Electricity Demystified, 2nd edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 12

1. In order for a diamagnetic material to repel an electromagnet with significant force, we must use a powerful electromagnet. In fact, for common diamagnetic materials, the electromagnet must produce an extremely high flux density. The correct choice is B.

2. Earnshaw proved that no set of fixed permanent magnets can produce levitation. If we want to obtain levitation with permanent magnets, then at least one of them must move or rotate. The answer is D.

3. In theory, a superconducting material offers no opposition whatsoever to the flow of electric current. In other words, its resistance is theoretically zero. The correct choice is A.

4. If we attach small wheels to the cars of a maglev train so that the cars roll along the track at low speed and rest on the wheels when the train is stationary, we can keep the cars from "crashing onto" or "grinding against" the track when the power source fails. Without some means of traction backup, a simple power interruption could cause a rude shock to passengers, if not an outright train wreck. The correct choice is B.

5. If we supply a rod-shaped electromagnet with AC at a frequency of 100 Hz, the magnetic polarity will reverse every 200th of a second. Remember: In an AC wave, the polarity reverses twice for each complete cycle. The answer is C.

6. The coil-and-rod arrangement described here is, in fact, a simple DC electromagnet. The retentivity of a particular DC electromagnet's core material equals the amount of flux density remaining in that material after the removal of coil current, divided by the maximum possible flux density that we can obtain inside the core material by driving a large current through the coil. In this case that ratio is 80 divided by 800, or 0.1. The correct choice is B.

7. We don't have enough information to determine the permeability of the core material described in Question 6. In order to do that, we'd have to know how much the core material concentrates the magnetic lines of flux, compared with the same coil and the same current with a vacuum inside. We haven't been given that data. The answer is D.

8. By definition, if we drive enough current through the coil (in the scenario of question 6) to produce 800 G of flux density inside the core material, we've saturated the core. That is to say, the core operates in a state of saturation. Obviously, increasing the current even further will not change that situation; the core will remain saturated. The correct choice is A.

9. In general, magnetic disks allow us to read data from, and write data to, the medium faster than we can do with magnetic tape. The reason for this increased read/write speed lies in the fact that on a disk, no two points are ever separated by more than the diameter of the medium (a few centimeters at most), while on a tape, two points can lie hundreds of meters apart (as measured along the tape itself). To get from one point to another, the read-write head must usually travel much farther along a tape than it must move over the surface of a disk. The correct choice is A.

10. High temperatures can adversely affect the data on a magnetic medium. Bright light or X rays do not interact with magnetic media. The correct choice is C.