Electricity Demystified, 2nd edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 5

1. When we connect two or more identical electrochemical cells in parallel with their polarities in agreement (plus-to-plus and minus-to-minus), the voltage across the combination equals the voltage produced by an individual cell. A typical "flashlight" cell produces 1.5 V DC, so the parallel combination would also produce 1.5 V DC. The correct choice is A.

2. If a current of 1.6 A flows for exactly four hours, we get 1.6 x 4.0, or 6.4, ampere-hours (Ah), as a result. That's the situation described in this question. The battery evidently has a flat discharge curve, because we're told that the current flows steadily and then abruptly stops. The correct choice is D.

3. An ideal cell always has a flat discharge curve (choice B). It might produce 1.018 V (choice A), but not necessarily. It might have large storage capacity (choice D), but not necessarily. An ideal cell would never have high internal resistance, so choice C is totally wrong. The answer is B.

4. Hydrogen has proven difficult to store. That problem, along with difficulty in transport, constitutes a disadvantage of hydrogen as a useful fuel source. The correct choice is C. Hydrogen produces water vapor when it burns (choice A), but that's not a problem. Hydrogen does not give rise to toxic compounds when it oxidizes, so choice B is wrong.

5. When discarded, anything containing mercury or mercury compounds can create a hazard by leaking out toxins over a period of time. That's a huge problem, and has contributed in large measure to the obsolescence of mercury cells and batteries. The correct choice is A. The other three choices don't describe characteristics of mercury cells and batteries, so they're all wrong.

6. When exposed to direct sunlight, a single silicon photovoltaic (PV) cell produces approximately 0.6 V DC. Three such cells in series, exposed to the same amount of light, would produce three times that much voltage, or 1.8 V DC. The correct choice is C.

7. We can connect the battery "either way" in a common flashlight, but not in a cell phone, a personal computer, or a transistor radio. The correct answer is D. However, the individual series-connected cells in the flashlight battery must have their polarities interconnected in the proper manner for series operation: plus-to-minus or minus-to-plus.

8. A grid-intertie PV electric energy system works with the utility company to provide electricity. When the PV system can't provide enough energy (such as on gloomy days or during the hours of darkness), the utility company sells the deficit to the user. When the PV system produces more than enough energy, the utility company buys the surplus from the user. Power inverters can convert the DC from the PV cells into usable AC electricity for use by common utility appliances. All three statements A, B, and C hold true, so the correct answer is D, "All of the above."

9. In a lead-acid electrochemical cell, the acid serves as the electrolyte, which "stores the energy" in chemical form until we need it as electricity. The correct choice is B.

10. If we place a high-resistance load across an alkaline cell, the cell delivers a small amount of current through the load resistance according to Ohm's Law. As we increase the load (that is, we reduce the load resistance), it demands more current from the cell. At a certain point, the load will draw so much current that the cell's internal resistance becomes significant, and the cell's output voltage decreases. We define the current at that critical threshold as the maximum deliverable current, so the correct choice is A. The cell's internal resistance doesn't go down; in fact it might actually increase, so B is wrong. Alkaline cells don't suffer from memory drain, nor do they reverse polarity, so choices C and D are both wrong.