Pearson Prentice Hall Physical Science: Concepts in Action

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Pearson Prentice Hall Physical Science Concepts
in Action

• Chapter 21
• Magnetism

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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

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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

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Magnetic Fields

• Def a magnetic field surrounds a magnet and can
  exert magnetic forces
• A magnetic field is strongest near a magnets
  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

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• 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)

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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

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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

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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

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• 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

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• 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

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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

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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

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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
  field

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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

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• 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

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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

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• 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)

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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