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Chapter 20. 0. Electric Fields and Forces. 20 Electric Fields and Forces. Slide 20-2. Slide 20-3. Slide 20-4. Slide 20-5. Electric Charges and Forces, Part I. Slide 20-12. Electric Charges and Forces, Part II. Slide 20-13. Electric Charges and Forces, Part III. Slide 20-14.

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chapter 20

Chapter 20

0

Electric Fields and Forces

slide9

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.

Slide 20-15

slide10

The Charge Model

Slide 20-16

slide15

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

Slide 20-21

slide16

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.

Slide 20-22

slide17

Coulomb’s Law

Slide 20-23

slide18

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?

Slide 20-24

slide19

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

Slide 20-25

slide20

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?

Slide 20-26

slide21

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

Slide 20-27

slide22

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:
      • to the right?
      • to the left?
      • zero?

Slide 20-28

slide23

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?

Slide 20-29

slide24

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

Slide 20-30

slide25

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?

Slide 20-31

slide26

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

Slide 20-32

slide27

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?

Slide 20-33

slide30

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?

Slide 20-36

slide31

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

Slide 20-37

slide32

Checking Understanding

Positive charges create an electric field in the space around themIn which case is the field at the black dot the largest?

Slide 20-38

slide33

Answer

Positive charges create an electric field in the space around themIn which case is the field at the black dot the largest?

C

Slide 20-39

slide34

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:
    • to the right?
    • to the left?
    • zero?

Slide 20-40

slide35

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?

Slide 20-41

slide36

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

Slide 20-42

slide37

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?

Slide 20-43

slide38

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

Slide 20-44

slide40

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?

Slide 20-46

slide41

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

Slide 20-47

slide42

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?

Slide 20-48

slide43

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

Slide 20-49

slide45

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?

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)

Slide 20-51

slide46

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?

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)

Slide 20-52

slide47

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?

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)

Slide 20-53

slide48

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?

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)

Slide 20-54

slide51

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?
    • The dipole moves to the right.
    • The dipole moves to the left.
    • The dipole rotates clockwise.
    • The dipole rotates counterclockwise.
    • The dipole remains motionless.

Slide 20-57

slide52

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?
    • The dipole moves to the right.
    • The dipole moves to the left.
    • The dipole rotates clockwise.
    • The dipole rotates counterclockwise.
    • The dipole remains motionless.

Slide 20-58

slide53

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?
    • The dipole moves to the right.
    • The dipole moves to the left.
    • The dipole rotates clockwise.
    • The dipole rotates counterclockwise.
    • The dipole remains motionless.

Slide 20-59

slide54

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?
    • The dipole moves to the right.
    • The dipole moves to the left.
    • The dipole rotates clockwise.
    • The dipole rotates counterclockwise.
    • The dipole remains motionless.

Slide 20-60

slide55

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?
    • The dipole moves to the right.
    • The dipole moves to the left.
    • The dipole rotates clockwise.
    • The dipole rotates counterclockwise.
    • The dipole remains motionless.

Slide 20-61

slide56

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?
    • The dipole moves to the right.
    • The dipole moves to the left.
    • The dipole rotates clockwise.
    • The dipole rotates counterclockwise.
    • The dipole remains motionless.

Slide 20-62

slide57

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.
    • Explain how a housefly could accumulate an electric charge by walking across a surface.
    • What magnitude and direction electric field would be necessary to “levitate” a housefly with a maximum charge? Could such a field exist in air?

Slide 20-63

slide58

Summary

Slide 20-64

slide59

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) Sphere charge (nC)

2.0 4.0

3.0 4.0

2.0 6.0

3.0 8.0

E. 4.0 9.0

Slide 20-65

slide60

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) Sphere charge (nC)

2.0 4.0

3.0 4.0

2.0 6.0

3.0 8.0

E. 4.0 9.0

Slide 20-66

slide61

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) Sphere charge (nC)

2.0 4.0

3.0 4.0

2.0 6.0

3.0 8.0

E. 4.0 9.0

Slide 20-67

slide62

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) Sphere charge (nC)

2.0 4.0

3.0 4.0

2.0 6.0

3.0 8.0

E. 4.0 9.0

Slide 20-68

slide63

Additional Example Problems

  • A positively charged particle is motionless in the center of a capacitor. Describe the subsequent motion of the charged particle.

Slide 20-69

slide64

Additional Example Problems

  • Determine the magnitude and the direction of the electric field at point A.
  • Determine the individual forces and the net force on charge B for each of the following cases.

Slide 20-70