suggested exercises: 25.1, 25.2, 25.6, 25.7, 25.19, 25.20
TRANSCRIPT
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Homework
Reading: Chap. 25 and Chap. 26
Suggested exercises: 25.1, 25.2, 25.6, 25.7,
25.10, 25.13, 25.14, 25.15, 25.17, 25.18,
25.19, 25.20, 25.21, 25.22, 25.24, 25.26, 25.27
Problems: 25.29, 25.31, 25.33, 25.36, 25.39,
25.43, 25.49, 25.51, 25.55, 25.58, (due:
Mon., Sept. 28)
Chapter 26. Electric Charges and Forces
The electric force is one of
the fundamental forces of
nature. Controlled electricity
is the cornerstone of our
modern, technological
society.
Chapter Goal: To develop
a basic understanding of
electric phenomena in terms
of charges, forces, and
fields.
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Chapter 26. Basic Concepts
• Electric charge and their interactions
• Coulomb’s law
• Conductor, insulator, semiconductor
• Electric field
Forces
What kinds of forces we have learnt so far?
Gravitational force (G)
Tension (EM)
Spring (elastic force) (EM)
Friction (EM)
Normal (support) (EM)
The Electromagnetic Force (EM)
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Charge Model
10-10 m 1012 m
Attraction between the positively charged nucleus and the negatively charged electrons
Charge Model
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Atoms and Electricity
• An atom consists of a very small and dense
nucleus surrounded by much less massive orbiting electrons.
• The nucleus is a composite structure consisting
of protons, positively charged particles, and neutral
neutrons.
• The atom is held together by the attractive electric
force between the positive nucleus and the negative
electrons.
• Electrons and protons have charges of opposite sign
but exactly equal magnitude, they carry the smallest unit of
charge.
• This atomic-level unit of charge, called the
fundamental unit of charge, is represented by the symbol e.
Charge Quantization
e = 1.60×10-19 C
where Np and Ne are the number of protons and electrons contained in
the object
• A macroscopic object has net charge
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Electric Charge
• A fundamental property of matter
• Charge is of two types: positive + and negative –
• Like charges repel; unlike charges attract
Unlike charges attract
Like charged repel
Electric Charges
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Material Properties
Conductors
Insulators
Semiconductors
Superconductors
Insulators
Insulators
--
--
Charges are isolated in insulators.
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
+
+
+
-
-
-
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Insulators
• The electrons in the insulator are
all tightly bound to the positive
nuclei and not free to move around.
• Charging an insulator by friction
leaves patches of molecular ions on
the surface, but these patches
are immobile.
--
-
--
--
insulator
The Electric Dipole
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The Electric Dipole
Conductors
--
--
-
-
-+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Charges move about easily in conductors.
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Conductors
• In metals, the outer atomic electrons
are only weakly bound to the nuclei.
• These outer electrons
become detached from their
parent nuclei and are free to
wander about through the entire solid.
• The solid as a whole
remains electrically neutral, but
the electrons are now like a negatively
charged liquid permeating an array
of positively charged ion cores.
Charges Transfer in Conductor
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Charges Transfer in Conductor
Transfer Charges to A Metal Ball
Before touching a solid ball Before touching a hallow ball
- --
-
-
--
-
--
-
-
-
-
-
-
-
-
--
-
-
After touching a solid ball
----
-----
---
-
---
--
-- -
--
-
After touching a hallow ball
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Thought Quiz:
How can a charged balloon stick to an uncharged wall?
-
-
-
-
-
-
+
++
Induction
-
--
Charge Polarization
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Charge Polarization
Electrostatic force
𝐹 =𝑞1𝑞2
4𝜋𝜀0𝑟2 𝑟
𝑞1
𝑞2
𝑟 𝑟
𝑭
Source charge
Test charge
• A vector Unit: Newton
• Determined by both source charge and test charge
• If 𝑞1𝑞2 > 0, the direction is 𝑟; if 𝑞1𝑞2 < 0, direction
is − 𝑟.
𝜀0 = 8.85 × 10−12𝐶2/𝑁 ∙ 𝑚2
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The Force Analysis for a Charged Car
+ -+
q1 q2q3
The Equilibrium Position for a Charged Car
+ ++
q1 q2q3
2L
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The Motion of a Charged Car
+ ++
Q q Q
2L
In-class Activity #1
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After removing the conducting wire, what happens to the
charge distribution of the metal ball? How about after removing
the charged rod?
In-Class Activity #2
In-class Activity #3
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In-class Activity #4
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In the figure, two tiny conducting balls of
identical mass m and identical charge q hand
from nonconducting threads of length L.
Assume that is so small that tan can be
replaced by its approximate equal, sin.
Show that, for equilibrium,
3/1
0
2
2
mg
Lqx
Example #3
The figure shows a long, nonconducting, massless rod of length L,
pivoted at its center and balanced with a block of weight W at a
distance x from the left end. At the left and right ends of the rod are
attached small conducting spheres with positive charges q and 2q,
respectively. A distance h directly beneath each of these sphere is a
fixed sphere with positive charge Q. (a) Find the distance x when
the rod is horizontal and balanced. (b) What value should h have so
that the rod exerts no vertical force on the bearing when the rod is
horizontal and balanced?
Example #4
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The Electric Field
rF ˆ4 2
0
21
r
q1
Source
charge
r̂
Pq2
Test
charge
The magnitude of the electrostatic
force is proportional to the test charge
2qF
There is a fundamental quantity that can represent this kind of
interaction: It should only depend on the source and distance:
Electric field: E
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The Electric Field
Superposition principle:
P
q1 q2 q3 qm
E = E1 + E2 + … + Em
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Electric Field of a Positive Point Charge
• The picture shows the electric field vector at
several points.
Visualization: Electric Field Lines
• Electric field lines show the direction of E at any point (tangent).
• The magnitude of E is proportional to the density of electric field lines.
• Electric field lines originate on positive charges and terminate on negative charges, unless they extend to infinity.
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Electric Field Lines for a Negative Point Charge
Electric Field Lines of Two Charged Particles
Act#1
& 2
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Electric Field Lines for a Infinitely large, nonconducting sheet
++
+ +
++
Problems to find electric fields
Use Coulomb's law to find the field
Superposition principle
Important: electric field is a vector, it
has magnitude and direction.
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The Electric Field
We begin our investigation of electric fields by postulating
a field model that describes how charges interact:
1.Some charges, which we will call the source
charges, alter the space around them by creating an
electric field.
2.A separate charge in the electric field experiences a
force exerted by the field.
Suppose probe charge q experiences an electric force Fon q
due to other charges.
The units of the electric field are N/C. The magnitude E of
the electric field is called the electric field strength.
The Electric Field of a Point Charge
The electric field at distance r from a point charge q is
where the unit vector for r points away from the charge to
the point at which we want to know the field.
This unit vector expresses the idea “away from q”.
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EXAMPLE 26.8 The electric field of a proton
QUESTIONS:
EXAMPLE 26.8 The electric field of a proton
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EXAMPLE 26.8 The electric field of a proton
In-class Activity #5
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In-class Activity #6
Chapter 26. Summary Slides
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General Principles
Important Concepts
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Important Concepts
Important Concepts
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Important Concepts
Chapter 26. Clicker Questions
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A. see if the object attracts a charged plastic rod.
B. see if the object repels a charged glass rod.
C. Both A and B.
D. Either A or B.
To determine if an object has “glass
charge,” you need to
To determine if an object has “glass
charge,” you need to
A. see if the object attracts a charged plastic rod.
B. see if the object repels a charged glass rod.
C. Both A and B.
D. Either A or B.
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A. qa = qb > qe > qc > qd
B. qa > qe > qd > qc > qb
C. qe > qa > qd > qb > qc
D. qd > qc > qe > qa = qb
E. qd > qc > qe > qa > qb
Rank in order, from most positive to most
negative, the charges qa to qe of these five
systems.
Rank in order, from most positive to most
negative, the charges qa to qe of these five
systems.
A. qa = qb > qe > qc > qd
B. qa > qe > qd > qc > qb
C. qe > qa > qd > qb > qc
D. qd > qc > qe > qa = qb
E. qd > qc > qe > qa > qb
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A.The leaves get closer together.
B. The leaves spread further apart.
C. The leaves don’t move.
D.One leaf moves higher, the other lower.
An electroscope is positively charged by
touching it with a positive glass rod. The
electroscope leaves spread apart and the
glass rod is removed. Then a negatively
charged plastic rod is brought close to the
top of the electroscope, but it doesn’t
touch. What happens to the leaves?
An electroscope is positively charged by
touching it with a positive glass rod. The
electroscope leaves spread apart and the
glass rod is removed. Then a negatively
charged plastic rod is brought close to the
top of the electroscope, but it doesn’t
touch. What happens to the leaves?
A.The leaves get closer together.
B. The leaves spread further apart.
C. The leaves don’t move.
D.One leaf moves higher, the other lower.
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A. FA on B > FB on A
B. FA on B < FB on A
C. FA on B = FB on A
Charges A and B exert repulsive forces on
each other. qA = 4qB. Which statement is
true?
Charges A and B exert repulsive forces on
each other. qA = 4qB. Which statement is
true?
A. FA on B > FB on A
B. FA on B < FB on A
C. FA on B = FB on A
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A.to the right.
B.to the left.
C.zero.
D.There’s not enough information to tell.
An electron is
placed at the
position marked
by the dot. The
force on the
electron is
An electron is
placed at the
position marked
by the dot. The
force on the
electron is
A.to the right.
B.to the left.
C.zero.
D.There’s not enough information to tell.
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Rank in order, from largest to smallest,
the electric field strengths E1 to E4 at
points 1 to 4.
A. E2 > E4 > E1 > E3
B. E1 = E2 > E3 = E4
C. E2 > E1 = E4 > E3
D. E2 > E1 > E4 > E3
E. E1 > E2 > E3 > E4
A. E2 > E4 > E1 > E3
B. E1 = E2 > E3 = E4
C. E2 > E1 = E4 > E3
D. E2 > E1 > E4 > E3
E. E1 > E2 > E3 > E4
Rank in order, from largest to smallest,
the electric field strengths E1 to E4 at
points 1 to 4.
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Chapter 26. Reading Quizzes
What is the SI unit of charge?
A. Coulomb
B. Faraday
C. Ampere
D. Ohm
E. Volt
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What is the SI unit of charge?
A. Coulomb
B. Faraday
C. Ampere
D. Ohm
E. Volt
A charge alters the space around it.
What is this alteration of space called?
A. Charged plasma
B. Charge sphere
C. Electric ether
D. Electric field
E. Electrophoresys
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A charge alters the space around it.
What is this alteration of space called?
A. Charged plasma
B. Charge sphere
C. Electric ether
D. Electric field
E. Electrophoresys
Topics:
• Developing a Charge Model
• Charge
• Insulators and Conductors
• Coulomb’s Law
• The Field Model
Chapter 26. Electric Charges and Forces
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If a negative charged rod is held near a
neutral metal ball, the ball is attracted to
the rod. This happens
A. because of magnetic effects.
B. because the ball tries to pull the
rod’s electrons over to it.
C. because the rod polarizes the metal.
D. because the rod and the ball
have opposite charges.
If a negative charged rod is held near a
neutral metal ball, the ball is attracted to
the rod. This happens
A. because of magnetic effects.
B. because the ball tries to pull the
rod’s electrons over to it.
C. because the rod polarizes the metal.
D. because the rod and the ball
have opposite charges.
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The electric field of a charge is defined
by the force on
A. an electron.
B. a proton.
C. a source charge.
D. a test charge.
The electric field of a charge is defined
by the force on
A. an electron.
B. a proton.
C. a source charge.
D. a test charge.