Chapter 34 - 34 Electromagnetic Waves CHAPTER OUTLINE 34.1...

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34 CHAPTER OUTLINE 34.1 Maxwell’s Equations and Hertz’s Discoveries 34.2 Plane Electromagnetic Waves 34.3 Energy Carried by Electromagnetic Waves 34.4 Momentum and Radiation Pressure 34.5 Production of Electromagnetic Waves by an Antenna 34.6 The Spectrum of Electromagnetic Waves Electromagnetic Waves ANSWERS TO QUESTIONS Q34.1 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 0 m 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. Q34.2 The Sun’s angular speed in our sky is our rate of rotation, 360 24 15 ° h h. In 8.3 minutes it moves west by θω ==° F H G I K J t 15 1 83 21 h h 60 min min bg af . . . This is about four times the angular diameter of the Sun. Q34.3 Energy moves. No matter moves. You could say that electric and magnetic fields move, but it is nicer to say that the fields at one point stay at that point and oscillate. The fields vary in time, like sports fans in the grandstand when the crowd does the wave. The fields constitute the medium for the wave, and energy moves. Q34.4 No. If a single wire carries DC current, it does not emit electromagnetic waves. In this case, there is a constant magnetic field around the wire. Alternately, if the cable is a coaxial cable, it ideally does not emit electromagnetic waves even while carrying AC current. Q34.5 Acceleration of electric charge. Q34.6 The changing magnetic field 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. Q34.7 A wire connected to the terminals of a battery does not radiate electromagnetic waves. The battery establishes an electric field, which produces current in the wire. The current in the wire creates a magnetic field. Both fields are constant in time, so no electromagnetic induction or “magneto-electric induction” happens. Neither field creates a new cycle of the other field. No wave propagation occurs. 291
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292 Electromagnetic Waves Q34.8 No. Static electricity is just that: static. Without acceleration of the charge, there can be no electromagnetic wave. Q34.9 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 disturbances 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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This homework help was uploaded on 04/13/2008 for the course PHYS 211 taught by Professor Shannon during the Spring '08 term at MSU Bozeman.

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Chapter 34 - 34 Electromagnetic Waves CHAPTER OUTLINE 34.1...

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