SM_chapter34

SM_chapter34 - 34 Electromagnetic Waves CHAPTER OUTLINE...

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34 Electromagnetic Waves CHAPTER OUTLINE 34.1 Displacement Current and the General Form of Ampère’s Law 34.2 Maxwell’s Equations and Hertz’s Discoveries 34.3 Plane Electromagnetic Waves 34.4 Energy Carried by Electromagnetic Waves 34.5 Momentum and Radiation Pressure 34.6 Production of Electromagnetic Waves by an Antenna 34.7 The Spectrum of Electromagnetic Waves ANSWERS TO QUESTIONS *Q34.1 Maxwell included a term in Ampère’s law to account for the contributions to the magnetic F eld by changing electric F elds, by treating those changing electric F elds as “displacement currents.” *Q34.2 No, they do not. SpeciF cally, Gauss’s law in magnetism prohibits magnetic monopoles. If magnetic monopoles existed, then the magnetic F eld lines would not have to be closed loops, but could begin or terminate on a magnetic monopole, as they can in Gauss’s law in electrostatics. Q34.3 Radio waves move at the speed of light. They can travel around the curved surface of the Earth, bouncing between the ground and the ionosphere, which has an altitude that is small when compared to the radius of the Earth. The distance across the lower forty-eight states is approximately 5 000 km, requiring a transit time of 51 0 10 6 2 × × m 31 0m s s 8 ~ . To go halfway around the Earth takes only 0.07 s. In other words, a speech can be heard on the other side of the world before it is heard at the back of a large room. Q34.4 Energy moves. No matter moves. You could say that electric and magnetic F elds move, but it is nicer to say that the F elds at one point stay at that point and oscillate. The F elds vary in time, like sports fans in the grandstand when the crowd does the wave. The F elds constitute the medium for the wave, and energy moves. Q34.5 The changing magnetic F eld of the solenoid induces eddy currents in the conducting core. This is accompanied by IR 2 conversion of electrically-transmitted energy into internal energy in the conductor. *Q34.6 (i) According to fL C = −− ()( ) , / 2 11 2 π to make f half as large, the capacitance should be made four times larger. Answer (a). (ii) Answer (b). *Q34.7 Answer (e). Accelerating charge, changing electric F eld, or changing magnetic F eld can be the source of a radiated electromagnetic wave. *Q34.8 (i) Answer (c). (ii) Answer (c). (iii) Answer (c). (iv) Answer (b). (v) Answer (b). *Q34.9 (i) through (v) have the same answer (c). 271
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272 Chapter 34 Q34.10 Sound The world of sound extends to the top of the atmosphere and stops there; sound requires a material medium. Sound propagates by a chain reaction of density and pressure dis- turbances recreating each other. Sound in air moves at hundreds of meters per second. Audible sound has frequencies over a range of three decades (ten octaves) from 20 Hz to 20 kHz. Audible sound has wavelengths of ordinary size (1.7 cm to 17 m). Sound waves are longitudinal.
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SM_chapter34 - 34 Electromagnetic Waves CHAPTER OUTLINE...

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