Download Pearson Prentice Hall Physical Science: Concepts in Action

Pearson Prentice Hall
Physical Science: Concepts in Action
Chapter 21
Magnetism
21.1 Magnets and Magnetic Fields
• Objectives:
• 1. Describe how magnetic poles
interact
• 2. Explain how a magnetic field
affects a magnet that enters the
field
• 3. Explain why some materials are
magnetic and others are not
Magnetic Pole Interaction
• Def: magnetic force is the force a magnet exerts
on another magnet, on iron or a similar metal, or
on moving charges
• Def: magnetic poles are regions where the
magnet’s force is the strongest
• All magnets have two poles : north and south
• If a magnet is cut, each piece will still have 2
opposite poles
• The poles exert a force on each other
• Like magnetic poles repel one another
• Opposite magnetic poles attract one another
Magnetic Fields
• Def: a magnetic field surrounds a magnet and can
exert magnetic forces
• A magnetic field is strongest near a magnet’s poles
• A magnetic field will attract or repel another
magnet that enters the field
• Magnetic field lines are used to represent the
magnetic field of a bar magnet
• field lines that are close together mean a strong
field
• The magnetic field gets weaker with distance from
the magnet
• Def: the magnetosphere is the magnetic field that
surrounds earth
• Earth can be imagined as a giant bar magnet
• Earth’s magnetic field has direction and strength
• Earth has one magnetic point in northeastern
Canada and an opposite point in Antarctica
• these magnetic poles are not the same as the
geographic poles
• At the northern pole, the compass needle would
point straight down
• At the southern pole, it would point straight up
• Earth’s magnetic field has changed direction
throughout geologic time (at least 20 reversals in
the past 5 million years)
Magnetic Materials
• Def: a magnetic domain is a region that has a very
large number of atoms with aligned magnetic fields
• Domains are microscopic magnetic regions
composed of a group of atoms whose magnetic
fields are aligned in a common direction
• Magnetic fields of atoms in a domain point in the
same direction
• Ferromagnetic materials like iron can be
magnetized because they contain magnetic
domains
• When a material is magnetized, most of its
magnetic domains are aligned
21.2 Electromagnetism
• Objectives:
• 1. Explain how an electric charge
can create a magnetic field
• 2. Describe how an
electromagnet is controlled
• 3. Explain how galvanometers
and electric motors work
Electric Charge Can Create a Magnetic
Field
• Electromagnetic force is a combination of
electricity and magnetism
• Electric force comes from charged particles
• Magnetic force usually comes from movement
of electrons in an atom
• Both electric and magnetic forces come from
electric charges
• Moving electric charges create a magnetic
field
• Use the right hand rule to find the
direction of the magnetic field
produced by a current
• Imagine holding the wire in your right
hand with your thumb in the direction
of the positive current, the direction
your fingers curl is the direction of the
magnetic field
• Def: A solenoid is a long wound coil of
insulated current carrying wire
• The magnetic field of each loop of wire
adds strength to the magnetic field
• Increasing the current also adds
strength to the magnetic field
• The magnetic field has north and south
poles and can be as strong as that of a
bar magnet
• Inserting a rod made of iron or other
potentially magnetic material increases
the magnetic field and creates an
electromagnet
Controlling Electromagnets
• Def: an electromagnet is a solenoid with
a ferromagnetic core
• Changing the current in an
electromagnet controls the strength
and direction of its magnetic field
• Electromagnetic devices such as
galvanometers and electric motors
change electrical energy into
mechanical energy
Galvanometers & Electric Motors
• Def: a galvanometer is a device that
uses a solenoid to measure small
amounts of current
• Example: the gas gauge in your car
• Def: an electric motor is device that
uses an electromagnet to turn an axle
• An actual motor has many loops of
wire around a central iron core to make
the motor stronger
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21.3 Electrical Energy Generation &
Transmission
Objectives:
1. Discuss how voltage is induced in a
conductor
2. Name two types of generators
3. Describe how a transformer changes
voltage and current
4. Name some sources of electrical
energy in the United States
Voltage Induction
• Def: electromagnetic induction is the process of
generating current by moving an electrical
conductor relative to a magnetic field
• In 1831, Michael Faraday discovered
electromagnetic induction
• Electromagnetic induction is the production of
current in a conducting circuit by a change in
the strength, position or orientation of an
external magnetic field
• Faraday’s law: An electric current can be
produced in a circuit by a changing magnetic
field
Two Types of Generators
• The two types of generators are AC generators and DC
generators
• Generators convert mechanical energy into electrical
energy
• For AC generators, a circuit is induced by a loop of wire
moving between two magnetic poles
• A charged particle moving in a magnetic field will
experience a force due to the magnetic field
• The force is at its maximum when the charge moves
perpendicular to the field, and at its minimum when
moving opposite the magnetic field lines
• When a wire full of charges is parallel to the field there is
no current induced in the wire
• Each half rotation of the loop inside the
generator reverses direction producing
alternating current (AC)
• Alternating current is an electric current that
changes direction at regular intervals
• Most of the world uses AC
• Def: a commutator is a device used to make
the current change direction every time the
loop rotates
• Devices called brushes connect the wires to
the commutator
• DC generators are similar to AC generators
except that commutators are used
Transformers Voltage and Current
• Def: A transformer is a device that can
change one AC voltage to a different AC
voltage
• Transformers have primary and secondary
coils around an iron core
• Transformers change voltage and current
by inducing a changing magnetic field in
one coil
• The changing field then induces an AC in
the second coil with a different number of
turns
• The voltage of the secondary coil depends
on the number of loops (turns) in the coil
• The transformer can be arranged for the
secondary voltage to be greater than the
primary (step up transformer) or less than
the primary (step down transformer)
• Step down transformers have fewer loops
than the primary coil
• Transformers are used in the transmission
of electrical energy to homes and
businesses (normally transmitted at 120 V)
Sources of Electrical Energy
• Most of the electrical energy generated in the
United States is produced using coal as an
energy source
• Other sources include water (hydroelectric),
nuclear energy, wind, natural gas and petroleum
• Def: a turbine is a device with fanlike blades that
turn when pushed (by water, wind or steam, for
example)
• Burning fossil fuels and nuclear power electrical
energy production require turbines to produce
electricity