Teach Yourself Electricity and Electronics, 5th edition
Stan Gibilisco
Explanations for Quiz Answers in Chapter 8
1. The magnetomotive force in ampere-turns (At) equals the current in amperes times the number of turns in the coil. In this case, we have a current of 200 mA or 0.200 A, and the coil has 100 turns. The magnetomotive force therefore equals 0.200 x 100, or 20.0 At. The correct choice is (c).
2. An AC electromagnet produces an alternating magnetic field whose polarity reverses twice with each complete wave cycle. The instantaneous magnetic-field intensity varies from moment to moment in time as the AC cycle proceeds, following along with the instantaneous current. The correct answer is (a).
3. Diamagnetic materials always exhibit permeability values less than 1. The correct answer is (d). Choice (a) is wrong, because there's no point in using a diamagnetic core in attempt to make a DC electromagnet. We could just as well (and more cheaply) wind a coil around a cardboard cylinder such as the sort left over when we've finished using a roll of paper towels! Choice (b) won't do. We could never make a permanent magnet from a sample of diamagnetic material; it simply wouldn't work. Choice (c) is absolutely wrong. It describes the characteristics of a ferromagnetic substance, not a diamagnetic substance..
4. When we have a constant flow of DC in a straight wire, we get circular magnetic flux curves around the wire. The centers of all the flux circles lie on the wire. The correct answer is (c).
5. If we want to build a powerful electromagnet, we should use a core material that exhibits high permeability. If we want the core to follow along with rapid changes in the intensity of the coil's magnetic field (as will happen if we drive AC through the coil), we should choose a core material with low retentivity. The correct answer is (b).
6. By definition, the permeability of a material equals the ratio of the magnetic flux density inside that material to the flux density in a vacuum, assuming the same external conditions. The information we have here does not tell us this ratio. Therefore, the correct choice is (a). The question does give us information enough to figure out the retentivity of the material, however; it equals 20/800, or 2.5%.
7. Our rod length s equals 10 cm, which we should convert to 0.10 m. The number of coil turns n equals 400. The coil core has permeability m = 1.5 x 105, and we're told that it has not reached a state of saturation. The flux density inside the core material is Bt = 0.30 T. We can approximate the coil current I (in amperes) using the formula

Bt = 1.2566 x 10-6 m nI / s

from the chapter text. Plugging in the numbers, we get

0.30 = 1.2566 x 10-6 x 1.5 x 105 x 400 x I / 0.10

which simplifies to

0.30 = 753.96 I

We can solve this equation to get

I = 0.30 / 753.96
= 0.00040 A
= 0.40 mA

The correct answer is (a).
8. Of the units listed here, the maxwell is the only one that expresses overall magnetic field quantity. The correct answer is (b).
9. Our distance r from the wire equals 2.50 m, and the wire carries a current of I = 500 mA = 0.500 A. We can input these values into the formula for flux density in teslas to obtain

Bt = 2 x 10-7 I / r
= 2.00 x 10-7 x 0.500 / 2.50
= 4.00 x 10-8 T
= 4.00 x 10-4 G

The correct choice is (d).
10. The flux density inside the coil varies in direct proportion to the current in the coil, and also in direct proportion to the number of coil turns, assuming that the coil length does not vary, and also assuming that the core material never reaches a state of saturation. In this situation, we increase the number of coil turns by a factor of 4 while leaving the current and the coil length the same. We've been assured that the core material never saturates. Therefore, we can expect the flux density inside the core to increase by a factor of 4. The correct answer is (b).
11. The term normally closed tells us that a relay closes an external circuit when its coil carries no current. In that sense, "normally" means "when no current flows in the coil." The correct answer is (c).
12. We define remanence, also called retentivity, as the extent to which a substance retains magnetism after we remove an external magnetic field. The correct answer is (b).
13. In magnetism, we define geomagnetic inclination as the vertical tilt of the geomagnetic lines of flux with respect to the earth's surface at a particular location. The correct answer is (a).
14. By definition, ferromagnetic materials exhibit permeability values greater than 1. Such media concentrate magnetic lines of flux, compared with the flux density in air or a vacuum under the same external conditions. An increased concentration of flux lines in a localized region translates to an increase in the magnetic-field intensity in that region. The correct answer is (b).
15. At the poles of any magnet, the lines of flux converge or diverge. This holds true for the entire planet earth, just as it holds true for a small bar magnet. The correct answer is (d).
16. The magnetomotive force produced by a solenoidal wire coil varies in direct proportion to the number of coil turns, and also in direct proportion to the current in the wire. The resistance of the wire, by itself, doesn't matter. The correct choice here is (a), because we can't solve the problem unless we know how much current flows in the coil.
17. Our solenoidal coil has 500 turns, and it carries 200 mA. We can convert the current to amperes, getting 0.200 A, and then multiply by the number of turns to get a magnetomotive force of 500 x 0.200 = 100 At. All of the choices here ask for gilberts. We must multiply ampere-turns by 1.257 to obtain gilberts. Therefore, this coil produces 100 x 1.257 = 125.7 Gb of magnetomotive force. Because we have input data accurate to only three significant figures, we should round this result off to 126 Gb. The correct choice is (d).
18. The magnetomotive force produced by a solenoidal coil does not depend on the permeability of the core material, but only on the current through the wire and the number of turns in the coil. Therefore, the correct answer is (b).
19. If we have two rod-shaped magnets with their magnetic poles at the ends, and then we bring the ends near each other, the rods will attract if the proximate poles oppose, and the rods will repel if the proximate poles are the same. The correct answer is (a).
20. We can expect a geomagnetic storm to take place shortly after the occurrence of a solar flare. Geomagnetic storms have nothing to do with declination, inclination, or volcanoes. The correct answer is (d).