poles facing out.
— If a south pole approaches, domains grow that have north
poles facing out.
Magnetism in solids
 Permanent
magnetism only exists in solids.
 Permanent
magnets and ferromagnetic materials
become demagnetized if the temperature gets too
 Even
the best magnetic materials are only able to
retain their magnetism only up to a few hundred
degrees Celsius.
Magnetism in solids
If you use the north end of
the magnet to pick up a nail,
the nail becomes
magnetized with its south
pole toward the magnet.
Because the nail itself
becomes a magnet, it can be
used to pick up other nails.
If you separate that first nail
from the bar magnet, the
entire chain demagnetizes
and falls apart.
Unit 5: Electricity and Magnetism
Chapter 15: Magnetism
Properties of Magnets
The Source of Magnetism
Earth's Magnetic Field
15.3 Investigation: Making a Model Maglev
Key Questions:
How can you make a model maglev train?
Use the Internet to conduct research about maglev trains.
Apply the engineering cycle to design, build, and test a
model maglev train.
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.
 Magnetite,
a magnetic
mineral made of iron
oxide, has been found in
bacteria and in the brains
of birds.
 Tiny
crystals of magnetite
may act like compasses
and allow these
organisms to sense the
magnetic field of Earth.
Earth as a magnet
 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
 That means the south
magnetic pole of the planet is
near the north geographic pole.
Declination and “true north”
 Because
Earth’s geographic north pole (true north)
and magnetic south pole are not located at the
exact same place, a compass will not point directly
to the geographic north pole.
 The
difference between the direction a compass
points and the direction of true north is called
magnetic declination.
Magnetic declination and “true” north
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 adjusted using a ring on the
 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
The large arrow points in the
direction you want to go.
Maps and declination
 Maps
often list the
declination for an area.
 To
go north, you must
walk in a direction 16
degrees west of the
direction the needle is
The source of Earth’s magnetism
 The
planet Earth has
a magnetic field that
comes from the core
of the planet itself.
The source of Earth’s magnetism
 Studies
of earthquake
waves reveal that the
Earth’s core is made of
hot, dense molten metals.
 Huge
electric currents
flowing in the molten iron
produce the Earth’s
magnetic field.
The source of Earth’s magnetism
 The
gauss is a unit used to measure the strength
of a magnetic field.
 The
magnetic field of Earth (.5 G) is weak
compared to the field near the ceramic magnets
you have in your classroom. (300- 1,000 G).
 For
this reason you cannot trust a compass to
point north if any other magnets are close by.
Earth’s magnetic trend
 Today,
Earth’s magnetic
field is losing
approximately 7% of its
strength every 100 years.
 If
this trend continues, the
magnetic poles will
reverse sometime in the
next 2,000 years.
Magnetism in stars and planets
 Like
Earth, other planets in the solar system also
have magnetic fields.
 In
the case of Jupiter, the magnetic field is very
strong compared to Earth’s and was mapped by the
Cassini spacecraft.
Magnetism in stars and planets
 Even
stars have magnetic fields.
 The
Sun’s uneven rotation twists
its magnetic field lines. Every so
often, the magnetic field lines
become so twisted they “snap”
and reconnect themselves.
 This
sudden change causes huge
solar storms where great
eruptions of hot gas flare up from
the Sun’s surface.
Magnetic Resonance Imaging
MRI is a powerful diagnostic
technology. An MRI scanner
makes a three-dimensional map
of the inside of the body.
As the name implies, MRI
technology uses magnets and
resonance to create images.