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Transcript
Electromagnetic Induction and Electromagnetic Waves Topics: • Electromagnetic induction • Lenz’s law • Faraday’s law • The nature of electromagnetic waves • The spectrum of electromagnetic waves 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? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-1 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-3 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-4 Electromagnetic Induction Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-8 Motional emf e = vlB Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-9 Induced Current in a Circuit Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-13 Magnetic Flux Fm = A × B = A B cos a The vector A is the area vector => it points perpendicular to the area Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-10 Checking Understanding A loop of wire of area A is tipped at an angle q to a uniform magnetic field B. The maximum flux occurs for an angle q = 0°. What angle q will give a flux that is ½ of this maximum value? A. q = 30° B. q = 45° C. q = 60° D. q = 90° Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-11 Answer A loop of wire of area A is tipped at an angle q to a uniform magnetic field B. The maximum flux occurs for an angle q = 0°. What angle q will give a flux that is ½ of this maximum value? C. q = 60° Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-12 Faraday’s Law Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-15 Lenz’s Law Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-17 Using Lenz’s Law Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-14 Checking Understanding A magnetic field goes through a loop of wire, as below. If the magnitude of the magnetic field is constant, 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. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-22 Answer A magnetic field goes through a loop of wire, as below. If the magnitude of the magnetic field is constant, what can we say about the current in the loop? C. The loop has no current. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-23 Checking Understanding 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. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-18 Answer 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. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-19 Checking Understanding 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. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-26 Answer 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. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-27 Checking Understanding 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. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-28 Answer 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. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-29 Checking Understanding 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. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-30 Answer 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. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-31 Additional Clicker Questions A bar magnet sits inside a coil of wire that is connected to a meter. The bar magnet is pulled out of the coil. What can we say about the current in the meter? A. The current goes from right to left. B. The current goes from left to right. C. There is no current in the meter. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-44 Answer A bar magnet sits inside a coil of wire that is connected to a meter. The bar magnet is pulled out of the coil. What can we say about the current in the meter? A. The current goes from right to left. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-45 Checking Understanding A long conductor carrying a current runs next to a loop of wire. The current in the wire varies as in the graph. Which segment of the graph corresponds to the largest induced current in the loop? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-19 Answer A long conductor carrying a current runs next to a loop of wire. The current in the wire varies as in the graph. Which segment of the graph corresponds to the largest induced current in the loop? E Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-20 Example Problems The figure shows a 10-cm-diameter loop in three different magnetic fields. The loop’s resistance is 0.1 Ω. For each situation, determine the magnitude and direction of the induced current. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-37 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) 3.Compute the emf in the loop while it is entering the field. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-32 Additional Examples 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. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-55 Eddy Currents Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-35 Induced Fields A changing magnetic field induces an electric field. A changing electric field induces a magnetic field too. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-38 Electromagnetic Waves Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 25-39