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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. – PowerPoint PPT presentation

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Title: 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
    magnets 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 magnets
  • 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
  • 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
  • Earths 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
  • Earths 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
  • 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

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

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
  • 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
  • Faradays law An electric current can be
    produced in a circuit by a changing magnetic

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
  • Devices called brushes connect the wires to the
  • 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
  • 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
  • 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