Exploring the Science of Electricity

1
Lesson

• Exploring the Science of Electricity

2
Interest Approach

• What is Electricity?
• Where does electricity come from?
• How does it get from its source to where we use
  it?

3
Interest Approach

• What happens when you walk across carpet in the
  winter and receive a shock when you touch
  someone or something metal?

4
Interest Approach

• Static electricity is actually a transfer of
  electrons from you to the object you touch

5
Student Learning Objectives

• Relate electricity to the structure of elements
  and atoms.
• Explain conductors, insulators, and
  semiconductors.
• Explain the conventional and electron theories of
  electrical current flow.

6
Student Learning Objectives

• Describe how electricity can be generated by
  friction, heat, light, chemical reactions, and
  magnetism.
• Describe the difference between and applications
  of direct current (DC) and alternating current
  (AC) electricity.

7
Terms

• Alternating current
• Atoms
• Battery
• Conductors
• Conventional theory
• Cycle
• Direct current
• Electricity
• Electrodes

• Electrolyte
• Electromagnet
• Electron theory
• Electrons
• Elements
• Free electrons
• Friction
• Fuel cell
• Hertz
• Insulators

8
Terms

• Magnetic induction
• Neutrons
• Photovoltaic effect
• Polarity
• Primary batteries
• Protons
• Secondary batteries
• Semiconductors

• Solar (photo) cell
• Synchronous alternator
• Thermocouple
• Thermopile
• Valence

9
Objective 1

• How does electricity relate to the structure to
  of elements and atoms?

10
Elements

• Elements are substances that cannot be broken
  down into simpler substances using ordinary
  chemical methods.
• All matter is composed of one or more elements.

11
Atoms

• Atoms are the smallest units of an element.
• They cannot be divided further without losing
  physical and chemical properties.
• Atoms are composed of protons, neutrons, and
  electrons.

12
Protons and Neutrons

• Protons have a positive electrical charge, while
  neutrons have no electrical charge.

13
Protons and Neutrons

• Protons and neutrons make up the nucleus, or
  center of each atom.
• Thus the nucleus of each atom has a positive
  electrical charge.

14
Electrons

• Electrons have a negative electrical charge and
  orbit the nucleus of an atom in rings or shells.
• Since unlike charges attract each other, the
  electrons are held in orbit by the positively
  charged nucleus.

15
Electrons

• The outer ring of electrons is called the valence
  ring.

16
Copper As A Conductor

• The element copper, has 29 protons in its nucleus
  and 29 electrons which orbit its nucleus.
• The electrical charges cancel each other.

17
Artwork supplied with permission of Interstate
Publishers, Inc.
18
Copper as a Conductor

• The electrons actually orbit the nucleus in
  rings.
• The first or inner ring can hold a maximum of 2
  electrons, the second ring, a maximum of 8
  electrons, the third ring, a maximum of 18, the
  fourth ring, a maximum of 32, the fifth ring, a
  maximum of 50, and the sixth, a maximum of 72.

19
Copper as a Conductor

• Generally, the rings closest to the nucleus are
  filled before additional rings are started.
• Thus, copper has 1 electron in the fourth ring or
  valence ring (2 8 18 1).

20
Copper as a Conductor

• The number of electrons in the valence ring has
  a key role in determining the electrical
  characteristics of the element.

21
Free Electrons moving between copper atoms
Artwork supplied with permission of Interstate
Publishers, Inc.
22
What are conductors, insulators, and
semiconductors?

• Objective 2

23
Conductors, Insulators, and Semiconductors

• Conductors, insulators, and semiconductors are
  important in electricity and electronics.
• The following will help explain each

24
Conductors

• Materials that allow electricity to flow through
  them easily.
• Copper, aluminum, silver, and gold are good
  conductors.

25
Conductors

• Generally, nearly all metals are good electrical
  conductors.
• Any element or material having atoms with three
  or fewer electrons in its valence ring will be a
  conductor.

26
Insulators

• Materials that do not allow electricity to flow
  through them under normal conditions.
• Materials such as rubber, plastic, porcelain, and
  glass are all good electrical insulators.

27
Insulators

• Any element or material composed of atoms having
  five or more electrons in the valence ring will
  be an insulator.
• Insulators are used to confine the flow of
  electricity to desired paths.

28
Semiconductors

• Materials that are neither good conductors nor
  good insulators.
• Semiconductors are manufactured from elements
  having atoms with four electrons in their valence
  rings.
• Silicon and germanium are widely used in making
  semi-conducting materials.

29
Objective 3

• What are the conventional and electron theories
  of electrical current flow?

30
Electricity

• Electricity is the flow of electrons from atom to
  atom in a conductor.
• There are two different theories to help describe
  electricity

31
Conventional Theory

• Says that electricity is the flow of positively
  charged particles through a conductor.
• This is the older theory and was developed before
  scientists discovered the existence of electrons.
  
• With this theory, it was assumed that current
  flow in an electrical circuit was from positive
  to negative.

32
The direction of current flow according to
theconventional theory.
Artwork supplied with permission of Interstate
Publishers, Inc.
33
Electron Theory

• This is the accepted model of electrical current
  flow, however, because of tradition, the
  conventional theory is still widely used.
• Either theory may be used as long as it is used
  consistently.

34
Electron Theory

• The electron theory says that electricity is the
  flow of electrons through a conductor.
• Remember conductors have three or fewer electrons
  in their valence ring.

35
Electron Theory

• When there are so few electrons in the valence
  ring they are not held very tightly.
• These free electrons can be dislodged if
  sufficient external force is applied.

36
Electron Theory

• An example of external force is a battery, which
  has a positive terminal and a negative terminal.

37
The direction of current flow according to the
electron theory.
Artwork supplied with permission of Interstate
Publishers, Inc.
38
Electron Theory

• Since unlike charges attract each other, if a
  copper wire were attached to the two terminals,
  the negatively charged free electrons would be
  dislodged and pushed by the negative terminal and
  pulled by the positive terminal of the battery,
  causing the electrons to flow.
• This would continue until the battery discharges.

39
How can electricity be generated by friction,
heat, light, chemical reactions, and magnetism?

• Objective 4

40
How can electricity be generated by friction,
heat, light, chemical reactions, and magnetism?

• An external force must be applied to cause free
  electrons to flow through a conductor.
• This force is the production or generation of
  electricity, which can be generated in the
  following ways

41
Friction

• Friction is caused when two or more materials rub
  against each other.

42
Friction

• When this happens, some of the free electrons
  from one material are transferred to the other
  material, causing one of the materials to have a
  positive electrical charge and the other to have
  a negative electrical charge.

43
Friction

• When the negatively charged material touches a
  material with a neutral or positive charge, the
  excess electrons will flow to the second object.

44
Friction

• This can be demonstrated when you walk across
  carpet in the wintertime.
• As you walk across the carpet, the soles of your
  shoes rub over the carpet which transfers
  electrons from the carpet to your body.

45
Friction

• When you touch someone else or a doorknob, the
  excess electrons discharge through your hands
  resulting in a static electricity shock.
• Friction is not a practical method of generating
  electricity.

46
Thermocouple

• Heat or temperature differences can be used to
  generate electricity using a thermocouple.
• A thermocouple consists of two dissimilar metals,
  such as iron and nickel, joined together to form
  two junctions.

47
(No Transcript)
48
Thermocouple

• When heat is applied to one of the junctions, the
  difference in temperature between the junctions
  causes free electrons to flow from the iron wire
  into the nickel wire and away from the hot
  junction toward the cold junction.

49
Thermocouple

• To increase output, several thermocouples are
  often combined to form a device called a
  thermopile.
• These are often used in flame detectors, furnace
  safety valves, and in precision heat measurement
  devices.

50
Photovoltaic Effect

• Light from the sun can also be used to generate
  electricity.
• It requires a solar (or photo) cell for
  converting the light into electricity through a
  process known as the photovoltaic effect.

51
Photovoltaic Effect

• Certain materials (such as gallium, silicon, and
  cadmium sulfide) will convert light energy into
  electrical energy through the photovoltaic
  effect.
• The solar cell is made of a thin disk of silicon
  to which other chemicals have been added.

52
Photovoltaic Effect

• When light strikes the disk, electrons move from
  one side to the other side.
• The electrons move through the conductors and
  provide electrical energy to power the circuit
  load(s).
• Solar energy is used to power to such things as
  calculators and electric fence controllers.

53
Chemical

• Reactions between certain chemicals can be used
  to produce electricity.

54
Chemical

• First, a battery is made of two or more chemical
  cells connected together.
• Each cell is composed of two dissimilar metal
  plates called electrodes.
• They are separated from each other and immersed
  in an electrolyte.

55
(No Transcript)
56
Chemical

• An electrolyte is a chemical solution that
  contains positively and negatively charged atoms
  called ions.

57
Chemical

• Using a chemical cell that is composed of a zinc
  plate, a copper plate, and an electrolyte
  solution of sulfuric acid and water as an
  example, the acid/water solution reacts with the
  zinc plate causing it to lose positively charged
  ions.

58
Chemical

• Each positive ion lost leaves behind two
  electrons.
• Thus, the zinc plate soon takes on a negative
  charge.
• It then becomes the negative terminal of the
  cell.

59
Chemical

• As the positive ions move through the
  electrolyte, they collide with the copper plate.
• The positive ions attract free electrons from
  the copper plate.

60
Chemical

• Thus, the copper plate soon takes on a positive
  charge due to this loss of electrons and becomes
  the positive terminal of the cell.

61
Chemical

• If we connect a conductor to the two terminals,
  electrons will flow through the conductor from
  the zinc plate to the copper plate.

62
Chemical

• This flow will continue until the difference in
  charge between the two plates has dissipated.
• Once this happens, the cell will be discharged.

63
Chemical

• Batteries may be classified as primary batteries
  (carbon-zinc, alkaline, and mercury) which cannot
  be recharged or as secondary batteries (lead-acid
  and nickel-cadmium) which can be recharged.

64
Chemical

• A second chemical reaction to produce electricity
  is a fuel cell, which is similar to a battery,
  but different in the way the chemicals are
  supplied.

65
Chemical

• In a battery, the chemicals are built in.
• In a fuel cell, the chemicals are pumped into the
  cell from an external source.
• Currently, fuel cells are limited to military and
  space applications.

66
Magnetism

• The final method of generating electricity
  discussed here will be that of magnetism.

67
Magnetism

• If a bar magnet is suspended freely from a
  string, the magnet will turn until one end points
  north and the other end points south.
• The end pointing north is said to be the north
  pole of the magnet and the end pointing south the
  south pole.

68
Magnetism

• An electromagnet may also be created using
  electricity.
• If electricity flows through an insulated
  conductor that is wound around a metal object,
  the metal object will become a magnet.

69
Magnetism

• Any time a conductor cuts across a magnetic field
  or a magnetic field cuts across a conductor,
  electricity will flow in the conductor.
• Electricity generated in this manner is produced
  through the process of magnetic induction.

70
Generation of Electricity Through Magnetism
71
(No Transcript)
72
Magnetism

• This would be a very impractical method of
  generating electricity.
• Electricity could also be generated by rotating a
  magnetic field around a stationary conductor or
  by rotating a conductor inside stationary
  magnetic field.

73
Magnetism

• As the rotating conductor cuts across the lines
  of magnetism in the stationary magnetic field,
  electrons will be forced to flow through the
  conductor.

74
Magnetism

• As the conductor continues to rotate, the
  conductor will travel parallel to the lines of
  magnetism and no electrons will flow.

75
Magnetism

• As the conductor continues in its rotation, it
  will cut across the lines of magnetism in the
  opposite direction, causing the electrons in the
  conductor to flow in the opposite direction.
• The cycle continues as the rotation continues.

76
Magnetism

• In a commercial electricity generation power
  plant, a synchronous alternator is used to
  produce electricity.
• This device has a rotating field winding and a
  stationary winding.

77
Magnetism

• A small amount of electricity is supplied to the
  alternators field windings, which produces a
  magnetic field around the windings (the field
  windings become an electromagnet).

78
Magnetism

• As the field windings are turned (by a turbine
  driven by an external power source), the magnetic
  field also turns, cutting across the alternators
  stationary windings.

79
Magnetism

• Since the conductors are cutting across a
  magnetic field, electricity is induced into the
  stationary conductors.

80
Magnetism

• Power plants may burn coal or use nuclear energy
  to make steam from heated water in order to turn
  the turbine, or they may use the kinetic energy
  of falling water to turn the turbine.

81
What is the difference between direct current and
alternating current and what are some
applications of each?

• Objective 5

82
AC/DC

• Electricity may be classified as direct current
  or alternating current depending on the pattern
  of flow of the electrons in the circuit.

83
Direct Current

• In direct current or DC electricity, the
  electrons flow in only one direction.

84
Direct Current

• Since electrons flow from the negative terminal
  to the positive terminal in an electrical
  circuit, sources of DC electricity must have a
  fixed polarity.

85
Direct Current

• This means one specific terminal is always
  negative while the other is always positive.
• Thermocouples and thermopiles, solar cells,
  batteries and fuel cells all produce DC
  electricity.

86
Direct Current

• A DC generator may also be used to produce DC
  electricity through magnetism.

87
Alternating Current

• With alternating current or AC, electricity flows
  first in one direction, stops, reverses and flows
  in the opposite direction.

88
Alternating Current

• Once this occurs, the electricity is said to have
  completed one cycle.
• AC electricity is the type generated by electric
  power plants and is what is used in homes,
  businesses, and other locations.

89
Alternating Current

• One cycle of electrical flow is produced with
  each revolution of the plants synchronous
  alternator.
• In the U.S., they turn at a speed of 60
  revolutions per second.
• Therefore, current generated in the U.S.
  completes 60 cycles per second.

90
Alternating Current

• The term hertz (Hz) represents one cycle per
  second, so in the U.S. our electricity is
  generated and delivered at 60 Hz.
• In several other countries, 50 Hz AC electricity
  is the standard.

91
Review / Summary

• 1. Relate electricity to the structure of
  elements and atoms.

92
Review / Summary

• 2. Explain conductors, insulators, and
  semiconductors.

93
Review / Summary

• 3. Explain the conventional and electron theories
  of electrical current flow.

94
Review / Summary

• 4. Describe how electricity can be generated by
  friction, heat, light, chemical reactions, and
  magnetism.

95
Review / Summary

• 5. Describe the difference between and
  applications of direct current (DC) and
  alternating current (AC) electricity.