Hobson Physics: Concepts & Connections 4e

CHAPTER 7 review questions Heating 1. What is heating? 2. In your own words, state the law of heating. 3. Give an example showing that thermal energy and temperature are really two different things. Heat Engines and Energy Quality 4. Give an example of each of these energy transformations: kinetic to thermal, gravitational to thermal, thermal to kinetic. 5. In your own words, state the law of heat engines. 6. What properties of the input and the exhaust are the most important to determining a heat engine’s efficiency? Describe the manner in which the efficiency depends on these properties. 7. Is the actual overall energy efficiency of an automobile closest to 98%, 90%, 40%, 10%, or 2%? What about a steam–electric power plant? 8. If a heat engine operated entirely without friction, would it then be 100% efficient? Explain. 9. In terms of energy, what happens when the motion of a rock swinging from a string “dies down”? In what sense is this behavior irreversible? The Law of Entropy 10. What is entropy? 11. In your own words, state the entropy form of the second law. 12. Which law or laws of physics distinguish between forward and backward in time? 13. When a growing leaf increases its organization, does it violate the second law? The Automobile and Transportation 14. Name two general types of heat engines of major social importance. 15. Why do we call it an internal combustion engine? Describe in your own words how it works. 16. Name two alternative fuels (not gasoline or diesel fuel) for the automobile. 17. What is the source of the largest inefficiency in an automobile’s operation? 18. Describe two different ways of measuring a transportation mode’s efficiency and give an appropriate measurement unit for each. 19. Why are trains more efficient than other transportation vehicles? The Steam–Electric Power Plant 20. In a steam–electric power plant, what is the purpose of the turbine? Generator? Condenser? Cooling tower? Stack? 21. What is thermal pollution? 22. What is the source of the most important inefficiency in a steam–electric power plant’s operation? 23. What part of a steam–electric power plant is analogous to the piston in an automobile? Exponential Growth 24. What is the difference between exponential and linear growth? 25. Your savings account grows at 7% per year. What is its doubling time? 26. Draw a typical life-history graph for a nonrenewable resource. Is any part approximately exponential? 27. Repeat the previous question, but for a renewable resource. conceptual exercises Heating 1. How would you describe the weather on a day when the temperature was 2. Give the approximate temperature, in ºC, of each of the following: your body; water boiling in an open pot; ice water; a nice day. 3. Which is larger, a Celsius degree or a Fahrenheit degree? 4. How does the flow of thermal energy through a closed window illustrate the second law? Which direction is this flow when it is cold outside? Hot outside? 5. Try to think of at least one technological device that causes thermal energy to flow “uphill,” from colder to hotter. Does this device violate the law of heating? Explain. 6. In the operation of a refrigerator, does thermal energy flow from hot to cold, or is it from cold to hot? Does this happen spontaneously, or is outside assistance required? Heat Engines 7. Is it possible to convert a given quantity of kinetic energy entirely into thermal energy? Is it possible to convert a given quantity of thermal energy entirely into kinetic energy? In each case, either give an example or explain why it is impossible. 8. Is it possible to convert a given quantity of chemical energy entirely into thermal energy? Is it possible to convert a given quantity of thermal energy entirely into chemical energy? In each case, either give an example or explain why it is impossible. 9. Which are not heat engines: natural-gas-burning power plant, hydroelectric power plant, ethanol-fueled automobile, bicycle, solar–thermal electric power plant, steam locomotive? 10. Which of the following are heat engines: nuclear power plant, diesel locomotive, electric locomotive, geothermal power plant, wind turbine (windmill for generating electricity), solar hot water heater? 11. What does the second law tell us about the efficiency of heat engines? 12. Can you think of any way to drive a ship across the ocean by using the ocean’s thermal energy without violating the second law? 13. Farswell Slick (see Concept Check 12 of Chapter 6) approaches you with plans for a revolutionary transportation system. He has noticed that when he drives an automobile without accelerating, all the input energy eventually shows up as thermal energy. Slick proposes to use this thermal energy to drive the car at a constant speed. The car will still need fuel, but only for accelerating. It will be possible to travel cross-country on only a few gallons of gasoline. He describes his scheme as a “computerized advanced-technology exhaust feedback afterburner.” Should you invest in Slick’s scheme? Explain. 14. On the Fahrenheit scale, what are the freezing and boiling points? Use your answer to calculate the number of Fahrenheit degrees in one Celsius degree. 15. Use the result of the preceding question to convert 10ºC to Fahrenheit. Convert 30ºC to Fahrenheit. 16. In one cycle of its operation, a heat engine does 100 J of work while exhausting 400 J of thermal energy. What is its energy input? Its efficiency? 17. In one cycle of its operation, a heat engine consumes 1500 J of thermal energy while performing 300 J of work. What is its efficiency? How much energy is exhausted in each cycle? Energy Quality and the Law of Entropy 18. When your book falls to the floor, is this a thermodynamically irreversible process? Is energy conserved? Does entropy increase? 19. When we say that the motion of a rock swinging on a string is irreversible, do we really mean that it is impossible to get the rock back to its starting condition? Explain. 20. When a block of wood slides down a sliding board, is this a thermodynamically irreversible process? Does this mean that it is impossible to make a block of wood slide up a sliding board? Explain. 21. As an egg develops into a chicken, its contents become more ordered. In light of what you have learned about the second law of thermodynamics, do you expect that this process violates the law of increasing entropy? Explain. 22. A pan of liquid water freezes when you place it outside on a cold day. Liquid water has greater molecular disorder than ice does. Is the freezing process then an exception to the law of entropy? Explain. 23. When orange juice and grapefruit juice are mixed, does entropy increase? The Automobile and Transportation 24. Describe the energy input for walking and bicycling. How do walking and bicycling illustrate the second law? 25. Suppose an automobile’s fuel could be made to burn hotter without harming the engine’s operation (for instance, without cracking the engine). Would you still get the same amount of useful work from each gallon of gasoline? 26. Suppose an automobile could run on hard wheels that were not squeezed by the weight of the car on the road. Would this alter the car’s efficiency? How might this affect the gas mileage? What kind of wheels and road might you suggest? 27. According to Figures 7.9, 7.10, and 7.11, which of the three main sectors of the U.S. economy (industry, residential–commercial, transportation) consumes the most oil? 28. One car has twice the gasoline mileage efficiency of a second car. Compare the amounts of pollution they produce when they both travel the same distance. 29. Out of every 100 barrels of gasoline, about how many actually go into driving a typical car down the road? 30. A bus carries 30 people 200 km using 300 liters of gasoline. Find its passenger-moving efficiency. The Steam–Electric Power Plant 31. Which type of generating plant would you expect to be more energy efficient, steam–electric or hydroelectric? Defend your answer. 32. Would it be more energy efficient to heat your home electrically or to heat it directly using a natural gas heater, assuming that the electricity comes from a steam–electric plant? 33. Which method of fueling your car is likely to be more energy efficient, and why: gasoline used in a standard car engine or electricity taken from a coal-fueled generating plant and stored in lightweight car batteries? Assume that the batteries convert electricity to work at 100% efficiency. 34. Out of every 100 tons of coal fed into an electric generating plant, roughly how many tons produce the electricity you can use at your home and how many go into waste energy? Use the approximate energy flows indicated in Figure 7.21. 35. For every 100 kilograms of coal entering a generating plant (recall that this much enters every second), about 15 kilograms of sulfur oxides and ash are removed, producing a significant solid-waste disposal problem. For a typical 1000 MW plant, how much of this solid waste is produced every day? Express your answer in tonnes 36. How would the annual pollution from two coal plants compare if the first plant is twice as energy efficient as the second? Assume that they both produce the same amount of electric power. Exponential Growth and Resource Use 37. A lily pond doubles its number of lilies every month. One day, you notice that 2% of the pond is covered by lilies. About how long will it be before the pond is entirely covered? 38. On June 1 there are a few water lilies in a pond, and they then double daily. By June 30 they cover the entire pond. On what day was the pond still 50% uncovered? 39. Company X increases its profits every year by $50 million. Is its growth in profits exponential? Company Y increases its profits by 1% every year. Is its growth in profits exponential? 40. According to Figure 7.25, did electric power production grow exponentially between 1910 and 1935? Estimate the number of kilowatt-hours produced in 1935, 1945, 1955, 1965, and 1975, and verify that production grew approximately exponentially between 1935 and 1975. 41. Which of the following are renewable energy resources: coal, firewood, nuclear power, wind, water behind a dam. 42. What is the original source of energy in each of the following energy resources: oil, firewood, wind, water behind a dam, geothermal, ocean-thermal electricity? Which of these are renewable resources? 43. The most recent world population doubling, to a total population of about 6 billion, has occurred in about 40 years. Making the (unrealistic!) assumption that this rate of population will continue for two centuries, what would the world’s population be two centuries from now? Such unrealistic assumptions are often useful in projecting future trends, because they give us a sense of what is likely or unlikely. For example, this exercise shows us that it is very unlikely that our present population growth will continue for two more centuries. 44. The most recent world population doubling has occurred in about 40 years. Suppose that the next doubling occurs also in 40 years, but that a new agricultural “green revolution” manages to also double food production. Then how many people will be starving 40 years from now, as compared to the number starving now? 45. Is the graph of Figure 7.29 an exponential curve? Explain. Figure 7.29 Is this an exponential curve? problems Heat Engines (You will need footnote 3 to solve some of these problems.) 1. If a heat engine’s efficiency is 30% and its work output is 2000 J, how much thermal energy must have been put into it? 2. If a heat engine’s efficiency is 20% and 1.5 million joules of energy are put into it, how much work does it do? 3. The steam entering the turbine in a coal-burning power plant is heated to 500ºC. The steam is cooled and condensed to water at 80ºC. Find the best possible efficiency of the power plant. Remember that you must convert Celsius temperatures to degrees Kelvin before using the formula. 4. A solar-heated steam–electric generating plant heats steam to 250ºC. After passing through the turbine, cooling towers cool the steam to 30ºC. Calculate the best possible efficiency of this power plant. Remember that you must convert Celsius temperatures to Kelvins before using the formula in footnote 3. 5. In the preceding question, suppose that for every 1000 J of thermal energy going into this plant, the cooling towers remove 750 J as exhaust. What is the actual efficiency of this power plant? 6. A coal-burning steam locomotive heats steam to 180ºC and exhausts it at 100ºC. During 1 s of operation, it consumes 500 million J of energy from the burning coal. According to Table 7.1, how much work can be obtained from this locomotive during 1 s of operation under ideal conditions (no friction or other imperfections)? 7. In the preceding question, how much work can be obtained under actual conditions? The Automobile and Transportation 8. You travel alone from New York to Los Angeles, about 2800 miles. Working from Tables 7.3 and 7.4, how many gallons of gasoline will you use if you travel by car (assuming your car gets average gasoline mileage for U.S. automobiles)? How many gallons if you travel by air? By bus? By train? 9. A 100-car freight train hauls 16,000 metric tonnes of freight from New York to Los Angeles, about 5000 km. How many trucks would be needed for this load, assuming that each truck carries 32 tonnes of freight? Working from Table 7.5, how many liters of gasoline are saved if this load is carried by train rather than by truck? The Steam–Electric Power Plant 10. Making estimates. In the United States, solar energy strikes a single square meter of ground at an average rate (averaged over day and night and over the different seasons) of 200 watts (200 joules/second). At what average rate does solar energy strike a football field (about 100 m by 30 m)? 11. Continuing the preceding question, a typical U.S. home consumes electricity at an average rate of 1 kW. How much surface area would be needed to provide this electric power, assuming a 10% conversion efficiency? What dimensions would a square-shaped photovoltaic collector need to cover this area? Exponential Growth 12. How much electric energy would have been produced in 1985, if the exponential growth of 1935–1975 had continued for another 10 years beyond 1975? If this growth had continued, roughly how many power plants would have been needed in 1985, as compared with 1975? 13. During 1985–1990, annual U.S. population growth was 0.8% per year, for Mexico it was 2.2%, and for Kenya (the highest) it was 4.2%. At these rates, how long will it take for the populations of each of these countries to double? 14. World population is now about 6 billion. The growth rate has been roughly 2% per year since the end of World War II (1945). If a 2% per year growth rate continued, when would world population be 12 billion? 15. Centerville, with a growth rate of 7% annually, is using its only sewage treatment plant at maximum capacity. If it continues its present growth rate, how many sewage treatment plants will it need 40 years from now? 16. During the 1980s, U.S. car and truck miles traveled increased by 4% per year, but the length of highway increased by only 0.1% per year. Find the doubling time for vehicle miles traveled and for miles of highway. 17. Continuing the preceding problem, suppose that these rates are maintained in the future. While vehicle miles double (a 100% increase), by roughly what percentage will the amount of highway increase? Roughly, how much worse will traffic congestion be at that time?