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Welcome to Physics 112

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Title: Electric Forces and Electric Fields Author: Michael Talbot Last modified by: Kegen McDermott Created Date: 7/4/2001 2:52:24 AM Document presentation format – 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
    introduced by Michael Faraday.
  • 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
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