SM_PDF_chapter24 - Electromagnetic Waves CHAPTER OUTLINE...

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661 Electromagnetic Waves CHAPTER OUTLINE 24.1 Displacement Current and the Generalized Ampère’s Law 24.2 Maxwell’s Equations 24.3 Electromagnetic Waves 24.4 Hertz’s Discoveries 24.5 Energy Carried by Electromagnetic Waves 24.6 Momentum and Radiation Pressure 24.7 The Spectrum of Electromagnetic Waves 24.8 Polarization 24.9 Context Connection The Special Properties of Laser Light ANSWERS TO QUESTIONS Q24.1 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 5 10 10 6 2 × × m 3 10 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. Q24.2 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. Q24.3 Acceleration of electric charge. Q24.4 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. Q24.5 No. Static electricity is just that: static. Without acceleration of the charge, there can be no electromagnetic wave. Q24.6 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. Light The universe of light fills the whole universe. Light moves through materials, but faster in a vacuum. Light propagates by a chain reaction of electric and magnetic fields recreating each other. Light in air moves at hundreds of millions of meters per second. Visible light has frequencies over a range of less than one octave, from 430 to 750 Tera hertz. Visible light has wavelengths of very small size (400 nm to 700 nm). Light waves are transverse. continued on next page
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662 Electromagnetic Waves Sound and light can both be reflected, refracted, or absorbed to produce internal energy. Both have amplitude and frequency set by the source, speed set by the medium, and wavelength set by both source and medium. Sound and light both exhibit the Doppler effect, standing waves, beats, interference, diffraction, and resonance. Both can be focused to make images. Both are described by
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