Sec 7.6 - 7 Waves: Sound, Radio, and Light I do not think...

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217 7 Waves: Sound, Radio, and Light I do not think that the wireless waves I have discovered will have any practical application. Heinrich Hertz (ca. 1890 ) 7.1 COMMUNICATING WITH SOUND, RADIO, AND LIGHT The Internet consists of millions of computers connected in such a way that they can exchange information (i.e., communicate). They do this using electromagnetic ( EM ) waves , which are carried either through the air, through metal wire, or through thin glass F bers, to the receiver’s computer. ±or example, the digital data that you down- load from the Internet might be sent from one city to another using light waves in an optical F ber. Then it might be converted into voltages and sent along a copper wire to a wireless router, where it would be converted into radio waves and broadcast to your computer. The data that you downloaded could represent music. After those data are reconstituted into an analog form, they can drive a stereo speaker, creating sound. The sound waves travel through the air and F nally arrive at your ears. We see that the con- cept of waves plays a key part in these phenomena. We will begin this discussion with the topic of harmonic motion and waves in general, and then move on to sound, radio, and light waves. Heinrich Hertz, the F rst person to generate wire- less EM (radio) waves, in 1888. Using cell phones on a sunny day. Radio waves, sound, and light all come together. TAF-K10173-08-1107-007.indd 217 TAF-K10173-08-1107-007.indd 217 4/24/09 9:28:17 PM 4/24/09 9:28:17 PM
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218 The Silicon Web: Physics for the Internet Age 7.2 SIMPLE HARMONIC MOTION A grandfather, or pendulum, clock “counts” time by the number of oscillations the pendulum makes. The simplest pendulum clock is in Figure 7.1 , showing a metal ball (called a bob) F xed to a rigid rod that is swinging from a low-friction pivot. When at rest, the bob is in the equilibrium position. When set into motion, the bob moves back and forth along an arc. The center picture shows the motion of the bob just after it has been set into motion. The distance the bob travels between its center position and its extreme position is called the amplitude of the motion. Remarkably, the time it takes for the bob to move between its two extreme positions is nearly the same regardless of whether the amplitude is small or is large. The story goes that 17-year-old Galileo noticed this behavior while in church, watching the motion of a swinging chandelier. As the service wore on, the pendulum’s amplitude grew smaller, yet the time between the plate reaching its extreme position remained the same (he timed the pendulum’s motion using his own heartbeat). If we graph the horizontal position of the pendulum versus time, we see a curve as shown in Figure 7.2 . By horizontal position, we mean the position of a shadow on the ± oor below the bob that would be produced by shining a light from above the pendu- lum. The shadow would move back and forth along a straight line. At any instant of time, the shadow’s distance from the equilibrium position would be given by the value of the oscillating curve shown in the graph. Any curve with this speciF
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This note was uploaded on 09/29/2011 for the course PHYS 222 taught by Professor Wade during the Spring '09 term at Edmonds Community College.

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Sec 7.6 - 7 Waves: Sound, Radio, and Light I do not think...

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