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Unformatted text preview: Chapter 21 Alternating Current Circuits and Electromagnetic Waves Quick Quizzes 1. (a), (c). The average power is proportional to the rms current which is non-zero even though the average current is zero. (a) is only valid for an open circuit, for which R . (b) and (d) can never be true because av i = for AC currents. 2. (b). Choices (a) and (c) are incorrect because the unaligned sine curves in Figure 21.9 mean the voltages are out of phase, and so we cannot simply add the maximum (or rms) voltages across the elements. (In other words, R L C V V V V + + even though R L C v v v v = + + .) 3. (b). Note that this is a DC circuit. However, changing the amount of iron inside the solenoid changes the magnetic field strength in that region and results in a changing magnetic flux through the loops of the solenoid. This changing flux will generate a back emf that opposes the current in the circuit and decreases the brightness of the bulb. The effect will be present only while the rod is in motion. If the rod is held stationary at any position, the back emf will disappear, and the bulb will return to its original brightness . 4. (b), (c). The radiation pressure (a) does not change because pressure is force per unit area. In (b), the smaller disk absorbs less radiation, resulting in a smaller force. For the same reason, the momentum in (c) is reduced. 5. (b), (d). The frequency and wavelength of light waves are related by the equation f c = or f c = , where c is the speed of light is a constant within a given medium. Thus, the frequency and wavelength are inversely proportional to each other, when one increases the other must decrease. 201 202 CHAPTER 21 Answers to Even Numbered Conceptual Questions 2. At resonance, L C X X = . This means that the impedance ( 29 2 2 L C Z R X X = +- reduces to Z = R . 4. The purpose of the iron core is to increase the flux and to provide a pathway in which nearly all the flux through one coil is led through the other. 6. The fundamental source of an electromagnetic wave is a moving charge. For example, in a transmitting antenna of a radio station, charges are caused to move up and down at the frequency of the radio station. These moving charges set up electric and magnetic fields, the electromagnetic wave, in the space around the antenna. 8. Energy moves. No matter moves. You could say that electric and magnetic fields move, but it is nicer to say that the fields 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. 10. The average value of an alternating current is zero because its direction is positive as often as it is negative, and its time average is zero. The average value of the square of the current is not zero, however, since the square of positive and negative values are always positive and cannot cancel....
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