Download The Magnetic Field

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reading about magnetism
Chapter 8: Magnetism
• Magnetism
– Natural Magnets
• Electricity and Magnetism
– Electromagnets
• Using and Producing Electricity
– Generators and Motors
• Production and Distribution of Electricity
– Transformers
Early Uses
• Thousands of years ago, people noticed that
a mineral called magnetite attracted other
pieces of magnetite and bits of iron.
• They discovered that when they rubbed small
pieces of iron with magnetite, the iron began
to act like magnetite.
• When these pieces were free to turn, one end
pointed north. Lodestone use for navigation.
Magnets
• Magnets attract objects made of iron or
steel, such as nails and paper clips.
• Magnets also can attract or repel other
magnets.
• Every magnet has two ends, or poles.
• One end is called the north pole and the other
is the south pole.
Magnets
• Two north poles or two south poles repel
each other. Like Repel
• North and south magnetic poles are attracted
to each other. Unlike Attract
The Magnetic Field
• Where the magnetic field lines are close
together, the field is strong.
• Field lines that
curve toward
each other show
attraction.
• Field lines that
curve away from
each other show
repulsion.
Making Magnetic Fields
• A magnet is surrounded by a magnetic
field that enables the magnet to exert a
magnetic force.
• A moving electric charge creates a
magnetic field.
Making Magnetic Fields
• Inside every magnet are moving charges.
• All atoms contain
negatively charged
particles called
electrons.
• Not only do these
electrons swarm
around the nucleus
of an atom, they
also spin.
Making Magnetic Fields
• Because of its movement, each electron
produces a magnetic field.
• A group of atoms, with their fields
pointing in the same direction, is called a
magnetic domain.
• Normally, these domains are oriented
randomly and their magnetic fields
cancel each other.
• When a strong magnet is brought near
the material, the domains line up, and
their magnetic fields add together.
Making Magnetic Fields
• Microscopic sections of iron and steel act as
tiny magnets.
• Normally,
these domains
are oriented
randomly and
their magnetic
fields cancel
each other.
Making Magnetic Fields
• When a strong
magnet is brought
near the material,
the domains line
up, and their
magnetic fields
add together.
Earth's Magnetic Field
• The shape of
Earth's magnetic
field is similar to
that of a huge bar
magnet tilted
about 11º from
Earth's geographic
north and south
poles.
The Compass
• A compass needle is
a small bar magnet
with a north and
south magnetic pole.
• In a magnetic field,
a compass needle
rotates until it is
aligned with the
magnetic field line
at its location.
The Compass
• Earth's magnetic field also causes a compass
needle to rotate.
• The north pole of the compass needle
points toward Earth's magnetic pole that is
in the north.
• The magnetic pole in the north is actually a
magnetic south pole.
Making Magnetic Fields
• Because of its movement, each electron
produces a magnetic field.
• A group of atoms, with their fields
pointing in the same direction, is called a
magnetic domain.
• Normally, these domains are oriented
randomly and their magnetic fields
cancel each other.
• When a strong magnet is brought near
the material, the domains line up, and
their magnetic fields add together.
Making Magnetic Fields
• When a strong
magnet is brought
near the material,
the domains line
up, and their
magnetic fields
add together.
Current Can Make a Magnet
• Magnetic fields are produced by moving
electric charges.
• When electric current flows in a wire, electric
charges move in the wire.
• As a result, a wire that contains an electric
current also is surrounded by a magnetic field.
Electromagnets
• A current-carrying
wire wrapped
around an iron
core is called an
electromagnet.
• An electric
doorbell uses an
electromagnet.
• By changing the current, the strength and
direction of the magnetic field of an
electromagnet can be change.
Using Electromagnets
• The magnetic field of an electromagnet is
turned on or off when the electric current is
turned on or off.
• By changing the current, the strength and
direction of the magnetic field of an
electromagnet can be change.
Solenoid and Electromagnets
Wire coils
around iron
rod
Battery
Iron
Using Electromagnets
• An electric doorbell uses an
electromagnet.
Galvanometer
page 234-235
S
N
N
S
S
Current
Magnets Push and Pull Currents
• Current-carrying
wires produce a
magnetic field.
• This magnetic
field behaves the
same way as the
magnetic field
that a magnet
produces.
Magnets Push and Pull Currents
• Two currentcarrying wires
can attract each
other as if they
were two
magnets.
Electric Motor
• Magnetic field like
the one shown will
push a currentcarrying wire
upward.
• Any device that
converts electric
energy into
kinetic energy is
a motor.
Electric Motor
• To keep a motor
running, the
current-carrying
wire is formed
into a loop so
the magnetic
field can force
the wire to spin
continually.
Using Magnets to Create Current
• If a wire is pulled
through a magnetic
field, the electrons in
the wire also move
downward.
• The magnetic field
exerts a force on the
moving electrons,
causing them to
move along the wire.
Using Magnets to Create Current
• A device called a generator uses a magnetic
field to turn motion into electricity.
• Electric motors and electric generators both
involve conversions between electric
energy and kinetic energy.
• When a wire is made to move through a
magnetic field, an electric current is
produced in the wire.
• In a generator, kinetic energy is changed into
electric energy.
The Magnetic Field
• Where the magnetic field lines are close
together, the field is strong.
• Field lines that
curve toward
each other show
attraction.
• Field lines that
curve away from
each other show
repulsion.
Making Magnetic Fields
• When a strong
magnet is brought
near the material,
the domains line
up, and their
magnetic fields
add together.
Solenoid and Electromagnets
Wire coils
around iron
rod
Battery
Iron
Electric Motor
• Magnetic field like
the one shown will
push a currentcarrying wire
upward.
• Any device that
converts electric
energy into
kinetic energy is
a motor.
Using Magnets to Create Current
• If a wire is pulled
through a magnetic
field, the electrons in
the wire also move
downward.
• The magnetic field
exerts a force on the
moving electrons,
causing them to
move along the wire.
Electric Generators
• In a generator, an
energy source
spins a wire loop
in a magnetic
field.
• Every half turn,
the current will
reverse direction.
Electric Generators
• This causes the current to alternate from
positive to negative.
• Such a current is called an alternating
current (AC).
• In the United States, electric current
change from positive to negative to
positive to positive 60 times each seconds.
Types of Current
• In a direct current (DC) electrons flow in
one direction. Wet cells and dry cells produce
DC current.
• In an alternating current, electrons change
their direction of movement many times
each second.
• Some generators are built to produce direct
current instead of alternating current.
Power Plants
• Electric generators produce almost all of the
electric energy used all over the world.
• Different energy sources are used to provide the
kinetic energy to rotate coils of wire in a
magnetic field.
• Burning fuels
• Coal, Natural Gas
• Moving Fluid
• hydroelectric, wind turbines
• Nuclear Energy
Power Plants
• Coal-burning power
plants are the most
common.
• This picture is deceiving.
• Problems
• Dirty to mine
• Dirty to burn
• We will run out
• Global Warming
Making, Distributing,
Using Electrical Energy
Voltage
• Voltage is a measure of how much energy the
electric charges in a current are carrying.
• Electrical Power is generated at a voltage of
about 2,400 Volts.
• Electrical Power is used at a voltage of about
240 Volts
• The electric transmission lines transmit
electric energy at a high voltage of about
230,000V.
• Device are needed to increase and reduce the
voltage.
Changing Voltage
• A transformer is a device that changes the
voltage of an alternating current with little
loss of energy.
• Transformers are used to increase the voltage
before transmitting an electric current through
the power lines. Step Up Transformer
• Other transformers are used to decrease the
voltage to the level needed for home or
industrial use. Step Down Transformer
Changing Voltage
Primary
Secondary
• A transformer
has two coils of
wire wrapped
around an iron
core.
• One coil is
connected to
an alternating
current source.
2
Primary
The Transformer Ratio
Secondary
• Whether a
transformer
increases or
decreases the
input voltage
depends on
the number of
coils on each
side of the
transformer.
The Transformer Ratio
• In a transformer the voltage is greater on the
side with more coils.
• If the number of coils on the input side is less
than the number on the output side, the
voltage is increased.
• If the number of coils on the input side is
more than the number on the output side, the
voltage is decreased.
Summary of E&M
Principle
Application
A current moving in a wire produces a
magnetic field around the wire
Solenoid, electromagnet,
doorbell, door locks, automatic
controls
The magnetic field of a permanent magnet
will exert a force on the magnetic field
around a current carrying wire.
Galvanometer- a device for
measuring electric current.
Electric motor
Stereo Speaker
A wire moving through the magnetic field of
a permanent magnet will produce an electric
current in the wire.
Generator
A changing magnetic field will “induce” a
current in a stationary wire.
Transformer
Schedule
Date
Activity
Monday 2/4
Review/Electrical Generators
Tuesday 2/5
Faraday Video
Wednesday 2/6
Magnetism Lab
Thursday 2/7
Transformers and Electrical Distribution
Friday 2/8
Transformer Problems
Monday 2/11
Notetaking worksheet
Tuesday 2/12
Chapter Review
Wednesday 2/13
Chapter 8 Test
Wednesday 2/20
DUKE Energy Field Trip
Assignment
8:00 – 10:30
Step Up Transformer
Step Down Transformer
Input
Output
Step Up/Step Down Transformers
V input
# input coils
=
V output
# output coils
Electric Power
production and distribution
Page 260
Produce
Heat
60%
Step Up
Transformer
Heat to
Steam
90%
High
Voltage
wires
Steam to
Rotation
75%
Step
Down
Transformer
Distribution
Lines
Rotation to
Electricity
95%
Step Down
Transformer
Household
Usage
Connecting Electricity
and Magnetism
• Electric charges and magnets are related to
each other.
• Moving electric charges produce magnetic
field.
• A moving magnetic field make electric
charges move.
Definitions 1
• Solenoid – a coil of wire carrying an electric
current which produces a magnetic field.
• Electromagnet- a coil of wire (solenoid) with
an iron core which produces a stronger
magnetic field.
• Applications – doorbells, actuators of all sorts
Definition 2
• Galvanometer- a device for measuring
electrical current. Also called an ampmeter.
• Electric motor- a device that uses a
electromagnet in the field of a permanent
magnet to produce rotational motion.
• A (DC) electric motor requires a commutator
to interrupt and change the direction of the
electric current
Definition 3
• Electric generator – a device that changes the motion
of a coil rotating in the field of a permanent magnet
to produce an electric current.
• The current produced by a generator changes
direction twice in each revolution of the coil. The
current produced by a generator is called Alternating
Current.
• With modifications a generator can produce direct
current.
Definition 4
• Transformer- a device that changes the
voltage of electric current.
• Primary coil – the coil through which the input
current flows.
• Secondary coil – the coil through which the
output current is produced.
• Core – the iron around which both the input
coil and the output coil are wound.
Example (Page 244)
Assignment- Applying Math 6
If the input voltage is 120 volts and the primary
coil (input) has 100 turns, what will the output
voltage be if there are 10 turns on the secondary
(output) coil?
V input
# input coils
120V
100
=
V output
# output coils
=
V output
10