phys1112ch24solutions - CHAPTER 24 ELECTROMAGNETIC WAVES...

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CHAPTER 24 ELECTROMAGNETIC WAVES CONCEPTUAL QUESTIONS _____________________________________________________________________________________________ 1. REASONING AND SOLUTION a. The intensity of a wave refers to the power P carried by the wave that passes perpendicularly through a surface in the path of the wave, divided by the area A of the surface. It is given for sound waves by Equation 16.8 and for electromagnetic waves by Equation 24.4. The concept of intensity applies to both sound waves and electromagnetic waves, because both types of wave transfer energy and, therefore, power away from their respective sources. b. In a transverse wave, the particles of the medium vibrate perpendicular to the direction of propagation of the wave. In the case of electromagnetic waves, there are no particle vibrations, but rather the electric and magnetic fields oscillate perpendicular to the direction of propagation of the wave. When a wave is produced or altered so that the vibrations or oscillations take place in a particular direction perpendicular to the wave velocity, the wave is said to be polarized . In a longitudinal wave, the notion of polarization has no meaning, as discussed in Section 24.6 of the text. Therefore, transverse waves can be polarized , while longitudinal waves cannot be polarized . Since sound waves are longitudinal waves, while electromagnetic waves are transverse waves, the concept of polarization applies only to electromagnetic waves. ____________________________________________________________________________________________ 2. REASONING AND SOLUTION Refer to Figure 24.2. Between the times indicated in parts c and d in the drawing, negative charges have moved to the top of the antenna, leaving a net positive charge of equal magnitude on the bottom of the antenna. Therefore, as the negative charges flow, the conventional current points toward the bottom of the antenna Using RHR-2, the magnetic field for the electromagnetic wave at P must point out of the page. ____________________________________________________________________________________________ 3. SSM REASONING AND SOLUTION A transmitting antenna is located at the origin of an x , y , z axis system, and broadcasts an electromagnetic wave whose electric field oscillates along the y axis. The wave travels along the + x axis. Three possible loops can be used with an LC-tuned circuit to detect the wave: One loop lies in the xy plane, another in the xz plane, and the third in the yz plane. The loop that will detect the electromagnetic wave must be oriented so that the normal to the loop is parallel to the magnetic field. Then, as the wave passes by the loop, the changing magnetic field penetrates the loop and results in an induced emf and current, as predicted by Faraday's law. Since the electromagnetic wave travels along the + x direction, and the electric field oscillations of the electromagnetic wave are along the y axis, the magnetic field oscillations will be along the z axis. For optimum reception, therefore, the loop should lie in the xy plane so that the normal to the loop is in the z direction.
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This note was uploaded on 04/14/2008 for the course PHY 1409 taught by Professor Vasut during the Spring '08 term at Baylor.

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phys1112ch24solutions - CHAPTER 24 ELECTROMAGNETIC WAVES...

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