Chapter 21

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Chapter 21 Electricity and Magnetism at Work

• All magnetism is created by a moving charge
• Electrons moving through a wire (electricity)
  created a magnetic field.
• A magnet can make the wire move by repelling or
  attracting the electricitys magnetic field.

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• The commutator in an electric motor changes the
  direction of the electric current every ½ turn to
  keep the coil turning (see pg. 678)
• The brushes are the contact point to the
  commutator.
• The many coils around an iron core is called an
  armature.

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• Galvanometer used to measure small currents. It
  uses the same principle as an electric motor.

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• Electric motor uses current to turn an axle
• Electric motor converts electrical energy into
  mechanical energy
• Mechanical energy is the energy of an object
  due to its movement or position.
• Work energy is mechanical energy

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• Generators
• Turning a coil of wire through a magnetic field
  causes electrons in the wire to move
• Turning the shaft of an electric motor will make
  electrons move through the wire
• This is generating electricity
• Electromagnetic induction the process of
  generating an electric current from the motion of
  a conductor through a field.

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• Generators convert mechanical energy into
  electrical energy
• Alternating current as the coil is turned, the
  electrons pulse back and forth as they are
  attracted to one magnetic pole and then the
  other.
• How can electrical contact be maintained to a
  turning shaft (by slip rings) see page 684
  brushes

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• Generating Electricity On a Big Scale (electrical
  power plants)
• Need a major power source such as water, wind,
  coal or nuclear

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• Wind turns blade on a windmill and water turns a
  turbine
• Coal and nuclear make steam which turns the
  turbine
• (see energy sources pg. 688)

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• Renewable Resource one that is replaceable by
  nature (wind, water, wood, solar)
• Non-renewable energy source one that is not
  replaced quickly (coal, petroleum, uranium)

Coal Mining Uranium Sample
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• Energy and Power
• Power - how fast energy is being supplied or
  consumed
• P V (I) I P / V
• 1000 watts 1 kilowatt
• Energy power (time)
• Energy kilowatts (hours)

watts
volts
amps
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• How much energy does a 300 watt light use if it
  is on for 10 hours?
• E P (time)
• Energy .3 kilowatts (10 hours)
• Energy 3 kilowatt hours
• Transporting electricity It is first brought to
  a very high voltage to reduce line loss, then it
  is brought to 120 volts for home use.
• Step-up transformer increases the voltage
• Step-down transformer decreases the voltage

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• Metals tend to give up electrons
• Non-metals and positive ions attract electrons
• All chemical reactions are competitions for
  electrons
• In a battery the metal that provides electrons is
  kept apart from the materials that accept
  electrons
• The electrons must travel through a wire to get
  from one to the other.

Electrochemical Cells and Batteries
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• A metal and a chemical that attracts electrons
  make a cell
• The voltage of the cell is the difference in
  attraction between the metal and the other
  compound
• The voltage of a cell is small (about 1.5 or 2
  volts).
• Cells are added in series to make a battery of
  higher voltage
• A 12 volt car battery is made from six two volt
  cells.

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• pg 691 practice problems 1 2 (Read War of
  the Currents on page 694)
• page 696 questions 1,2,4
• page 701 questions 1-4