Electricity Demystified, 2nd edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 9

1. Coaxial cable has two conductors: a central wire and a surrounding cylindrical or tubular shield. Banana plugs and alligator clips are both designed for single-wire conductors, so choices B and D do not apply. The D-shell design is meant for cables having more than two conductors, so choice A doesn't  apply either. We can attach a phono plug to the end of a length of coaxial cable, however, so choice C is correct.

2. Silver constitutes the best known electrical conductor at room temperature, so D is the correct choice. Copper and aluminum conduct quite well, and steel fairly well, but none of them have resistance-per-unit-length values as low as silver does.

3. The narrower of the two blades in a polarized utility plug, intended for use at a nominal 117 V RMS AC in the United States, connects to the "hot" wire in the cord. That is, it carries single-phase AC. The correct choice is A. The wider blade goes to ground, so choice B is wrong. A single connector can't carry more than one phase of AC, so choice C is wrong. Although we might run DC through a common polarized utility plug, we would never run DC through a standard utility circuit (as the question specifically says), so choice D won't work.

4. The D-shell design is irrelevant to this application, so choice A is wrong. Banana plugs and phono plugs lack the clamping action needed to maintain a physical and electrical connection to a length of exposed wire, so choices B and C are both wrong. Alligator clips will work well, however. They'll "bite" down on the wire and stay there. The correct choice is D.

5. For the transfer of radio-frequency (RF) signals, such as the connection between a radio receiver or transmitter and its antenna, coaxial cable represents the best choice of the four given here. The correct answer is B.

6. Stranded wire stretches less, not more, easily than solid wire made of the same substance and having the same overall diameter, so choice A is wrong. Stranded wire breaks less, not more, easily than solid wire does, so choice C is wrong. Stranded wire adheres more easily to solder than solid wire does, so choice D is incorrect as well. The only choice left is B, and it makes a true statement. If we have a length of stranded wire and an equal length of solid wire, both having the same gauge (in this case AWG No. 12) and both made of the same material (in this case pure copper), the length of stranded wire has slightly greater resistance than the length of solid wire does.

7. If we want to make a temporary connection and we aren't concerned with physical strength, a twist splice will work well. The correct choice is A. We can't expect a twist splice to hold up very well under mechanical stress, so choice B is wrong. Choice C is completely false; twist splices are easy to make. Choice D is also wrong, because we can make a twist splice perfectly well if one or both wires are stranded.

8. Given an unlimited supply of wire made of a certain material and having a uniform diameter (gauge) all along its length, the resistance varies in direct proportion to the length. If we increase the length from 1000 meters (m) to 1414 m, that's a factor of 1414/1000, or 1.414. If the resistance of a 1000-m span equals 2.000 ohms, the resistance of a 1414-m span will equal 2.000 x 1.414, or 2.828 ohms. The correct choice is C.

9. Given wire made of a certain material and having a uniform gauge, we can state its resistance per unit length without regard to how much of the wire we use. We're told that the wire in question exhibits 2000 micro-ohms per meter. That's the resistance-per-unit-length value for any span of that wire, no matter how long. The correct choice is B.

10. A good solder joint has a shiny, smooth appearance. In contrast, a cold solder joint typically looks dull or rough. The correct choice is D.