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Chapter 20
Electric
Fields and
Forces
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PowerPoint® Lectures for
College Physics: A Strategic Approach, Second Edition
20 Electric Fields and Forces
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Slide 20-2
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Slide 20-3
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Slide 20-4
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Slide 20-5
Electric Charges and Forces, Part I
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Slide 20-12
Electric Charges and Forces, Part II
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Slide 20-13
Electric Charges and Forces, Part III
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Slide 20-14
Visualizing Charge
• Charges on an insulator do not move.
• Charges on a conductor adjust until
there is no net force on any charge.
We call this electrostatic equilibrium.
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Slide 20-15
The Charge Model
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Slide 20-16
Electric Charges and Forces, Part IV
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Slide 20-17
Charging by Contact
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Slide 20-18
Insulators and Conductors
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Slide 20-19
An Induced Electric Dipole
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Slide 20-20
Conceptual Example Problem
Two spheres are touching each other. A charged rod is brought
near. The spheres are then separated, and the rod is taken away.
In the first case, the spheres are aligned with the rod, in the
second case, they are perpendicular. After the charged rod is
removed, which of the spheres is:
i) Positive
ii) Negative
iii) Neutral
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Slide 20-21
Conceptual Example Problems
If a charged plastic rod is brought near an uncharged metal rod on
an insulating stand, an uncharged metal ball near the other end of
the metal rod is attracted to this end of the rod. Explain the motions
of charges that give rise to this force.
Describe a procedure by which you could give two identical metal
spheres exactly equal charges.
Describe a procedure by which you could give two identical metal
spheres charges of opposite sign but exactly equal magnitude.
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Slide 20-22
Coulomb’s Law
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Slide 20-23
Checking Understanding
A small, positive charge is placed at the black dot.
In which case is the force on the small, positive charge the largest?
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Slide 20-24
Answer
A small, positive charge is placed at the black dot.
In which case is the force on the small, positive charge the largest?
C
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Slide 20-25
Checking Understanding
A small, positive charge is placed at the black dot.
In which case is the force on the small, positive charge the
smallest?
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Slide 20-26
Answer
A small, positive charge is placed at the black dot.
In which case is the force on the small, positive charge the
smallest?
D
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Slide 20-27
Conceptual Example Problem
All charges in the diagrams below are equal magnitude. In each
case, a small positive charge is placed at the blank dot. In which
cases is the force on this charge:
A. to the right?
B. to the left?
C. zero?
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Slide 20-28
Checking Understanding
All charges in the diagrams below are of equal magnitude. In each
case, a small, positive charge is placed at the black dot.
In which case is the force on the small, positive charge the
largest?
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Slide 20-29
Answer
All charges in the diagrams below are of equal magnitude. In each
case, a small, positive charge is placed at the black dot.
In which case is the force on the small, positive charge the
largest?
C
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Slide 20-30
Checking Understanding
All charges in the diagrams below are of equal magnitude. In each
case, a small, positive charge is placed at the black dot.
In which case is the force on the small, positive charge the
smallest?
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Slide 20-31
Answer
All charges in the diagrams below are of equal magnitude. In each
case, a small, positive charge is placed at the black dot.
In which case is the force on the small, positive charge the
smallest?
D
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Slide 20-32
Example Problem
Two 0.10 g honeybees each acquire a charge of +23 pC as they fly
back to their hive. As they approach the hive entrance, they are 1.0
cm apart. What is the magnitude of the repulsive force between the
two bees? How does this force compare with their weight?
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Slide 20-33
The Electric Field
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Slide 20-34
The Electric Field of a Point Charge
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Slide 20-35
Checking Understanding
Positive charges create an electric field in the space around them.
In which case is the field at the black dot the smallest?
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Slide 20-36
Answer
Positive charges create an electric field in the space around them.
In which case is the field at the black dot the smallest?
D
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Slide 20-37
Checking Understanding
Positive charges create an electric field in the space around them
In which case is the field at the black dot the largest?
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Slide 20-38
Answer
Positive charges create an electric field in the space around them
In which case is the field at the black dot the largest?
C
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Slide 20-39
Conceptual Example Problem
All charges in the diagram below are of equal magnitude. In each of
the four cases, two charges lie along a line, and we consider the
electric field due to these two charges at a point along this line
represented by the black dot. In which of the cases is the field:
A. to the right?
B. to the left?
C. zero?
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Slide 20-40
Checking Understanding
All charges in the diagram below are of equal magnitude. In each of
the four cases below, two charges lie along a line, and we consider
the electric field due to these two charges at a point along this line
represented by the black dot. In which case is the magnitude of the
field at the black dot the largest?
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Slide 20-41
Answer
All charges in the diagram below are of equal magnitude. In each of
the four cases below, two charges lie along a line, and we consider
the electric field due to these two charges at a point along this line
represented by the black dot. In which case is the magnitude of the
field at the black dot the largest?
A
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Slide 20-42
Checking Understanding
All charges in the diagram below are of equal magnitude. In each of
the four cases below, two charges lie along a line, and we consider
the electric field due to these two charges at a point along this line
represented by the black dot. In which case is the magnitude of the
field at the black dot the smallest?
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Slide 20-43
Answer
All charges in the diagram below are of equal magnitude. In each of
the four cases below, two charges lie along a line, and we consider
the electric field due to these two charges at a point along this line
represented by the black dot. In which case is the magnitude of the
field at the black dot the smallest?
D
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Slide 20-44
Dipole and Uniform Electric Fields
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Slide 20-45
Checking Understanding
A set of electric field lines is directed as below. At which of the
noted points is the magnitude of the field the greatest?
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Slide 20-46
Answer
A set of electric field lines is directed as below. At which of the
noted points is the magnitude of the field the greatest?
A
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Slide 20-47
Checking Understanding
A set of electric field lines is directed as below. At which of the
noted points is the magnitude of the field the smallest?
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Slide 20-48
Answer
A set of electric field lines is directed as below. At which of the
noted points is the magnitude of the field the smallest?
D
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Slide 20-49
Electric Field Lines
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Slide 20-50
Checking Understanding
Two parallel plates have charges of equal magnitude but opposite
sign. What change could be made to increase the field strength
between the plates?
A.
B.
C.
D.
E.
Increase the magnitude of the charge on both plates
Decrease the magnitude of the charge on both plates
Increase the distance between the plates
Decrease the distance between the plates
Increase the area of the plates (while keeping the magnitude of
the charges the same)
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Slide 20-51
Answer
Two parallel plates have charges of equal magnitude but opposite
sign. What change could be made to increase the field strength
between the plates?
A.
B.
C.
D.
E.
Increase the magnitude of the charge on both plates
Decrease the magnitude of the charge on both plates
Increase the distance between the plates
Decrease the distance between the plates
Increase the area of the plates (while keeping the magnitude of
the charges the same)
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Slide 20-52
Checking Understanding
Two parallel plates have charges of equal magnitude but opposite
sign. What change could be made to decrease the field strength
between the plates?
A.
B.
C.
D.
E.
Increase the magnitude of the charge on both plates
Decrease the magnitude of the charge on both plates
Increase the distance between the plates
Decrease the distance between the plates
Decrease the area of the plates (while keeping the
magnitude of the charges the same)
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Slide 20-53
Answer
Two parallel plates have charges of equal magnitude but opposite
sign. What change could be made to decrease the field strength
between the plates?
A.
B.
C.
D.
E.
Increase the magnitude of the charge on both plates
Decrease the magnitude of the charge on both plates
Increase the distance between the plates
Decrease the distance between the plates
Decrease the area of the plates (while keeping the
magnitude of the charges the same)
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Slide 20-54
Conductors and Electric Fields
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Slide 20-55
Forces and Torques on Charges in Electric Fields
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Slide 20-56
Checking Understanding
A dipole is held motionless in a uniform electric field. For the
situation below, when the dipole is released, which of the following
describes the subsequent motion?
A. The dipole moves to the right.
B. The dipole moves to the left.
C. The dipole rotates clockwise.
D. The dipole rotates counterclockwise.
E. The dipole remains motionless.
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Slide 20-57
Answer
A dipole is held motionless in a uniform electric field. For the
situation below, when the dipole is released, which of the following
describes the subsequent motion?
A. The dipole moves to the right.
B. The dipole moves to the left.
C. The dipole rotates clockwise.
D. The dipole rotates counterclockwise.
E. The dipole remains motionless.
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Slide 20-58
Checking Understanding
A dipole is held motionless in a uniform electric field. For the
situation below, when the dipole is released, which of the following
describes the subsequent motion?
A. The dipole moves to the right.
B. The dipole moves to the left.
C. The dipole rotates clockwise.
D. The dipole rotates counterclockwise.
E. The dipole remains motionless.
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Slide 20-59
Answer
A dipole is held motionless in a uniform electric field. For the
situation below, when the dipole is released, which of the following
describes the subsequent motion?
A. The dipole moves to the right.
B. The dipole moves to the left.
C. The dipole rotates clockwise.
D. The dipole rotates counterclockwise.
E. The dipole remains motionless.
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Slide 20-60
Checking Understanding
A dipole is held motionless in a uniform electric field. For the
situation below, when the dipole is released, which of the following
describes the subsequent motion?
A. The dipole moves to the right.
B. The dipole moves to the left.
C. The dipole rotates clockwise.
D. The dipole rotates counterclockwise.
E. The dipole remains motionless.
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Slide 20-61
Answer
A dipole is held motionless in a uniform electric field. For the
situation below, when the dipole is released, which of the following
describes the subsequent motion?
A. The dipole moves to the right.
B. The dipole moves to the left.
C. The dipole rotates clockwise.
D. The dipole rotates counterclockwise.
E. The dipole remains motionless.
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Slide 20-62
Example Problem
A housefly walking across a clean surface can accumulate a
significant positive or negative charge. In one experiment, the
largest positive charge observed was 73 pC. A typical housefly
has a mass of 12 mg.
A. Explain how a housefly could accumulate an electric charge by
walking across a surface.
B. What magnitude and direction electric field would be necessary
to “levitate” a housefly with a maximum charge? Could such a
field exist in air?
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Slide 20-63
Summary
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Slide 20-64
Additional Questions
A small sphere is suspended from a string in a uniform electric
field. Several different cases of sphere mass and sphere charge
are presented in the following table. In which case is the angle at
which the sphere hangs the largest?
Sphere mass (g)
A.
2.0
B.
3.0
C.
2.0
D.
3.0
E.
4.0
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Sphere charge (nC)
4.0
4.0
6.0
8.0
9.0
Slide 20-65
Answer
A small sphere is suspended from a string in a uniform electric
field. Several different cases of sphere mass and sphere charge
are presented in the following table. In which case is the angle at
which the sphere hangs the largest?
Sphere mass (g)
A.
2.0
B.
3.0
C.
2.0
D.
3.0
E.
4.0
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Sphere charge (nC)
4.0
4.0
6.0
8.0
9.0
Slide 20-66
Additional Questions
A small sphere is suspended from a string in a uniform electric
field. Several different cases of sphere mass and sphere charge
are presented in the following table. In which case is the angle at
which the sphere hangs the smallest?
Sphere mass (g)
A.
2.0
B.
3.0
C.
2.0
D.
3.0
E.
4.0
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Sphere charge (nC)
4.0
4.0
6.0
8.0
9.0
Slide 20-67
Answer
A small sphere is suspended from a string in a uniform electric
field. Several different cases of sphere mass and sphere charge
are presented in the following table. In which case is the angle at
which the sphere hangs the smallest?
Sphere mass (g)
A.
2.0
B.
3.0
C.
2.0
D.
3.0
E.
4.0
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Sphere charge (nC)
4.0
4.0
6.0
8.0
9.0
Slide 20-68
Additional Example Problems
1. A positively charged particle is motionless in the center of a
capacitor. Describe the subsequent motion of the charged
particle.
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Slide 20-69
Additional Example Problems
2. Determine the magnitude and the direction of the electric field
at point A.
3. Determine the individual forces and the net force on charge B
for each of the following cases.
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Slide 20-70