Transcript Standard EPS Shell Presentation

ELECTRICITY AND MAGNETISM
Chapter Twenty-Two: Electricity
and Magnetism
22.1 Properties of Magnets
22.2 Electromagnets
22.3 Electric Motors
Chapter 22.1 Learning Goals
Identify properties of magnetic
materials and use interactions
between magnets to explain
attraction and repulsion.
Describe the source of Earth’s
magnetism.
Explain how a compass works.
Investigation 22A
Magnetism
Key Question:
How do magnets and compasses work?
22.1 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.
22.1 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.
22.1 Properties of Magnets
Whether the two magnets attract or
repel depends on which poles face
each other.
22.1 Properties of Magnets
Magnetic forces can pass through
many materials with no apparent
decrease in strength.
22.1 Properties of Magnets
Magnetic forces are used
in many applications
because they are relatively
easy to create and can be
very strong.
Large magnets create
forces strong enough to
lift a car or a moving train.
22.1 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.
22.1 Magnetic fields
A special kind of
diagram is used to
map the magnetic
field.
The force points
away from the north
pole and towards the
south pole.
22.1 Magnetic fields
You can actually see
the pattern of the
magnetic field lines
by sprinkling
magnetic iron filings
on cardboard with a
magnet underneath.
22.1 Magnetic field lines
A compass needle is a
magnet that is free to
spin.
Because the needle
aligns with the local
magnetic field, a
compass is a great
way to “see” magnetic
field lines.
22.1 Geographic and magnetic
poles
The planet Earth
has a magnetic
field that comes
from the core of
the planet itself.
22.1 Geographic and magnetic
poles
The names of
Earth’s poles were
decided long before
people understood
how a compass
needle worked.
The compass needle’s “north” end is actually
attracted to Earth’s “south” magnetic pole!
22.1 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.
22.1 Declination and “true north”
Magnetic declination is measured in
degrees and is indicated on
topographical maps.
22.1 Declination and “true north”
Magnetic declination is
measured in degrees and
is indicated on
topographical maps.
Most good compasses
contain an adjustable
ring with a degree scale
used compensate for
declination.
22.1 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.
22.1 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.
22.1 Earth’s magnetism
Today, Earth’s
magnetic field is
losing approximately
7 percent of its
strength every 100
years.
If this trend
continues, the
magnetic poles will
reverse sometime in
the next 2,000 years.
Chapter Twenty-Two: Electricity
and Magnetism
22.1 Properties of Magnets
22.2 Electromagnets
22.3 Electric Motors
Chapter 22.2 Learning Goals
Define electromagnet.
Build a simple electromagnet.
Compare permanent magnets and
electromagnets.
Investigation 22B
Electromagnets
Key Question:
How are electricity and magnetism related?
22.2 Electomagnets
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.
22.2 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.
22.2 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.
A toaster tray is
pulled down by an
electromagnet while
bread is toasting.
22.2 Doorbells
A doorbell contains
an electromagnet.
When the button of
the bell is pushed, it
sends current
through the
electromagnet.
22.2 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.
22.2 Building an electromagnet
 There are two ways to
increase the current in a
simple electromagnet:
1. Apply more voltage by
adding a second battery.
2. Add more turns of wire
around the nail.
Why do these two
techniques work?
22.2 Similarities in permanent
and electromagnets
 The charged electrons in atoms
behave like small loops of current.
 Electric current through loops of
wire creates an electromagnet.
 Atomic-scale electric currents
create a permanent magnet.
22.2 Magnetic materials
 Atoms act like
tiny magnets.
Permanent magnets have
their atoms aligned,
creating the magnetic
forces we observe.
22.2 Magnetic materials
 In iron, the atoms are free to rotate
and easily align their individual
north and south poles.
22.2 Nonmagnetic materials
 The atoms in
non-magnetic
materials, like
plastic, are not
free to move or
change their
magnetic
orientation.
Chapter Twenty-Two: Electricity
and Magnetism
22.1 Properties of Magnets
22.2 Electromagnets
22.3 Electric Motors
Chapter 22.3 Learning Goals
Describe the role of electromagnetism
in the function of electric motors and
generators.
Contrast energy transformations in
electric motors and generators.
Explain the principle of
electromagnetic induction.
22.3 Electric motors and
generators
Permanent magnets and
electromagnets work together to make
electric motors and generators.
The secret is in the ability of an
electromagnet to reverse its north and
south poles.
22.3 Electric motors
Around the edge
of a disk are
several magnets,
their alternating
north and south
poles facing out.
22.3 Electric motors
To make the disk
spin, you bring a
permanent magnet
close to its edge.
The free magnet
attracts one of the
magnets in the disk
and repels the next
one.
The disk is a “rotor”
because it rotates.
22.3 Electric motors
In a working electric
motor, an
electromagnet
replaces the magnet
you reversed with
your fingers.
The electromagnet
switches its poles to
make the rotor keep
turning.
22.3 Electric motors
As the rotor spins,
a commutator
reverses the
direction of the
current in the
electromagnet.
22.3 Electric motors
Motors have three
parts:
1. A rotor with magnets
that alternate.
2. One or more fixed
magnets around the
rotor.
3. A commutator that
switches the direction
of current to keep the
rotor spinning.
22.3 Battery run electric motors
 An electric motor that runs from
batteries has the same three parts.
 The permanent magnets are on the
outside, and the electromagnets turn
in the rotor.
22.3 Battery run electric motors
 A simple battery
powered motor
has three
electromagnets.
22.3 Electromagnetic induction
 Motors transform electrical energy into
mechanical energy.
 Electric generators do the opposite.
 They transform mechanical energy into
electrical energy.
 The process of using a moving magnet to
create electric current is called
electromagnetic induction.
22.3 Electromagnetic induction
 A moving magnet produces a current
in a coil of wire.
22.3 Generating electricity
 A generator converts mechanical
energy into electrical energy using
the law of induction.
 As long as the disk is spinning, there is a
changing magnetic field through the coil
and electric current is created.
Investigation 22C
Electromagnetic Forces
Key Question:
How does an electric motor work?
Under the Sea
Have you ever wondered
what it would be like to take
a “walk” on the seafloor?
Engineer Jim Varnum gets to
go “down under” regularly,
because he is a pilot of an
extremely complex robot
called Jason II. This robot
dives deep into the ocean
and is used to take pictures
and collect data from the
seafloor.