1. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electromagnetic induction Lenz’s law Faraday’s law The nature of electromagnetic waves The spectrum of electromagnetic waves Electromagnetic Induction and Electromagnetic Waves Topics: Sample question: The ultraviolet view of the flowers on the right shows markings that cannot be seen in the visible region of the spectrum. Whose eyes are these markings intended for? Slide 25-1

2. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Lenz’s Law Slide 25-13 Lenz’s law There is an induced current in a closed, conducting loop if and only if the magnetic flux through the loop is changing. The direction of the induced current is such that the induced magnetic field opposes the change in the flux.

3. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Using Lenz’s Law Slide 25-14

4. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A magnetic field goes through a loop of wire, as below. If the magnitude of the magnetic field is increasing, what can we say about the current in the loop? A.The loop has a clockwise current. B.The loop has a counterclockwise current. C.The loop has no current. Slide 25-18 Checking Understanding

5. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A magnetic field goes through a loop of wire, as below. If the magnitude of the magnetic field is increasing, what can we say about the current in the loop? B.The loop has a counterclockwise current. Slide 25-19 Answer

6. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A magnetic field goes through a loop of wire, as below. If the magnitude of the magnetic field is decreasing, what can we say about the current in the loop? A.The loop has a clockwise current. B.The loop has a counterclockwise current. C.The loop has no current. Slide 25-20 Checking Understanding

7. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A magnetic field goes through a loop of wire, as below. If the magnitude of the magnetic field is decreasing, what can we say about the current in the loop? A.The loop has a clockwise current. Slide 25-21 Answer

8. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A battery, a loop of wire, and a switch make a circuit below. A second loop of wire sits directly below the first. Immediately after the switch is closed, what can we say about the current in the lower loop? A.The loop has a clockwise current. B.The loop has a counterclockwise current. C.The loop has no current. Slide 25-26 Checking Understanding

9. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A battery, a loop of wire, and a switch make a circuit below. A second loop of wire sits directly below the first. Immediately after the switch is closed, what can we say about the current in the lower loop? A.The loop has a clockwise current. Slide 25-27 Answer

10. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A battery, a loop of wire, and a switch make a circuit below. A second loop of wire sits directly below the first. Long after the switch is closed, what can we say about the current in the lower loop? A.The loop has a clockwise current. B.The loop has a counterclockwise current. C.The loop has no current. Slide 25-28 Checking Understanding

11. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A battery, a loop of wire, and a switch make a circuit below. A second loop of wire sits directly below the first. Long after the switch is closed, what can we say about the current in the lower loop? C.The loop has no current. Slide 25-29 Answer

12. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A battery, a loop of wire, and a switch make a circuit below. A second loop of wire sits directly below the first. Immediately after the switch is reopened, what can we say about the current in the lower loop? A.The loop has a clockwise current. B.The loop has a counterclockwise current. C.The loop has no current. Slide 25-30 Checking Understanding

13. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A battery, a loop of wire, and a switch make a circuit below. A second loop of wire sits directly below the first. Immediately after the switch is reopened, what can we say about the current in the lower loop? B.The loop has a counterclockwise current. Slide 25-31 Answer

14. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-32 Into the Field A five-turn rectangular loop is moved through a uniform field at 2 m/s as shown below. 1.What is the maximum magnetic flux through the loop during its motion through the field? The loop is 5 cm long and 3 cm wide. 2.The loop takes 100 ms to completely enter the field. Sketch a graph of the magnetic flux through the loop in the interval from t=0 to t=150 ms. Label values of flux. (Assume the loop begins to enter the magnetic field at t = 0 s)

15. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 2.The outer coil of wire is 10 cm long, 2 cm in diameter, wrapped tightly with one layer of 0.5-mm-diameter wire, and has a total resistance of 1.0 Ω. It is attached to a battery, as shown, that steadily decreases in voltage from 12 V to 0 V in 0.5 s, then remains at 0 V for t > 0.5 s. The inner coil of wire is 1 cm long, 1 cm in diameter, has 10 turns of wire, and has a total resistance of 0.01 Ω. It is connected, as shown, to a current meter. a.As the voltage to the outer coil begins to decrease, in which direction (left-to-right or right-to-left) does current flow through the meter? Explain. b.Draw a graph showing the current in the inner coil as a function of time for 0 ≤ t ≤ 1 s. Include a numerical scale on the vertical axis. Slide 25-55 Additional Examples

16. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Generators A generator is a device that converts mechanical energy to electric energy. Rather than move a straight wire through a magnetic field, it is more practical to rotate a coil of wire. As the coil rotates, one edge always moves upward through the electric field while the other edge moves downward. The motion of the wire induces a current, which is then removed by brushes that press up against rotating slip rings.

17. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Generators

18. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Generators As the coil in a generator rotates, the sense of emf changes, giving a sinusoidal variation of emf as a function of time. The alternating sign of the voltage produces an alternating current, AC.

19. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Electric Generators A sinusoidal emf is induced in the rotating loop ( N is the number of turns, and A the area of the loop): (21-5)