Teach Yourself Electricity and Electronics, 5th edition
Stan Gibilisco
Explanations for Quiz Answers in Chapter 30
1. Capacitive proximity sensors are intended for close range, so choice (a) won't work. Dynamic transducers don't work with IR, so (b) is wrong. Optical shaft encoders measure the angular displacements of rotatable controls, so they're irrelevant here; choice (c) is wrong! The correct choice is (d), "An acoustic direction finder with multiple ultrasonic beacons." That scheme will work in a large enclosed space such as a gymnasium.
2. The water droplets in clouds reflect radar signals, and all thunderstorms (including those that produce tornadoes) contain clouds. Tornadoes often produce "hook-shaped" echoes on radar displays because of rotation in the cloud complexes. Therefore, we might use a radar set to identify and locate a tornado in our general vicinity at night, when visual observation is impractical. The correct choice is (a). The other three choices are absolutely wrong. Sonar sets, fluxgate magntometers, and capacitive proximity sensors all have uses entirely different from radar applications.
3. A fluxgate magnetometer uses magnetic receptors and a microcomputer to sense the presence of, and detect changes in, an external magnetic field. Capacitive proximity sensors, epipolar navigation systems, synchros, and selsyns don't operate that way. The correct choice is (d), "None of the above."
4. If we want to control and display the compass direction (or azimuth) in which an antenna points, we can employ a synchro. In fact, most electromechanical antenna rotators use synchros. The answer is (c).
5. An elastomer pad exhibits variable resistance (and therefore variable conductance) depending on the extent to which we compress it. The correct choice is (a).
6. A radar display showing azimuth (compass direction) and range (distance from the observing station) has the same geometric configuration as a polar coordinate system. The correct choice is (d).
7. An angular displacement transducer can handily, and directly, define or measure the angle through which an antenna rotator turns, and therefore the compass direction in which the antenna points. The correct choice is (a). When we talk about "displacement," we can refer to either linear (straight-line) motion or rotational motion. If any doubt exists as to whether a particular device measures linear or angular quantities, we should clearly specify which one!
8. A fluxgate magnetometer determines location on the basis of the orientations of the lines of flux from external magnets. The correct choice is (b).
9. Electric motors, electric generators, and selsyns all transform energy from electrical to mechanical form or vice-versa, so they're all electromechanical transducers. A fluxgate magnetometer consists of only magnetic receptors and a computer; it has no moving parts of its own. The correct choice is (d).
10. A low-frequency RDF (radio direction-finding) system uses a small loop or loopstick antenna to pick up the signals. The correct choice is (b).
11. A robot can employ a back-pressure sensor to regulate or limit the torque produced by an electric motor such as the one that drives a drill. The correct choice is (b). The other three choices are "way off"; visible light, magnetic flux orientation, and acoustic output have nothing to do with the regulation of torque.
12. An AF voltage between the plates of an electrostatic transducer causes changes in the intensity of the electric field between the plates. This varying field exerts a fluctuating mechanical force between the plates. In response to the varying force, the flexible plate moves in and out, creating sound waves in the air. The correct choice is (c).
13. All antennas are RF transducers, so (a) can serve as a plausible answer. A   back-pressure sensor and an elastomer (choices b and c) both constitute electromechanical transducers, so these choices won't work. An epipolar navigation system operates with visible light (or sometimes IR) but not RF, so choice (d) is wrong. The correct choice is therefore (a).
14. When sound waves strike the diaphragm of a dynamic transducer, the diaphragm moves back and forth. It's attached to the coil, so the coil moves along with it. A fixed magnet runs along the coil axis, so as the coil moves relative to the magnetic field, electric currents flow in the wire. These currents follow the waveform of the sound. The correct choice is (a).
15. When a stepper motor shaft has come to rest and DC continues to flow in the motor coils, the shaft tends to remain fixed, resisting applied torque. As the strength of the DC increases, so does the shaft's resistance to any external attempt to turn it. The correct choice is (b).
16. Black surfaces, especially those with a matte texture such as a black sheet of paper, do not reflect enough visible light to allow a photoelectric proximity sensor to operate effectively. The correct choice is (b). A gray cloth curtain (a) and an orange basketball (c) reflect and scatter incident light rays quite well, so a photoelectric proximity sensor would have no difficulty detecting them.
17. Using an epipolar navigation system, we can determine our location, and even define our speed and direction, from a single vantage point. This feature gives epipolar navigation a big advantage over other location and navigation methods. The correct answer is (a).
18. A loran system operates at radio frequencies below 2 MHz, much lower than the frequency of a radar set or the GPS (both of which work in the microwave range at many gigahertz) and also far lower than the frequency of a photoelectric proximity sensor (which works at visible-light frequencies of many terahertz). The correct choice is (d).
19. A dynamic transducer operates on the basis of varying magnetic forces between a magnet and a current-carrying coil of wire. The correct choice is (c). Sonar works with acoustic waves, so (a) is wrong. Loran works with RF signals, so (b) is wrong. A capacitive proximity sensor works on the basis of interelectrode capacitance, not magnetic fields or forces, so (d) is wrong.
20. We can detect or generate ultrasonic waves using a piezoelectric transducer, which is a form of acoustic transducer. The correct choice is (b). An engineer wouldn't try to use this type of transducer to detect magnetic fields, RF energy at any frequency, or motor torque.