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Do you wonder why ac is used at all Isn t it a lot more complicated than dc Well, ac may be more complicated in theory, but in practice it is a lot simpler to use when it is necessary to provide electricity to a large number of people. Alternating current lends itself well to being transformed to lower or higher voltages, according to the needs of electrical apparatus. It is not so easy to change dc voltages. Electrochemical cells produce dc directly, but they are impractical for the needs of large populations. Serving millions of consumers requires the immense power of falling or flowing water, the ocean tides, wind, fossil fuels, controlled nuclear reactions, or geothermal heat. All of these energy sources can be used to drive turbines that turn ac generators. Technology is advancing in the realm of solar-electric energy; someday a significant part of our electricity might come from photovoltaic power plants. These would generate dc. High voltages could be attained by connecting giant arrays of solar panels in series. But there would be a problem transforming this voltage down to manageable levels for consumer use. Thomas Edison is said to have favored dc over ac for electrical power transmission in the early days, as the electric utilities were first being devised and constructed. His colleagues argued that ac would work better. But perhaps Edison knew something that his contemporaries did not. There is one advantage to dc in utility applications, and it involves the transmission of energy over great distances using wires. Direct currents, at extremely high voltages, are transported more efficiently than alternating currents. The wire has less effective resistance with dc than with ac, and there is less energy lost in the magnetic fields around the wires. Direct-current high-tension transmission lines are being considered for future use. Right now, the main problem is expense. Sophisticated power-conversion equipment is needed. If the cost can be brought within reason, Edison will be vindicated.

Refer to the text in this chapter if necessary. A good score is at least 18 correct. Answers are in the back of the book. 1. Which of the following can vary with ac, but never with dc (a) Power (b) Voltage (c) Frequency (d) Amplitude 2. The length of time between a point in one cycle and the same point in the next cycle of an ac wave is the (a) frequency. (b) magnitude. (c) period. (d) polarity. 3. On a spectrum analyzer, an ac signal having only one frequency component looks like (a) a single pip. (b) a sine wave. (c) a square wave. (d) a sawtooth wave. 4. The period of an ac wave, in seconds, is (a) the same as the frequency in hertz. (b) not related to the frequency in any way. (c) equal to 1 divided by the frequency in hertz. (d) equal to the peak amplitude in volts divided by the frequency in hertz. 5. The sixth harmonic of an ac wave whose period is 1.000 millisecond (1.000 ms) has a frequency of (a) 0.006 Hz. (b) 167.0 Hz. (c) 7.000 kHz. (d) 6.000 kHz. 6. A degree of phase represents (a) 6.28 cycles. (b) 57.3 cycles. (c) 1 60 of a cycle. (d) 1 360 of a cycle.

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7. Suppose that two ac waves have the same frequency but differ in phase by exactly 1 20 of a cycle. What is the phase difference between these two waves (a) 18 (b) 20 (c) 36 (d) 5.73 8. Suppose an ac signal has a frequency of 1770 Hz. What is its angular frequency (a) 1770 rad/s (b) 11,120 rad/s (c) 282 rad/s (d) Impossible to determine from the data given 9. A triangular wave exhibits (a) an instantaneous rise and a defined decay. (b) a defined rise and an instantaneous decay. (c) a defined rise and a defined decay, and the two are equal. (d) an instantaneous rise and an instantaneous decay. 10. Three-phase ac (a) has sawtooth waves that add together in phase. (b) consists of three sine waves in different phases. (c) is a sine wave with exactly three harmonics. (d) is of interest only to physicists. 11. If two perfect sine waves have the same frequency and the same amplitude, but are in opposite phase, the composite wave (a) has twice the amplitude of either input wave alone. (b) has half the amplitude of either input wave alone. (c) is complex, but has the same frequency as the originals. (d) has zero amplitude (that is, it does not exist), because the two input waves cancel each other out. 12. If two perfect sine waves have the same frequency and the same phase, the composite wave (a) is a sine wave with an amplitude equal to the difference between the amplitudes of the two input waves. (b) is a sine wave with an amplitude equal to the sum of the amplitudes of the two original waves. (c) is not a sine wave, but has the same frequency as the two input waves. (d) has zero amplitude (that is, it does not exist), because the two input waves cancel each other out.