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Chapter 25 Electromagnetic Induction and Electromagnetic Waves © 2010 Pearson Education, Inc. PowerPoint® Lectures for College Physics: A Strategic Approach, Second Edition 25 Electromagnetic Induction and Electromagnetic Waves © 2010 Pearson Education, Inc. Slide 25-2 © 2010 Pearson Education, Inc. Slide 25-3 © 2010 Pearson Education, Inc. Slide 25-4 Electromagnetic Induction © 2010 Pearson Education, Inc. Slide 25-11 Motional emf © 2010 Pearson Education, Inc. Slide 25-12 Induced Current in a Circuit © 2010 Pearson Education, Inc. Slide 25-13 Magnetic Flux © 2010 Pearson Education, Inc. Slide 25-14 Magnetic Flux video that helped Steve R. of PH204 summer session II 2012 Ok wow, I'm super special and I completly forgot to add the link for the video lol. Here it is: http://www.youtube.com/watch?v=pB7oZNBIqq c © 2010 Pearson Education, Inc. 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. B. C. D. 30 45 60 90 © 2010 Pearson Education, Inc. Slide 25-15 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? A. B. C. D. 30 45 60 90 © 2010 Pearson Education, Inc. Slide 25-16 Lenz’s Law © 2010 Pearson Education, Inc. Slide 25-17 © 2010 Pearson Education, Inc. Slide 25-18 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? © 2010 Pearson Education, Inc. 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 © 2010 Pearson Education, Inc. Slide 25-20 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. © 2010 Pearson Education, Inc. Slide 25-21 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? A. The loop has a clockwise current. B. The loop has a counterclockwise current. C. The loop has no current. © 2010 Pearson Education, Inc. Slide 25-22 Checking Understanding 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. © 2010 Pearson Education, Inc. Slide 25-23 Answer 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. © 2010 Pearson Education, Inc. Slide 25-24 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. © 2010 Pearson Education, Inc. Slide 25-25 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? A. The loop has a clockwise current. B. The loop has a counterclockwise current. C. The loop has no current. © 2010 Pearson Education, Inc. Slide 25-26 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. Just before 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. © 2010 Pearson Education, Inc. Slide 25-27 Answer A battery, a loop of wire, and a switch make a circuit below. A second loop of wire sits directly below the first. Just before 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. © 2010 Pearson Education, Inc. Slide 25-28 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. © 2010 Pearson Education, Inc. Slide 25-29 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. B. The loop has a counterclockwise current. C. The loop has no current. © 2010 Pearson Education, Inc. Slide 25-30 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. © 2010 Pearson Education, Inc. Slide 25-31 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. B. The loop has a counterclockwise current. C. The loop has no current. © 2010 Pearson Education, Inc. Slide 25-32 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. © 2010 Pearson Education, Inc. Slide 25-33 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? A. The loop has a clockwise current. B. The loop has a counterclockwise current. C. The loop has no current. © 2010 Pearson Education, Inc. Slide 25-34 Eddy Currents © 2010 Pearson Education, Inc. Slide 25-35 Faraday’s Law © 2010 Pearson Education, Inc. Slide 25-36 Faraday’s Law of Induction Wikipedia Farady’s Law of Induction © 2010 Pearson Education, Inc. 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. A coil used to produce changing magnetic fields in a TMS (transcranial magnetic field stimulation) device produces a magnetic field that increases from 0 T to 2.5 T in a time of 200 s. Suppose this field extends throughout the entire head. Estimate the size of the brain and calculate the induced emf in a loop around the outside of the brain. © 2010 Pearson Education, Inc. Slide 25-37 Induced Fields A changing magnetic field induces an electric field. A changing electric field induces a magnetic field too. © 2010 Pearson Education, Inc. Slide 25-38 Electromagnetic Waves © 2010 Pearson Education, Inc. Slide 25-39 Checking Understanding A plane electromagnetic wave has electric and magnetic fields at all points in the plane as noted below. With the fields oriented as shown, the wave is moving A. B. C. D. E. into the plane of the paper. out of the plane of the paper. to the left. to the right. toward the top of the paper. © 2010 Pearson Education, Inc. Slide 25-40 Answer A plane electromagnetic wave has electric and magnetic fields at all points in the plane as noted below. With the fields oriented as shown, the wave is moving A. B. C. D. E. into the plane of the paper. out of the plane of the paper. to the left. to the right. toward the top of the paper. © 2010 Pearson Education, Inc. Slide 25-41 Intensity of an Electromagnetic Wave © 2010 Pearson Education, Inc. Slide 25-42 Example Problems: Electromagnetic Waves Carry Energy Inside the cavity of a microwave oven, the 2.4 GHz electromagnetic waves have an intensity of 5.0 kW/m2. What is the strength of the electric field? The magnetic field? A digital cell phone emits a 1.9 GHz electromagnetic wave with total power 0.60 W. At a cell phone tower 2.0 km away, what is the intensity of the wave? (Assume that the wave spreads out uniformly in all directions.) What are the electric and magnetic field strengths at this distance? © 2010 Pearson Education, Inc. Slide 25-43 Polarization © 2010 Pearson Education, Inc. Slide 25-44 Example Problem Light passed through a polarizing filter has an intensity of 2.0 W/m2. How should a second polarizing filter be arranged to decrease the intensity to 1.0 W/m2? © 2010 Pearson Education, Inc. Slide 25-45 The Electromagnetic Spectrum © 2010 Pearson Education, Inc. Slide 25-46 The Photon Model of Electromagnetic Waves Ephoton = hf h = 6.63 ´10 J · s -34 © 2010 Pearson Education, Inc. Slide 25-47 Example Problems A gamma ray has a frequency of 2.4 1020 Hz. What is the energy of an individual photon? A typical digital cell phone emits radio waves with a frequency of 1.9 GHz. What is the wavelength, and what is the energy of individual photons? If the phone emits 0.60 W, how many photons are emitted each second? © 2010 Pearson Education, Inc. Slide 25-48 Example Problem: The Microwave Oven Inside the cavity of a microwave oven, the 2.4 GHz electromagnetic waves have an intensity of 5.0 kW/m2. A. B. What is the strength of the electric field? The magnetic field? © 2010 Pearson Education, Inc. Slide 25-49 Thermal Emission Spectrum © 2010 Pearson Education, Inc. Slide 25-50 Understanding Global Warming with Wien’s Law and the Stephan-Boltzmann Law and spectal absorption in the atmosphere. © 2010 Pearson Education, Inc. Understanding Global Warming with Wien’s Law and the Stephan-Boltzmann Law and spectal absorption in the atmosphere. © 2010 Pearson Education, Inc. Hunting with Thermal Radiation © 2010 Pearson Education, Inc. Slide 25-51 Seeing the Universe in a Different Light © 2010 Pearson Education, Inc. Slide 25-52 Summary © 2010 Pearson Education, Inc. Slide 25-53 Summary © 2010 Pearson Education, Inc. Slide 25-54 Additional Questions A bar magnet sits inside a coil of wire that is connected to a meter. The bar magnet is at rest in 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. © 2010 Pearson Education, Inc. Slide 25-55 Answer A bar magnet sits inside a coil of wire that is connected to a meter. The bar magnet is at rest in 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. © 2010 Pearson Education, Inc. Slide 25-56 Additional 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. © 2010 Pearson Education, Inc. Slide 25-57 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. B. The current goes from left to right. C. There is no current in the meter. © 2010 Pearson Education, Inc. Slide 25-58 Additional Questions A bar magnet sits inside a coil of wire that is connected to a meter. The bar magnet is completely out of the coil and at rest. 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. © 2010 Pearson Education, Inc. Slide 25-59 Answer A bar magnet sits inside a coil of wire that is connected to a meter. The bar magnet is completely out of the coil and at rest. 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. © 2010 Pearson Education, Inc. Slide 25-60 Additional Questions A bar magnet sits inside a coil of wire that is connected to a meter. The bar magnet is reinserted into 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. © 2010 Pearson Education, Inc. Slide 25-61 Answer A bar magnet sits inside a coil of wire that is connected to a meter. The bar magnet is reinserted into 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. © 2010 Pearson Education, Inc. Slide 25-62 Additional Questions A typical analog cell phone has a frequency of 850 MHz, a digital phone a frequency of 1950 MHz. Compared to the signal from an analog cell phone, the digital signal has A. B. C. D. longer wavelength and lower photon energy. longer wavelength and higher photon energy. shorter wavelength and lower photon energy. shorter wavelength and higher photon energy. © 2010 Pearson Education, Inc. Slide 25-63 Answer A typical analog cell phone has a frequency of 850 MHz, a digital phone a frequency of 1950 MHz. Compared to the signal from an analog cell phone, the digital signal has A. B. C. D. longer wavelength and lower photon energy. longer wavelength and higher photon energy. shorter wavelength and lower photon energy. shorter wavelength and higher photon energy. © 2010 Pearson Education, Inc. Slide 25-64 Additional Questions A radio tower emits two 50 W signals, one an AM signal at a frequency of 850 kHz, one an FM signal at a frequency of 85 MHz. Which signal has more photons per second? A. The AM signal has more photons per second. B. The FM signal has more photons per second. C. Both signals have the same photons per second. © 2010 Pearson Education, Inc. Slide 25-65 Answer A radio tower emits two 50 W signals, one an AM signal at a frequency of 850 kHz, one an FM signal at a frequency of 85 MHz. Which signal has more photons per second? A. The AM signal has more photons per second. B. The FM signal has more photons per second. C. Both signals have the same photons per second. © 2010 Pearson Education, Inc. Slide 25-66 Additional Example Problems Two metal loops face each other. The upper loop is suspended by plastic springs and can move up or down. The lower loop is fixed in place and is attached to a battery and a switch. Immediately after the switch is closed, A. B. Is there a force on the upper loop? If so, in which direction will it move? Explain your reasoning. Is there a torque on the upper loop? If so, which way will it rotate? Explain your reasoning. © 2010 Pearson Education, Inc. Slide 25-67 Additional Example Problems 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. B. 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. 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. © 2010 Pearson Education, Inc. Slide 25-68