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Chapter 8: Magnetism
• Magnetism– Natural Magnets
• Electricity and Magnetism– Electromagnets
• Using and Producing Electricity– Generators and Motors
• Production and Distribution of Electricity– Transformers
• 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.
Early Uses
• Magnets attract objects made of iron or steel, such as nails and paper clips.
• Magnets also can attract or repel other magnets.
Magnets
• Every magnet has two ends, or poles.
• One end is called the north pole and the other is the south pole.
• Two north poles or two south poles repel each other. Like Repel
• North and south magnetic poles are attracted to each other. Unlike Attract
Magnets
• Where the magnetic field lines are close together, the field is strong.
• Field lines that curve toward each other show attraction.
The Magnetic Field
• Field lines that curve away from each other show repulsion.
• 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.
Making Magnetic Fields
• Not only do these electrons swarm around the nucleus of an atom, they also spin.
• Because of its movement, each electron produces a magnetic field.
Making Magnetic Fields
• 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.
• Microscopic sections of iron and steel act as tiny magnets.
Making Magnetic Fields
• 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
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.
• A compass needle is a small bar magnet with a north and south magnetic pole.
The Compass
• In a magnetic field, a compass needle rotates until it is aligned with the magnetic field line at its location.
• Earth's magnetic field also causes a compass needle to rotate.
The Compass
• 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.
• Because of its movement, each electron produces a magnetic field.
Making Magnetic Fields
• 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.
• When a strong magnet is brought near the material, the domains line up, and their magnetic fields add together.
Making Magnetic Fields
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.
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 current-carrying wires can attract each other as if they were two magnets.
Electric Motor• Magnetic field like
the one shown will push a current-carrying 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.
• In a generator, kinetic energy is changed into electric energy.
• When a wire is made to move through a magnetic field, an electric current is produced in the wire.
• Where the magnetic field lines are close together, the field is strong.
• Field lines that curve toward each other show attraction.
The Magnetic Field
• Field lines that curve away from each other show repulsion.
• When a strong magnet is brought near the material, the domains line up, and their magnetic fields add together.
Making Magnetic Fields
Electric Motor• Magnetic field like
the one shown will push a current-carrying 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
Voltage• Voltage is a measure of how much energy
the electric charges in a current are carrying.
• The electric transmission lines transmit electric energy at a high voltage of about 230,000V.
• Device are needed to increase and reduce the voltage.
• Electrical Power is generated at a voltage of about 2,400 Volts.
• Electrical Power is used at a voltage of about 240 Volts
Changing Voltage
• 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
• A transformer is a device that changes the voltage of an alternating current with little loss of energy.
Changing Voltage• A transformer
has two coils of wire wrapped around an iron core.
• One coil is connected to an alternating current source. Primary Secondary
The Transformer Ratio• Whether a
transformer increases or decreases the input voltage depends on the number of coils on each side of the transformer.
22
Primary Secondary
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&MPrinciple 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 motorStereo 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
ScheduleDate Activity Assignment
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 8:00 – 10:30
Electric Power production and distribution
Page 260
Produce Heat60%
Heat to Steam90%
Steam to Rotation
75%
Rotation to Electricity
95%
Step Up Transformer
High Voltage wires
Step Down
Transformer
Distribution Lines
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.
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