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2. Standardized Test Prep
Resources
Chapter Presentation
Visual Concepts
Transparencies
Sample Problems
Links as follows.
Chapter Presentation: “Table of Contents”
Visual Concepts: all Visual Concepts in order of occurrence
Transparencies: all sheets which instruct pickup of transparency art
Standardized Test Prep: First page of “Standardized Test Prep”
Sample Problems: all sample problems in order of occurrence
Standardized Test Prep

3. Chapter 20 Table of Contents Section 1 Electricity from Magnetism
Electromagnetic Induction
Table of Contents
Section 1 Electricity from Magnetism
Section 2 Generators, Motors, and Mutual Inductance
Section 3 AC Circuits and Transformers
Section 4 Electromagnetic Waves
Link each section title to the first page of that section (“Objectives”)

4. Section 1 Electricity from Magnetism
Chapter 20
Objectives
Recognize that relative motion between a conductor and a magnetic field induces an emf in the conductor.
Describe how the change in the number of magnetic field lines through a circuit loop affects the magnitude and direction of the induced electric current.
Apply Lenz’s law and Faraday’s law of induction to solve problems involving induced emf and current.

5. Electromagnetic Induction
Section 1 Electricity from Magnetism
Chapter 20
Electromagnetic Induction
Electromagnetic induction is the process of creating a current in a circuit by a changing magnetic field.
A change in the magnetic flux through a conductor induces an electric current in the conductor.
The separation of charges by the magnetic force induces an emf.

6. Electromagnetic Induction in a Circuit Loop
Section 1 Electricity from Magnetism
Chapter 20
Electromagnetic Induction in a Circuit Loop
Insert High-Res image from Figure 1, page 708

7. Electromagnetic Induction, continued
Section 1 Electricity from Magnetism
Chapter 20
Electromagnetic Induction, continued
The angle between a magnetic field and a circuit affects induction.
A change in the number of magnetic field lines induces a current.
Insert High-Res art from Figure 3 page 709

8. Ways of Inducing a Current in a Circuit
Section 1 Electricity from Magnetism
Chapter 20
Ways of Inducing a Current in a Circuit
Pick up Visual Concepts: Chapter 20, Section 1: Ways of Inducing a Current in a Circuit #70689

9. Characteristics of Induced Current
Section 1 Electricity from Magnetism
Chapter 20
Characteristics of Induced Current
Lenz’s Law
The magnetic field of the induced current is in a direction to produce a field that opposes the change causing it.
Note: the induced current does not oppose the applied field, but rather the change in the applied field.

10. Lenz's Law for Determining the Direction of the Induced Current
Section 1 Electricity from Magnetism
Chapter 20
Lenz's Law for Determining the Direction of the Induced Current
Pick up Visual Concepts: Chapter 20, Section 1: Lenz's Law for Determining the Direction of the Induced Current #70690

11. Characteristics of Induced Current, continued
Section 1 Electricity from Magnetism
Chapter 20
Characteristics of Induced Current, continued
The magnitude of the induced emf can be predicted by Faraday’s law of magnetic induction.
Faraday’s Law of Magnetic Induction
The magnetic flux is given by FM = ABcosq.

12. Chapter 20 Sample Problem Induced emf and Current
Section 1 Electricity from Magnetism
Chapter 20
Sample Problem
Induced emf and Current
A coil with 25 turns of wire is wrapped around a hollow tube with an area of 1.8 m2. Each turn has the same area as the tube. A uniform magnetic field is applied at a right angle to the plane of the coil. If the field increases uniformly from 0.00 T to 0.55 T in 0.85 s, find the magnitude of the induced emf in the coil. If the resistance in the coil is 2.5 Ω, find the magnitude of the induced current in the coil.

13. Sample Problem, continued
Section 1 Electricity from Magnetism
Chapter 20
Sample Problem, continued
Induced emf and Current
1. Define
Given:
∆t = 0.85 s A = 1.8 m2 q = 0.0º
N = 25 turns R = 2.5 Ω
Bi = 0.00 T = 0.00 V•s/m2
Bf = 0.55 T = 0.55 V•s/m2
Unknown:
emf = ?
I = ?

14. Sample Problem, continued
Section 1 Electricity from Magnetism
Chapter 20
Sample Problem, continued
Induced emf and Current
1. Define, continued
Diagram: Show the coil before and after the change in the magnetic field.

15. Sample Problem, continued
Section 1 Electricity from Magnetism
Chapter 20
Sample Problem, continued
Induced emf and Current
2. Plan
Choose an equation or situation. Use Faraday’s law of magnetic induction to find the induced emf in the coil.
Substitute the induced emf into the definition of resistance to determine the induced current in the coil.

16. Sample Problem, continued
Section 1 Electricity from Magnetism
Chapter 20
Sample Problem, continued
Induced emf and Current
2. Plan, continued
Rearrange the equation to isolate the unknown. In this example, only the magnetic field strength changes with time. The other components (the coil area and the angle between the magnetic field and the coil) remain constant.

17. Sample Problem, continued
Section 1 Electricity from Magnetism
Chapter 20
Sample Problem, continued
Induced emf and Current
3. Calculate
Substitute the values into the equation and solve.

18. Sample Problem, continued
Section 1 Electricity from Magnetism
Chapter 20
Sample Problem, continued
Induced emf and Current
4. Evaluate
The induced emf, and therefore the induced current, is directed through the coil so that the magnetic field produced by the induced current opposes the change in the applied magnetic field. For the diagram shown on the previous page, the induced magnetic field is directed to the right and the current that produces it is directed from left to right through the resistor.

19. Chapter 20 Objectives Describe how generators and motors operate.
Section 2 Generators, Motors, and Mutual Inductance
Chapter 20
Objectives
Describe how generators and motors operate.
Explain the energy conversions that take place in generators and motors.
Describe how mutual induction occurs in circuits.

20. Generators and Alternating Current
Section 2 Generators, Motors, and Mutual Inductance
Chapter 20
Generators and Alternating Current
A generator is a machine that converts mechanical energy into electrical energy.
Generators use induction to convert mechanical energy into electrical energy.
A generator produces a continuously changing emf.

21. Induction of an emf in an AC Generator
Section 2 Generators, Motors, and Mutual Inductance
Chapter 20
Induction of an emf in an AC Generator
Insert High-Res image from TR109

22. Function of a Generator
Section 2 Generators, Motors, and Mutual Inductance
Chapter 20
Function of a Generator
Pick up Visual Concepts: Chapter 20, Section 2:Function of a Generator #70691

23. Generators and Alternating Current, continued
Section 2 Generators, Motors, and Mutual Inductance
Chapter 20
Generators and Alternating Current, continued
Alternating current is an electric current that changes direction at regular intervals.
Alternating current can be converted to direct current by using a device called a commutator to change the direction of the current.

24. Comparing AC and DC Generators
Section 2 Generators, Motors, and Mutual Inductance
Chapter 20
Comparing AC and DC Generators
Pick up Visual Concepts: Chapter 20, Section 2: Comparing AC and DC Generators #70692

25. Section 2 Generators, Motors, and Mutual Inductance
Chapter 20
Motors
Motors are machines that convert electrical energy to mechanical energy.
Motors use an arrangement similar to that of generators.
Back emf is the emf induced in a motor’s coil that tends to reduce the current in the coil of a motor.

26. Section 2 Generators, Motors, and Mutual Inductance
Chapter 20
DC Motors
Pick up Visual Concepts: Chapter 20, Section 2: DC Motors #70693

27. Chapter 20 Mutual Inductance
Section 2 Generators, Motors, and Mutual Inductance
Chapter 20
Mutual Inductance
The ability of one circuit to induce an emf in a nearby circuit in the presence of a changing current is called mutual inductance.
In terms of changing primary current, Faraday’s law is given by the following equation, where M is the mutual inductance:

28. Chapter 20 Mutual Inductance
Section 2 Generators, Motors, and Mutual Inductance
Chapter 20
Mutual Inductance
Pick up Visual Concepts: Chapter 20, Section 2: Mutual Inductance #70694

29. Section 3 AC Circuits and Transformers
Chapter 20
Objectives
Distinguish between rms values and maximum values of current and potential difference.
Solve problems involving rms and maximum values of current and emf for ac circuits.
Apply the transformer equation to solve problems involving step-up and step-down transformers.

30. Chapter 20 Effective Current
Section 3 AC Circuits and Transformers
Chapter 20
Effective Current
The root-mean-square (rms) current of a circuit is the value of alternating current that gives the same heating effect that the corresponding value of direct current does.
rms Current

31. Effective Current, continued
Section 3 AC Circuits and Transformers
Chapter 20
Effective Current, continued
The rms current and rms emf in an ac circuit are important measures of the characteristics of an ac circuit.
Resistance influences current in an ac circuit.

32. Chapter 20 rms Current Section 3 AC Circuits and Transformers
Pick up Visual Concepts: Chapter 20, Section 2: RMS Current #70441

33. Chapter 20 Sample Problem rms Current and emf
Section 3 AC Circuits and Transformers
Chapter 20
Sample Problem
rms Current and emf
A generator with a maximum output emf of 205 V is connected to a 115 Ω resistor. Calculate the rms potential difference. Find the rms current through the resistor. Find the maximum ac current in the circuit.
1. Define
Given:
∆Vrms = 205 V R = 115 Ω
Unknown:
∆Vrms = ? Irms = ? Imax = ?

34. Sample Problem, continued
Section 3 AC Circuits and Transformers
Chapter 20
Sample Problem, continued
rms Current and emf
2. Plan
Choose an equation or situation. Use the equation for the rms potential difference to find ∆Vrms.
∆Vrms = ∆Vmax
Rearrange the definition for resistance to calculate Irms.
Use the equation for rms current to find Irms.
Irms = Imax

35. Sample Problem, continued
Section 3 AC Circuits and Transformers
Chapter 20
Sample Problem, continued
rms Current and emf
2. Plan, continued
Rearrange the equation to isolate the unknown. Rearrange the equation relating rms current to maximum current so that maximum current is calculated.

36. Sample Problem, continued
Section 3 AC Circuits and Transformers
Chapter 20
Sample Problem, continued
rms Current and emf
3. Calculate
Substitute the values into the equation and solve.
4. Evaluate The rms values for emf and current are a little more than two-thirds the maximum values, as expected.

37. Section 3 AC Circuits and Transformers
Chapter 20
Transformers
A transformer is a device that increases or decreases the emf of alternating current.
The relationship between the input and output emf is given by the transformer equation.

38. Chapter 20 Transformers Section 3 AC Circuits and Transformers
Pick up Visual Concepts: Chapter 20, Section 3: Transformer #70717

39. Transformers, continued
Section 3 AC Circuits and Transformers
Chapter 20
Transformers, continued
The transformer equation assumes that no power is lost between the primary and secondary coils. However, real transformers are not perfectly efficient.
Real transformers typically have efficiencies ranging from 90% to 99%.
The ignition coil in a gasoline engine is a transformer.

40. A Step-Up Transformer in an Auto Ignition System
Section 3 AC Circuits and Transformers
Chapter 20
A Step-Up Transformer in an Auto Ignition System
Insert High-Res image from TR112

41. Section 4 Electromagnetic Waves
Chapter 20
Objectives
Describe what electromagnetic waves are and how they are produced.
Recognize that electricity and magnetism are two aspects of a single electromagnetic force.
Explain how electromagnetic waves transfer energy.
Describe various applications of electromagnetic waves.

42. Propagation of Electromagnetic Waves
Section 4 Electromagnetic Waves
Chapter 20
Propagation of Electromagnetic Waves
Electromagnetic waves travel at the speed of light and are associated with oscillating, perpendicular electric and magnetic fields.
Electromagnetic waves are transverse waves; that is, the direction of travel is perpendicular to the the direction of oscillating electric and magnetic fields.
Electric and magnetic forces are aspects of a single force called the electromagnetic force.

43. Electromagnetic Waves
Section 4 Electromagnetic Waves
Chapter 20
Electromagnetic Waves
Pick up Visual Concepts: Chapter 20, Section 4: Electromagnetic Waves #70120

44. Propagation of Electromagnetic Waves, continued
Section 4 Electromagnetic Waves
Chapter 20
Propagation of Electromagnetic Waves, continued
All electromagnetic waves are produced by accelerating charges.
Electromagnetic waves transfer energy. The energy of electromagnetic waves is stored in the waves’ oscillating electric and magnetic fields.
Electromagnetic radiation is the transfer of energy associated with an electric and magnetic field. Electromagnetic radiation varies periodically and travels at the speed of light.

45. The Sun at Different Wavelengths of Radiation
Section 4 Electromagnetic Waves
Chapter 20
The Sun at Different Wavelengths of Radiation
Insert High-Res image from TR113

46. Propagation of Electromagnetic Waves, continued
Section 4 Electromagnetic Waves
Chapter 20
Propagation of Electromagnetic Waves, continued
High-energy electromagnetic waves behave like particles.
An electromagnetic wave’s frequency makes the wave behave more like a particle. This notion is called the wave-particle duality.
A photon is a unit or quantum of light. Photons can be thought of as particles of electromagnetic radiation that have zero mass and carry one quantum of energy.

47. The Electromagnetic Spectrum
Section 4 Electromagnetic Waves
Chapter 20
The Electromagnetic Spectrum
The electromagnetic spectrum ranges from very long radio waves to very short-wavelength gamma waves.
The electromagnetic spectrum has a wide variety of applications and characteristics that cover a broad range of wavelengths and frequencies.

48. The Electromagnetic Spectrum, continued
Section 4 Electromagnetic Waves
Chapter 20
The Electromagnetic Spectrum, continued
Radio Waves
longest wavelengths
communications, tv
Microwaves
30 cm to 1 mm
radar, cell phones
Infrared
1 mm to 700 nm
heat, photography
Visible light
700 nm (red) to 400 nm (violet)
Ultraviolet
400 nm to 60 nm
disinfection, spectroscopy
X rays
60 nm to 10–4 nm
medicine, astronomy, security screening
Gamma Rays
less than 0.1 nm
cancer treatment, astronomy

49. The Electromagnetic Spectrum
Section 4 Electromagnetic Waves
Chapter 20
The Electromagnetic Spectrum
Insert High-Res image from TR114

50. Chapter 20 Multiple Choice
Standardized Test Prep
Multiple Choice
1. Which of the following equations correctly describes Faraday’s law of induction?

51. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
1. Which of the following equations correctly describes Faraday’s law of induction?

52. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
2. For the coil shown at right, what must be done to induce a clockwise current?
F. Either move the north pole of a magnet down into the coil, or move the south pole of the magnet up and out of the coil.
G. Either move the south pole of a magnet down into the coil, or move the north pole of the magnet up and out of the coil.
H. Move either pole of the magnet down into the coil.
J. Move either pole of the magnet up and out of the coil.

53. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
2. For the coil shown at right, what must be done to induce a clockwise current?
F. Either move the north pole of a magnet down into the coil, or move the south pole of the magnet up and out of the coil.
G. Either move the south pole of a magnet down into the coil, or move the north pole of the magnet up and out of the coil.
H. Move either pole of the magnet down into the coil.
J. Move either pole of the magnet up and out of the coil.

54. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
3. Which of the following would not increase the emf produced by a generator?
A. rotating the generator coil faster
B. increasing the strength of the generator magnets
C. increasing the number of turns of wire in the coil
D. reducing the cross-sectional area of the coil

55. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
3. Which of the following would not increase the emf produced by a generator?
A. rotating the generator coil faster
B. increasing the strength of the generator magnets
C. increasing the number of turns of wire in the coil
D. reducing the cross-sectional area of the coil

56. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
4. By what factor do you multiply the maximum emf to calculate the rms emf for an alternating current?

57. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
4. By what factor do you multiply the maximum emf to calculate the rms emf for an alternating current?

58. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
5. Which of the following correctly describes the composition of an electromagnetic wave?
A. a transverse electric wave and a magnetic transverse wave that are parallel and are moving in the same direction
B. a transverse electric wave and a magnetic transverse wave that are perpendicular and are moving in the same direction
C. a transverse electric wave and a magnetic transverse wave that are parallel and are moving at right angles to each other
D. a transverse electric wave and a magnetic transverse wave that are perpendicular and are moving at right angles to each other

59. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
5. Which of the following correctly describes the composition of an electromagnetic wave?
A. a transverse electric wave and a magnetic transverse wave that are parallel and are moving in the same direction
B. a transverse electric wave and a magnetic transverse wave that are perpendicular and are moving in the same direction
C. a transverse electric wave and a magnetic transverse wave that are parallel and are moving at right angles to each other
D. a transverse electric wave and a magnetic transverse wave that are perpendicular and are moving at right angles to each other

60. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
6. A coil is moved out of a magnetic field in order to induce an emf. The wire of the coil is then rewound so that the area of the coil is increased by 1.5 times. Extra wire is used in the coil so that the number of turns is doubled. If the time in which the coil is removed from the field is reduced by half and the magnetic field strength remains unchanged, how many times greater is the new induced emf than the original induced emf ?
F. 1.5 times
G. 2 times
H. 3 times
J. 6 times

61. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
6. A coil is moved out of a magnetic field in order to induce an emf. The wire of the coil is then rewound so that the area of the coil is increased by 1.5 times. Extra wire is used in the coil so that the number of turns is doubled. If the time in which the coil is removed from the field is reduced by half and the magnetic field strength remains unchanged, how many times greater is the new induced emf than the original induced emf ?
F. 1.5 times
G. 2 times
H. 3 times
J. 6 times

62. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
Use the passage below to answer questions 7–8.
A pair of transformers is connected in series, as shown in the figure below.
7. From left to right, what are the types of the two transformers?
A. Both are step-down transformers.
B. Both are step-up transformers.
C. One is a step-down transformer; and one is a step-up transformer.
D. One is a step-up transformer; and one is a step-down transformer.

63. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
Use the passage below to answer questions 7–8.
A pair of transformers is connected in series, as shown in the figure below.
7. From left to right, what are the types of the two transformers?
A. Both are step-down transformers.
B. Both are step-up transformers.
C. One is a step-down transformer; and one is a step-up transformer.
D. One is a step-up transformer; and one is a step-down transformer.

64. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
Use the passage below to answer questions 7–8.
A pair of transformers is connected in series, as shown in the figure below.
8. What is the output potential difference from the secondary coil of the transformer on the right?
F. 400 V
G V
H V
J V

65. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
Use the passage below to answer questions 7–8.
A pair of transformers is connected in series, as shown in the figure below.
8. What is the output potential difference from the secondary coil of the transformer on the right?
F. 400 V
G V
H V
J V

66. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
9. What are the particles that can be used to describe electromagnetic radiation called?
A. electrons
B. magnetons
C. photons
D. protons

67. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
9. What are the particles that can be used to describe electromagnetic radiation called?
A. electrons
B. magnetons
C. photons
D. protons

68. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
10. The maximum values for the current and potential difference in an ac circuit are 3.5 A and 340 V, respectively. How much power is dissipated in this circuit?
F. 300 W
G. 600 W
H W
J W

69. Multiple Choice, continued
Chapter 20
Standardized Test Prep
Multiple Choice, continued
10. The maximum values for the current and potential difference in an ac circuit are 3.5 A and 340 V, respectively. How much power is dissipated in this circuit?
F. 300 W
G. 600 W
H W
J W

70. Chapter 20 Short Response
Standardized Test Prep
Short Response
11. The alternating current through an electric toaster has a maximum value of 12.0 A. What is the rms value of this current?

71. Short Response, continued
Chapter 20
Standardized Test Prep
Short Response, continued
11. The alternating current through an electric toaster has a maximum value of 12.0 A. What is the rms value of this current?
Answer:
8.48 A

72. Short Response, continued
Chapter 20
Standardized Test Prep
Short Response, continued
12. What is the purpose of a commutator in an ac generator?

73. Short Response, continued
Chapter 20
Standardized Test Prep
Short Response, continued
12. What is the purpose of a commutator in an ac generator?
Answer:
It converts ac to a changing current in one direction only.

74. Short Response, continued
Chapter 20
Standardized Test Prep
Short Response, continued
13. How does the energy of one photon of an electromagnetic wave relate to the wave’s frequency?

75. Short Response, continued
Chapter 20
Standardized Test Prep
Short Response, continued
13. How does the energy of one photon of an electromagnetic wave relate to the wave’s frequency?
Answer:
The energy is directly proportional to the wave’s frequency (E = hf ).

76. Short Response, continued
Chapter 20
Standardized Test Prep
Short Response, continued
14. A transformer has 150 turns of wire on the primary coil and turns on the secondary coil. If the input potential difference across the primary is 120 V, what is the output potential difference across the secondary?

77. Short Response, continued
Chapter 20
Standardized Test Prep
Short Response, continued
14. A transformer has 150 turns of wire on the primary coil and turns on the secondary coil. If the input potential difference across the primary is 120 V, what is the output potential difference across the secondary?
Answer:
6.0  104 V

78. Chapter 20 Extended Response
Standardized Test Prep
Extended Response
15. Why is alternating current used for power transmission instead of direct current? Be sure to include power dissipation and electrical safety considerations in your answer.

79. Extended Response, continued
Chapter 20
Standardized Test Prep
Extended Response, continued
15. Answer:
For electric power to be transferred over long distances without a large amount of power dissipation, the electric power must have a high potential difference and low current. However, to be safely used in homes, the potential difference must be lower than that used for long-distance power transmission. Because of induction, the potential difference and current of electricity can be transformed to higher or lower values, but the current must change continuously (alternate) for this to happen.

80. Extended Response, continued
Chapter 20
Standardized Test Prep
Extended Response, continued
Base your answers to questions 16–18 on the information below.
A device at a carnival’s haunted house involves a metal ring that flies upward from a table when a patron passes near the table’s edge. The device consists of a photoelectric switch that activates the circuit when anyone walks in front of the switch and of a coil of wire into which a current is suddenly introduced when the switch is triggered.
16. Why must the current enter the coil just as someone comes up to the table?

81. Extended Response, continued
Chapter 20
Standardized Test Prep
Extended Response, continued
Base your answers to questions 16–18 on the information below.
A device at a carnival’s haunted house involves a metal ring that flies upward from a table when a patron passes near the table’s edge. The device consists of a photoelectric switch that activates the circuit when anyone walks in front of the switch and of a coil of wire into which a current is suddenly introduced when the switch is triggered.
16. Why must the current enter the coil just as someone comes up to the table?
Answer: The change in current in the coil will produce a changing magnetic field, which will induce a current in the ring. The induced current produces a magnetic field that interacts with the magnetic field from the coil, causing the ring to rise from the table.

82. Extended Response, continued
Chapter 20
Standardized Test Prep
Extended Response, continued
Base your answers to questions 16–18 on the information below.
A device at a carnival’s haunted house involves a metal ring that flies upward from a table when a patron passes near the table’s edge. The device consists of a photoelectric switch that activates the circuit when anyone walks in front of the switch and of a coil of wire into which a current is suddenly introduced when the switch is triggered.
17. Using Lenz’s law, explain why the ring flies upward when there is an increasing current in the coil?

83. Extended Response, continued
Chapter 20
Standardized Test Prep
Extended Response, continued
Base your answers to questions 16–18 on the information below.
A device at a carnival’s haunted house involves a metal ring that flies upward from a table when a patron passes near the table’s edge. The device consists of a photoelectric switch that activates the circuit when anyone walks in front of the switch and of a coil of wire into which a current is suddenly introduced when the switch is triggered.
17. Using Lenz’s law, explain why the ring flies upward when there is an increasing current in the coil?
Answer: According to Lenz’s law, the magnetic field induced in the ring must oppose the magnetic field that induces the current in the ring. The opposing fields cause the ring, which can move freely, to rise upward from the coil under the table’s surface.

84. Extended Response, continued
Chapter 20
Standardized Test Prep
Extended Response, continued
Base your answers to questions 16–18 on the information below.
A device at a carnival’s haunted house involves a metal ring that flies upward from a table when a patron passes near the table’s edge. The device consists of a photoelectric switch that activates the circuit when anyone walks in front of the switch and of a coil of wire into which a current is suddenly introduced when the switch is triggered.
18. Suppose the change in the magnetic field is 0.10 T/s. If the radius of the ring is 2.4 cm and the ring is assumed to consist of one turn of wire, what is the emf induced in the ring?

85. Extended Response, continued
Chapter 20
Standardized Test Prep
Extended Response, continued
Base your answers to questions 16–18 on the information below.
A device at a carnival’s haunted house involves a metal ring that flies upward from a table when a patron passes near the table’s edge. The device consists of a photoelectric switch that activates the circuit when anyone walks in front of the switch and of a coil of wire into which a current is suddenly introduced when the switch is triggered.
18. Suppose the change in the magnetic field is 0.10 T/s. If the radius of the ring is 2.4 cm and the ring is assumed to consist of one turn of wire, what is the emf induced in the ring?
Answer: 1.8  10–4 V

86. Ways of Inducing a Current in a Circuit
Section 1 Electricity from Magnetism
Chapter 20
Ways of Inducing a Current in a Circuit
Insert High-Res image from TR114