Magnetism and Electromagnetism

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Magnetism and Electromagnetism

• SCI 201

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Driving Questions

• What kinds of forces do magnets exert?
• What kinds of materials do magnets attract or
  repel?
• Can magnets exert forces through some substances
  and not others?
• How does the strength of a magnet vary with
  distance?
• How can objects exert forces on one another
  without being in contact?

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Magnetism Stations

• Go through the magnetism stations and complete
  the hand-out
• What can you say for sure about magnets and
  magnetism now and what evidence do you have?

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Magnetism Inquiry

• How can the strength of the force of attraction
  between two magnets be measured?
• Does distance affect the strength of a magnet? If
  so, how?

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The basics of magnetism

• Named for Magnesia, an island in the Aegean Sea
  gt2000 years ago
• Lodestones or magnetite, Fe2O3
• Chinese used magnets as compasses in the 12th
  century
• 1750 in England, John Mitchell shows that
  magnetic poles obey the inverse-square law
• 1820 Oersted discovered that magnetism is related
  to electricity Ampere later proposed that
  electric currents are the source of all magnetism

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Magnetic Forces and Poles

• Like the electrostatic force it can repel or
  attract
• regions of magnets called magnetic poles give
  rise to magnetic forces
• All magnets have both a north and south pole
• Like poles repel, opposite poles attract
• No magnetic monopoles exist in nature

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Magnetic Fields

• The space around a magnet is altered the
  alteration is called a magnetic field
• Lines of force always in closed loops
• Bar magnet
• Horseshoe magnet
• Unlike poles
• Like poles

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Magnetic Field of a Bar Magnet
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Earth has a magnetic field
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Magnetic Field of a Horseshoe Magnet
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Driving Questions

• What is the nature of magnetism?
• What is the relationship between electricity and
  magnetism?
• What applications exist that utilize the
  relationship between electricity and magnetism?

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Magnetic materials
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Magnetic materials (contd)

• Magnetic force is due to the motion of electric
  charges
• Magnetism is produced by two motions of
  electrons their spin and their orbiting motion
  about the nucleus
• The space around a moving electric charge is
  altered the alteration is called a magnetic
  field
• In many substances, magnetism produced cancels
  out
• In some materials such cancellation does not
  occur called Ferromagnetic materials such as
  iron, nickel, cobalt, and alloys e.g. alnico an
  alloy of aluminum nickel - cobalt

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Electromagnetism

• 1820, Oersted finds that a current carrying wire
  will cause a compass needle to deflect
• Implication the current carrying wire itself
  behaves like a magnet (since it can exert a force
  on another magnet like a permanent magnet would)
• Lets use field theory to understand whats
  happening

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Magnetic Field for a coil of wire called a
solenoid
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Magnetic Field for Solenoid with an Iron Core
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Make an electromagnet

• materials
• battery
• switch
• long wire made into a solenoid
• nail
• washers

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Inquiry

• In what way, if any, is the number of coils in
  the electromagnet related to its strength?
• Propose an experiment to test a hypothesis.

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Another interesting application electric motors

• Principle underlying electric motors
• Lets build one from simple materials
• battery
• insulated wire
• Small, cylindrical magnet
• Small nail
• Aluminum foil (optional)

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Electromagnetism (contd)

• 1820, Oersted finds that a current carrying wire
  will cause a compass needle to deflect so there
  was a discovery waiting to be made
• 1832, Michael Faraday found that a changing
  magnetic field could produce an electric current
  electromagnetic induction
• Principle underlying electric generators

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Contrasting Electric motors and generators

• Electric motors convert electrical energy into
  mechanical energy the motion of electrons into
  the motion of something else
• Electric generators convert mechanical energy
  into electrical energy the motion of something
  into the motion of electrons

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Driving Questions

• What is the nature of magnetism?
• What is the relationship between electricity and
  magnetism?
• What applications exist that utilize the
  relationship between electricity and magnetism?