Transcript Chapter 15: Magnetism

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
work?
Objectives:

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
nearby.
 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
poles.
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
strong.
 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
magnets.
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
strength.
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
15.1
Properties of Magnets
15.2
The Source of Magnetism
15.3
Earth's Magnetic Field
15.2 Investigation: Electromagnets
Key Questions:
How are electricity and magnetism related?
Objectives:

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
battery.
Why do these two techniques work?
Doorbells
 Some
doorbells contain an
electromagnet.
 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
forces.
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
completely.

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
zero.
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
domains.
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
hot.
 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
15.1
Properties of Magnets
15.2
The Source of Magnetism
15.3
Earth's Magnetic Field
15.3 Investigation: Making a Model Maglev
Train
Key Questions:
How can you make a model maglev train?
Objectives:

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
 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
field.
 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
compass.
 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.
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
pointing.
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.