1. Electric Fields and Forces
Chapter 20
Electric Fields and Forces

2. 20 Electric Fields and Forces
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3. Slide 20-3

4. Virtual photons, the longitudinal part of the EM field are responsible for the static, or somewhat static, electric field.
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5. Slide 20-5

6. Electric Charges and Forces, Part I
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7. Electric Charges and Forces, Part II
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8. Electric Charges and Forces, Part III
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9. 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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10. The Charge Model Slide 20-16
Figured out by Benjamin Franklin, the smartest American who ever was so far.
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11. Electric Charges and Forces, Part IV
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12. Charging by Contact
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13. Insulators and Conductors
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14. An Induced Electric Dipole
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15. 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
(i B
ii) A
iii) C, D
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16. 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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17. Coulomb’s Law
In this version of the law you have to put in the direction or sign by hand.
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18. 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?
Answer: C
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19. 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
Answer: C
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20. 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?
Answer: D
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21. Answer A small, positive charge is placed at the black dot.
In which case is the force on the small, positive charge the smallest?
Answer: D D
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22. 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?
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23. 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?
Answer: C
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24. 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
Answer: C
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25. 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?
Answer: D
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26. 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
Answer: D
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27. 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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28. The Electric Field
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29. The Electric Field of a Point Charge
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30. 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?
Answer: D
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31. Answer
Positive charges create an electric field in the space around them. In which case is the field at the black dot the smallest?
Answer: D D
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32. 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?
Answer: C
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33. 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
Answer: C
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34. 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?
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35. 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?
Answer: A
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36. 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
Answer: A
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37. 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?
Answer: D
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38. 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?
Answer: D D
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39. Dipole and Uniform Electric Fields
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40. 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?
Answer: A
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41. 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
Answer: A, when the electric field lines are the closest together the magnitude of the filed is the greatest. Michael Faraday
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42. 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?
Answer: D, when the field lines are the least dense the magnitude of the filed is the smallest. Michael Faraday
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43. 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
Answer: D
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44. Electric Field Lines
Answer: D, Huh?
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45. 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)
Answer: A
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46. 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)
Answer: A
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47. 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)
Answer: B
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48. 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)
Answer: B
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49. Conductors and Electric Fields
Answer: B, Huh?
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50. Forces and Torques on Charges in Electric Fields
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51. 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.
Answer: E
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52. 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.
Answer: E
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53. 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.
Answer: D
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54. 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.
Answer: D
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55. 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.
Answer: C
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56. 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.
Answer: C
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57. 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?
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58. Summary
Answer: C, Huh?
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59. 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) E
Answer: C
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60. 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) E
Answer: C
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61. 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) E
Answer: B
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62. 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) E
Answer: B
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63. Additional Example Problems
A positively charged particle is motionless in the center of a capacitor. Describe the subsequent motion of the charged particle.
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64. 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.
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