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## Welcome to Physics 112

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### Welcome to Physics 112 Instructor: Mike Talbot Web Page : delta.edu/mttalbot/physics112 Electric Forces and Electric Fields Chapter 15 Introduction Greek ... – PowerPoint PPT presentation

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Title: Welcome to Physics 112

1
Welcome to Physics 112
• Instructor Mike Talbot
• Web Page
• delta.edu/mttalbot/physics112

2
Electric Forces and Electric Fields
• Chapter 15

3
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4
Introduction
• Greek contributions (700 BC)
• Friction generates electricity
• Rub a piece of amber with wool

61
5
Introduction
• Charged particles
• What are they?
• What colors are they?

6
Introduction
• Coulombs Law
• What does it say?

7
Introduction
• Electric fields
• What is a gravitational field?
• What is an electric field?

8
Properties of Electric Charges
• An electroscope may be used to demonstrate the
existence of electrostatic forces.
• It detects and identifies charges produced by
such things as
• Plastic combs
• Glass and silk
• Hard rubber and wool
• Balloons
• 62, 15.1

9
Properties of Electric Charges
• Walking across a carpet in the winter can also
generate static electricity.

10
Properties of Electric Charges
• Dust accumulates on a TV screen because of the
large static charge on the glass.

11
Properties of Electric Charges
• What should you do if a power line falls on your
car while you are in it?

12
Properties of Electric Charges
• How many kinds of charge exist?

13
Properties of Electric Charges
• How many kinds of charge exist?
• Two

14
Properties of Electric Charges
• Two kinds of charge ( and -)
• Named by Benjamin Franklin

15
Properties of Electric Charges
• Law of Charges
• 155

16
Properties of Electric Charges
• Law of Charges
• At least two particles are involved
• Like charges repel, opposites attract.
• Motion may result

17
Properties of Electric Charges
• Law of Charges
• At least two particles are involved
• Like charges repel, opposites attract.
• Motion may result
• Planetary model of the atom
• 125

18
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19
Properties of Electric Charges
• Are atoms neutral?

20
Properties of Electric Charges
• What is meant by the conservation of charge?

21
Properties of Electric Charges
• What is meant by the quantization of charge (e)

22
Properties of Electric Charges
• Demonstrating the quantization of charge (e)
• The Millikan Oil-Drop Experiment
• Measured the elementary charge on an electron
• 15.21

23
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24
Insulators and Conductors
• Materials may be classified by their ability to
conduct electricity.

25
Insulators and Conductors
• Materials may be classified by their ability to
conduct electricity.
• Conductors (many free electrons)
• Insulators (few free electrons)

26
Insulators and Conductors
• Materials may be classified by their ability to
conduct electricity.
• Conductors (many free electrons)
• Insulators (few free electrons)
• Semiconductors

27
Insulators and Conductors
• What are some examples of good conductors?

28
Insulators and Conductors
• What are some examples of good insulators?

29
Insulators and Conductors
• What is an example of a semiconductor?
• Where are semiconductors used?
• 225

30
Charging by Friction
31
Charging by conduction
• 15.3

32
Charging by induction
• 15.4

33
Grounding
• 127

34
Insulators and Conductors
• Polarization of charge

35
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36
Coulombs Law
• An electric force has three properties
• 15.6

37
Coulombs Law
• An electric force has three properties
• It is attractive or repulsive depending upon the
sign of the charges.

38
Coulombs Law
• An electric force has three properties
• It is attractive or repulsive depending upon the
sign of the charges.
• It is directly proportional to the product of the
magnitudes of the charges (q1.q2).

39
Coulombs Law
• An electric force has three properties
• It is attractive or repulsive depending upon the
sign of the charges.
• It is directly proportional to the product of the
magnitudes of the charges (q1.q2).
• It is inversely proportional to the square of the
separation (r2).

40
Coulombs Law
• Coulombs formula

41
Coulombs Law
• Definitions
• Coulomb
• The amount of charge that has passed a given
point in one second when one ampere of current is
flowing

42
Coulombs Law
• Definitions
• Ampere
• One coulomb of charge passing a given point in
one second
• Analogy Like water flowing through a pipe

43
Coulombs Law
• Coulombs constant
• ke 8.9875 x 10 9 N.m2/C2

44
Coulombs Law
• Charges and masses of particles
• Table 15.1 (pg. 501)

45
Coulombs Law
• Reminders
• Force is a vector quantity
• r is the distance between centers
• Newtons Third Law applies
• The Coulomb force is a field force just like ?

46
Coulombs Law
• Reminders
• Force is a vector quantity.
• r is the distance between centers.
• Newtons Third Law applies.
• The Coulomb force is a field force just like
gravity.

47
Coulombs Law
• The two field force formulas are mathematically
identical.

48
Coulombs Law
• The two field force formulas are mathematically
identical.
• F k q1q2/r2

49
Coulombs Law
• The two field force formulas are mathematically
identical.
• F k q1q2/r2
• F G m1m2/r2

50
Coulombs Law
• Differences between electrical and gravitational
forces
• Gravity only attracts

51
Coulombs Law
• Differences between electrical and gravitational
force
• Gravity only attracts
• Gravity is weaker

52
Coulombs Law
• The Superposition Principle may be used to find
the resultant force between two charges
• In a straight line
• Not in a straight line
• Trigonometry is required

53
• QUESTIONS
• 1 7
• Pg. 525

54
The Electric Field
• The concept of an electric field was first
• 15.10, 15.11

55
The Electric Field
• Description of an electric field
• 157, 158, 159, 161

56
The Electric Field
• Effects upon particles entering an electric field
• Charged particles
• 156

57
The Electric Field
• Effects upon particles entering an electric field
• Charged particles
• Uncharged particles

58
The Electric Field
• Electric field strength
• E F/qo
• Gravitational field strength
• ?

59
The Electric Field
• Electric field strength
• E F/qo
• Gravitational field strength
• g F/m

60
The Electric Field
• The magnitude of the electric field due to charge
q can also be found by using

61
The Electric Field
• The electric field is a vector quantity
• Magnitude (N/C)
• Direction

62
The Electric Field
• Direction (defined)
• The direction of the force on a small test
charge

63
Electric Field Lines
• Point in the direction of the electric field.
• 160

64
Electric Field Lines
• Point in the direction of the electric field
• Are tangent to the electric field vector (E)

65
Electric Field Lines
• Point in the direction of the electric field
• Are tangent to the electric field vector (E)
• The number of lines per unit area are
proportional to the electric field strength
• 15.16

66
Electric Field Lines
• Point in the direction of the electric field
• Are tangent to the electric field vector (E)
• The number of lines per unit area are
proportional to the electric field strength
• Are always directed away from a charge

67
Electric Field Lines
• Point in the direction of the electric field
• Are tangent to the electric field vector (E)
• The number of lines per unit area are
proportional to the electric field strength
• Are always directed away from a charge
• Are always closer together near the charge

68
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69
Electric Field Lines
• For an electric dipole
• The number of lines leaving a charge must equal
the number of lines entering a - charge

70
Electric Field Lines
• For an electric dipole
• The number of lines leaving one charge must equal
the number of lines entering a second charge
• The lines can never cross

71
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72
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73
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74
What is the ratio of q1 to q2?
75
Conductors In Electrostatic Equilibrium
• In electrostatic equilibrium
• There is no net motion of charges within the
conductor
• 15.20

76
Conductors in Electrostatic Equilibrium
• Properties of an isolated Conductor

77
Conductors in Electrostatic Equilibrium
• Properties of an isolated Conductor
• The electric field is zero everywhere inside the
conductor

78
Conductors in Electrostatic Equilibrium
• Properties of an isolated Conductor
• The electric field is zero everywhere inside the
conductor
• Any excess charge resides on the surface

79
Conductors in Electrostatic Equilibrium
• Properties of an isolated Conductor
• The electric field is zero everywhere inside the
conductor
• Any excess charge resides on the surface
• The outside electric field is always
perpendicular to the surface

80
Conductors in Electrostatic Equilibrium
• Properties of an isolated Conductor
• The electric field is zero everywhere inside the
conductor
• Any excess charge resides on the surface
• The outside electric field is always
perpendicular to the surface
• The charge tends to accumulate at the sharpest
points

81
Conductors In Electrostatic Equilibrium
• More applications involving static electricity
• Lightning rods
• Tesla coils
• Van De Graaff generators

82
Parallel Plate Capacitor
• The device consists of plates of positive and
negative charge
• The total electric field between the plates is
given by
• The field outside the plates is zero

83
The Van De Graaff Generator
• How does it work?
• 15.23

84
Van de Graaff Generator
• An electrostatic generator designed and built by
Robert J. Van de Graaff in 1929
• Charge is transferred to the dome by means of a
rotating belt
• Eventually an electrostatic discharge takes place

85
The Oscilloscope
• Creates a visual display of waveforms from
external applications

86
The Oscilloscope
• Uses a CRT (cathode ray tube) similar to those
found in radar systems, computers, televisions,
and hospital monitors
• 15.24

87
The Oscilloscope
• How does it work?
• Electron gun
• Filament
• Cathode
• Anode
• Horizontal and vertical deflection plates
• Screen

88
• QUESTIONS
• 9 - 13
• Pg. 525