Physics, electromagnetism, charge model, charge, Insulators, conductors, Coulomb's Law, & the field model

1. General Physics
Physics 120
Chapter 26: ELECTRIC
CHARGES & FORCES
Antelope Valley College
Math & Sciences Dept
George Cross

2. ANNOUNCEMENTS

3. TODAYS LECTURE
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Introductions
How this Class Will Run
Syllabus
Class Safety
Chapter 26

4. TODAYS LECTURE
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Introductions
How this Class Will Run
Syllabus
Class Safety
Chapter 26

6. Physics 120
TEXT: PHYSICS for Scientists and Engineers
Second Edition: A Strategic Approach; Knight
Lecture: 6:00 – 10:05 PM Monday, SC3 168
Office:
6:00 – 6:50 PM Wednesday, SC2 136
Lab:
7:00 – 10:05 PM Wednesday, SC2 136
Course Goals – Learn and Understand
•Electrostatics, electric fields and potentials
•Resistance, capacitance, current, DC circuits
•Magnetic fields, electromagnetic induction
•Electromagnetic fields and waves, AC circuits
•Help you to Love Physics (Well, at least like it) as much as I do
•Have Fun while we learn

7. How We Learn
• Repetition (minimum of 7 times before it is
locked into memory)
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Read Chapter Before Coming to Class
Listen in Class
Discuss in Study Groups
Do Your Homework
• Tell it to Someone Else
– Participate in Study Groups
– Explain it to Someone
• Participation
– Participate in the Class Discussion & Activities
– Participate in Lab

8. How This Class Will Run
• It won’t work to sit in class and expect me to
pour knowledge into you. You must follow the
way we learn. Therefore:
– Read Chapter Before You Come to Class This will
help you to:
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Remember the material
Understand the lecture
Participate in the class and group discussion
Pass the Chapter Quiz at the beginning of class
– Participate in Class
– This Class Will not be a Typical Lecture
– This Class Will be a Combination of Lecture,
Demonstration, Discussion, Group Work with a
Workbook and so Forth that is Coupled with a Lab

10. Why Study Physics? What Good Will it Be in
My Life or Career?
• Physics is the science of how things work
• Physics plays a part in every technology
that has been devised by man
• Physics is infused into every aspect of
your life
– What you physically see/observe
– What you physically experience
– The consequences to your physical actions or
inaction
– What happens to the objects around you

11. Physics Can Help You Work In State Of The Art
Technological Laboratories Like This

12. Physics Can Help Prevent You From Doing Stupid
Things With Serious Health Consequences
Inertia Works!

13. TODAYS LECTURE
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Introductions
How this Class Will Run
Syllabus
Class Safety
Chapter 26

14. Warning About Disobeying Rules of Conduct

15. TODAYS LECTURE
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Introductions
How this Class Will Run
Syllabus
Class Safety
Chapter 26

16. WARNING: Carrying Calculators, Rulers etc.
Los Angeles County Teacher Arrested in New York Airport
A public school teacher was arrested today at John F. Kennedy International Airport as
he attempted to board a flight in possession of a ruler, a protractor, a compass, a slide
rule, and a calculator. At a morning press conference, the Attorney General said he
believes the man is a member of the notorious Al-Gebra Movement.
He did not identify the man, who has been charged by the FBI with carrying weapons of
Math Instruction.
“Al-Gebra is a problem for us,” the Attorney General said. “They derive solutions by
means and extremes, and sometimes go off on tangents in search of absolute values.”
They use secret code names like “X” and “Y” and refer to themselves as “unknowns.”
Their variable nature makes everyone leery of their actions as sometimes the equation
does not add up. We have determined that these operatives belong to a common
denominator with coordinates in every country. An examination of the matrix of their
operation, will allow the FBI to determine their function, and plan methods to FOIL their
maneuvers.
When asked to comment on the arrest, President Obama said, “If God had wanted us to
have better Weapons of Math Instruction, He would have given us more fingers and
toes.”

17. TODAYS LECTURE
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•
•
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Introductions
How this Class Will Run
Syllabus
Class Safety
Chapter 26

18. CHAPTER 26 QUIZ

19. 1. Which of the following is not true? The electric force
___ 1. decreases with the inverse of the square of the distance between two
charged particles.
___ 2. between an electron and a proton is much stronger than the
gravitational
force between them.
___ 3. between two protons separated by a distance d is larger than that
between
two electrons separated by the same distance d.
___ 4. may be either attractive or repulsive.
___ 5. Is the coolest new sci-fi tech TV show.

20. 2. A material that permits electric charge to move through it is called a(n)
___ 1. insulator.
___ 2. conductor.
___ 3. capacitor.
___ 4. incapacitator.
___ 5. inductor.

21. 3. When the electric charge on each of two charged particles is
doubled, the
electric force between them is
___ 1. doubled.
___ 2. quadrupled.
___ 3. the same.
___ 4. totally awesome!
___ 5. none of the above

22. 4. In any reaction involving charged particles, the total charge before
and
after the reaction is always the same. This relationship is known as
___ 1. quantization of charge.
___ 2. conservation of charge.
___ 3. status quo law of charge
___ 4. the law of induction.
___ 5. not covered in the reading assignment

23. 5. The net charge on any charged object is always an integer multiple
of the charge of the electron (e =1.6 x 1019 Coulombs). This relationship is known as
___ 1. quantization of charge.
___ 2. Coulomb’s Law.
___ 3. the law of induction.
___ 4. conservation of charge.
___ 5. Murphy’s Law
___ 6. not covered in the reading assignment

24. 1. Which of the following is not true? The electric force
___ 1. decreases with the inverse of the square of the distance between two
charged particles.
___ 2. between an electron and a proton is much stronger than the
gravitational
force between them.
_X_ 3. between two protons separated by a distance d is larger than that
between
two electrons separated by the same distance d.
___ 4. may be either attractive or repulsive.
_X_ 5. Is the coolest new sci-fi tech TV show.

25. 2. A material that permits electric charge to move through it is called a(n)
___ 1. insulator.
_X_ 2. conductor.
___ 3. capacitor.
___ 4. incapacitator.
___ 5. inductor.

26. 3. When the electric charge on each of two charged particles is
doubled, the
electric force between them is
___ 1. doubled.
_X_ 2. quadrupled.
___ 3. the same.
___ 4. totally awesome!
___ 5. none of the above

27. 4. In any reaction involving charged particles, the total charge before
and
after the reaction is always the same. This relationship is known as
___ 1. quantization of charge.
_X_ 2. conservation of charge.
___ 3. status quo law of charge
___ 4. the law of induction.
___ 5. not covered in the reading assignment

28. 5. The net charge on any charged object is always an integer multiple
of the charge of the electron (e =1.6 x 1019 Coulombs). This relationship is known as
_X_ 1. quantization of charge.
___ 2. Coulomb’s Law.
___ 3. the law of induction.
___ 4. conservation of charge.
___ 5. Murphy’s Law
___ 6. not covered in the reading assignment

29. Einstein said,
He also said that the
development of the
theory of electromagnetism
was “the most important
event in physics since
Newton’s time.”

30. ELECTRICITY & MAGNETISM
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Electric Charges & Forces
Electric Field
Gauss’s Law
Electric Potential
Potential & Field
Current & Resistance
Fundamentals of Circuits
Magnetic Field
Electromagnetic Induction
Electromagnetic Fields & Waves
AC Circuits

31. TODAYS LECTURE
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Developing a Charge Model
Charge
Insulators & Conductors
Coulomb’s Law
The Field Model

32. • Electricity Powers Your Modern Life
– Entertainment
• Audio
• Video
– Communication
• TV
• Radio
• Cell phones & Land Lines
– Transportation
– Light & heat

33. Developing a Charge Model
• Rubbing things together produces charges
& forces
• Three states of charge: a) positive, b)
negative and c) neutral (equal mix of
positive and negative charges)
• Like charges repel, opposites attract
• Force between charged objects is a long
distance force that decreases with
distance
• Charge can be transferred by contact

34. Electric Properties of Materials
• Charge can be transferred by contact
between objects
• Two types of materials: a) conductors
(conduct charge easily), b) insulators (do
not conduct charges easily)
• Both conductors and insulators can be
charged
• Conductors give up charge easily
• Insulators do not give up charge easily

35. Charge
• Positive charge & negative charge –
named by Benjamin Franklin
• Franklin named the charge on the glass
rod positive
• Any charge that repels the glass rod is
also positive and any that attracts the
glass rod is negative
• By convention, electrons are negatively
charged and protons are positively
charged
• Developed without knowledge of atoms

36. Electron orbital frequencies
~ 1015 rev/minute
This atom is neutral in charge
since it has the same number
of protons and
electrons.
e is called the
fundamental charge
~10-14 m
•Protons
•1.67x10-27 kg
•+e
•Electrons
•9.11x10-31 kg
•-e

37. The Micro/Macro Connection
• Electrons & protons – basic charges of
ordinary matter
– Particles
– Follow Newton’s Laws of Motion
• Charge represented by symbol “q”
• Object is charged if the total number of
electrons and protons are unequal
• Objects charge will always be an integer
multiple of e (charge quantization)

38. The Micro/Macro Connection
• Objects acquire charge by gaining or
loosing (ionization) electrons
– Atoms missing electrons – positive ions
– Atoms with extra electrons – negative ions
• Friction causes charging (ionization)
– Molecular bonds broken at surface
– One part of molecule looses an electron
– One part of molecule gains an electron
– Ions are on opposing objects
• Fur (positive) & rubber rod (negative)
• Silk (negative) & glass rod (positive)

40. Charge Conservation
• Laws of Conservation
– Conservation of Energy
– Conservation of linear and rotational momentum
– Conservation of charge
• Law of Conservation of Charge:
Charge is neither created or destroyed.
Charge can be transferred from one object
to another as electrons and ions move
about, but the total amount of charge
remains constant.
(Net charge of fur and rubber rod is zero)

41. Example of charge diagrams. Draw a simple two-dimensional cross-section
Of the object/s. Draw only the net charge (neutral should show no charges).
Charge on conductor will only be at the surface. Conserve charge from one
diagram to the next if multiple diagrams are needed.

42. Insulators & Conductors
• Insulator - Electrons tightly bound to
nucleus and cannot move about
• Conductor – Outer electrons (valence
electrons) are weakly bound. When
atoms come together to form a solid,
these valence electrons are shared and
are then free to move about or wander
through the entire solid. Similar to an
array of positive ion cores with negatively
charged fluid moving about in between
(sea of electrons)

43. Charging an insulator
By friction leaves patches
of charge on surface.
These patches of charge
are not mobile and the
charge does not spread
out.
Electrons in a metal are
highly mobile.
In a metal the mobile valence
electrons are called charge
carriers.

44. Insulators & Conductors – Cont.
• Conductors
– Metals – charge conductors are electrons
– Ionic solutions – charge conductors are
positive and negative ions
– Plasma – charge conductors are electrons
and nuclei

45. Charging
Metal objects usually cannot be charged by rubbing, but by coming into
contact with a charged object.

46. Electrons in a conductor are free to move.
When charge is transferred, electrons repel
each other and rapidly move to redistribute
themselves accordingly (entire sea shifted
to side and then redistributes). This response
Is virtually instantaneous.
Other than this brief adjustment period when
charging is happening, the charges in a
conductor are in static equilibrium. This means
that the charges are at rest because there is no
net force acting on any of them.
“Electrostatic Equilibrium”
In an isolated conductor, any excess charge is
located on the surface of the conductor.

47. Electroscope

48. Since the electroscope is metal, all excess charges move as far
apart as possible. As a result, the leaves become charged as well
And repel each other since they are of like charge.

49. Discharging
• Charging happens as a result of friction
• Discharging happens by contact
(touching)
– A conductor, or reasonable conductor
touching a charged object removes or
conducts away charge
– Things that can remove charge
• Water – most water contains ions making it a good
conductor
• Humans – mostly salt water – therefore, good
conductor
• Grounding straps
• Air – poor conductor but some charged ions which
will gradually discharge an object exposed to the
air
• Earth – “grounded”

50. Human – fair conductor. Charges will
quickly spread over entire conductive
area which now includes the human.
Not much charge will be left on the metal
when person removes their hand.
Insulators will not give up their charge
easily since the charges are not free to
move. They may, but in the small area
in contact with the discharging object.
An object connected to the earth will share
its charge with entire earth and will
become completely discharged.
Grounding objects prevents the build up of
excess charge in electric and electronic
circuits.

51. Charge Polarization
• Charging happens as a result of friction
• Discharging happens by contact
(touching)
– A conductor, or reasonable conductor
touching a charged object removes or
conducts away charge
– Things that can remove charge
• Water – most water contains ions making it a good
conductor
• Humans – mostly salt water – therefore, good
conductor
• Grounding straps
• Air – poor conductor but some charged ions which
will gradually discharge an object exposed to the
air
• Earth – “grounded”

52. Charge Polarization

53. Charge Polarization
Charge polarization is the slight separation of positive and negative charges
in a neutral object.

54. Charge Polarization
• Equilibrium position for electrons is such
that force of nearby charged object and
force from positive nuclei is balanced
– Nuclei exert restoring force on electrons
– Actual position shift is typically < 10-15m

55. Polarization Force
Polarization force arises due to separation of charge in the metal

56. The Electric Dipole
Why does a charged object attract neutral insulators?
Also an example of polarization force.
This slight separation of charge is called an electric dipole.

58. A charged object can pick up paper because it:
•Polarizes atoms in paper
•Exerts attractive polarization
force on each atom

59. Charging by Induction

60. Coulomb’s Law
• Basic law of electric force
• Explains why
– Adding more charge increases the
repulsive/attractive force
– Increasing the distance reduces the
repulsive/attractive force
• Some debate as to whether he discovered
the inverse square law or made a good
guess, but he was correct
K is the electrostatic constant
Law also says that the force is an action/
reaction pair acting along the line joining the
two charges and that the force is repulsive
For like charges and attractive for opposites.

61. Coulomb’s Law – Cont.
Similar to Newton’s Law of Gravitation
but:
Charges can be positive or negative
therefore the absolute value signs give
us the magnitude. The second part of
Coulomb’s Law tells us the direction of
the forces on the respective charges.

62. •
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Coulomb’s Law & Units of Charge
Coulomb had no unit of charge so he
could not define the electrostatic constant
(numerical value depends on units of
distance and charge)
Fundamental unit of charge:
e = 1.60x10-19 C (C is for Coulomb)
or… 1C = 6.25x1018 protons
Typical charge on rubber rod is 10-9C to 107
C or an excess of 1010 or 1012 electrons
K = 8.99x109 Nm2/C2

63. Coulomb’s Law – Cont.
• We can make Coulomb’s Law more useful
for later chapters by rewriting it
– Define permittivity constant εo (epsilon zero)
– εo = 1/4πK = 8.85x10-12 C2/Nm2
Now let’s use Coulomb’s Law!

69. The Field Model
• Electric & magnetic forces are long range forces
• Despite the success of Newton’s Law of
Gravitation, scientists of the time still had great
difficulty with forces at a distance
• Faraday postulated that the object producing the
field changed the space around it in some way
and that the behavior of a charged object was in
response to the change in the space & affects all
space
• Faraday also postulated that the effects of field
took a finite time (not instantaneous) to
propagate

71. The Field Model – Cont.
• Faraday was not taken seriously at first –
many scientists thought it was just a
pictorial gimmick
• Over time scientists realized that the
concept of field was required to
understand the behavior of
electromagnetics
• Put on a mathematical basis in 1865 by
James Clerk Maxwell
– Explained all known electromagnetic behavior
in four equations known as “Maxwell’s
Equations” (Chapter 35)

72. Gravity is a Field Force
•r is measured from the center of mass to
center of mass
•Force acts as if all mass concentrated at
center of mass
•Action reaction force pair
•Force is in direction of center of mass of object
exerting the force

73. Gravity is a Field Force
All objects of mass are
gravitational sinks field lines are in toward
Center of mass

75. Gravity is a Field Force

76. Gravity is a Field Force
This is approximately true for all locations near the
Earth’s surface.

77. The Electric Field Model
• We will call some charges source charges
(electric field source) and others sink
charges
• A separate charge in the electric field
experiences a force exerted by the field
• Force proportional to strength of field
• “E” is called the electric field strength

78. How Do We Know There is an Electric Field?
Put a point charge at location of
interest and see if there is a force
on it

79. The Electric Field Model
• From any given charge there is an electric
field vector defined for every point in
space
• If q of the test charge is positive, the force
will be in direction of field vector at that
point
• Electric field does not depend on size of
test charge – force dependant upon q, so
field is independent.
• Fon q = qE

80. Point charge q creates field.
Point charge q’ to test the field
made by q.
By measuring the force on q’
we can determine the
direction of E at that point.
We note that no matter where
we place q’, the field will
always point out away from q.
F = (1/4πεo)qq’/r2
E = F/q’ = (1/4πεo)q/r2

81. Positive Charge: Field Source
Notice how the arrows for
the field get smaller the
Farther out they go. This
Is due to the fact that the
field for a point charge
drops with the square
of the distance.
Arrows are only a
representation of the
field. The field exists at
all points. The lengths are
representative of the field
strength at that point (Iwhere
the dot is).

82. The Electric Field of a Point Charge
• We will use vector notation to make our
equation a little more useful and allow use
of positive or negative charges

83. Position (Location) Vectors

84. Electric Field Vectors

85. Negative Charge: Field Sink

86. Backup Slides