Chapter 21 Electromagnetism

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Chapter 21 Electromagnetism

• 21-1 Magnetism from Electricity
• Electromagnetism relationship between
  magnetism and electricity
• Oersted (1820) Danish physicist who first
  discovered that an electric current produces a
  magnetic field.

Example of Oersteds Discovery
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• Solenoid long coil of wire with a current
  passing through wrapped into many loops creating
  a magnetic field. Acts like a magnet when a
  current is passing through. Strongest at the ends
  like a magnet.
• Increase the strength of the magnetic field
• increasing the amount of coils and/or
• 2) increasing the amount of current.
• OR by adding magnetic material between the wires.
• An electro-magnet solenoid with magnetic
  material (iron nail) inside the coils. Greatly
  increases the strength.
• Strong temporary magnets

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• Just as an electric current exerts a force on a
  magnet. A magnetic field exerts a force on an
  electric current (charged particles)
• Practical devices Electric motors
• converts electrical energy into mechanical
  energy. Contains a coil of wire connected to a
  cylinder (armature). Armature free to spin
  between the poles of a magnet.

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Galvanometers instruments used to detect small
currents (coil of wire connected to an electric
circuit and a needle)Examples ammeters and
voltmeters
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21-2 Electricity from Magnetism

• Faraday 1831 Englishman and Henry an American
  first to prove that electricity can be produced
  from magnetism.
• Electromagnetic Induction - Faraday noticed that
  changing the magnetic field produced an
  electrical current
• induced current.

Faradays experiment
Another experiment illustrating electromagnetic
induction
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• An electric current will be produced (induced) in
  a circuit exposed to a changing magnetic field.
• The direction of the current depends on the
  direction of the moving magnet.

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• Generators converts mechanical energy into
  electrical energy opposite of a motor
  (electrical energy into mechanical energy)
• Consist of loops of wire connected to a rotating
  rod.

AC generator
DC generator
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• In large generators turbines provide the
  mechanical energy to turn the axles. Either
  moving wind, water or steam turn the turbines.
• Wind power, hydroelectric power, nuclear power or
  electricity from burning of fossil fuels - coal
  and oil.

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• Transformers devices that increase or decrease
  the voltage of alternating current. In US 60
  hertz
• Consists of 2 coils wrapped around an iron core.
  AC current passes through first coil, inducing a
  current in second wire.
• Step-up transformers when primary coil has less
  loops than second coil. Voltage in second coil
  greater than first coil step-up transformer
  (increases voltage)

• Step-down transformers more loops in first
  coil, less in second voltage in secondary coil
  less.

• Power can be conserved if voltage is stepped up
  before transmission and stepped down before being
  used by consumer

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