ENVS/Phys 105 HW Questions, Fall Last revised 8/21/13

Transcription

1 Monday, Sept. 2: HW #1: Chapter 1 QUESTIONS (p ) 1. Identify the principal energy sources used worldwide and classify them as either renewable or non-renewable. 12. Find the price of oil today in terms of dollars per barrel. Compare to last year at this time. Do the same for the price of gas per gallon. 24. A significant amount of time always occurs between book publication and available data for such things as energy use. From current data on the Web, find the present (or most recent) numbers for world and U.S. consumption of energy. Cite your URLs. Don t forget to cite!! Friday, Sept. 6: HW #2: Chapter 1 4. (a) What is exponential growth? (b) Today the United States has the equivalent of 400 standard-sized 1000 MW power plants. If electrical energy consumption continues to rise at the present rate of 2% per year, how many additional power plants will be needed in 35 years to meet those needs? 5. If the world's population is increasing at an annual rate of 1.3%, and there were 5 billion people in the year 1986, then in what year will the world's population be 10 billion? ADDITIONAL EXPONENTIAL PROBLEMS (Chap. 1) Stolen from Energy by Priest 1. From 1935 to about 1975 the electric energy in the US grew at an exponential rate of 7% per year. What was the doubling time? The doubling time for oil consumption in the same period was 20 years. What was the growth rate? 3. In two days a moth caterpillar can consume an amount of food equal to 70,000 times its birth weight. Assuming that it consumes at an exponential rate, show that about 16 doubling periods are required. What would be the doubling time? Wednesday, Sept. 11: HW #3: Chapter 2 PROBLEMS 20. Calculate the work done in lifting a 4-lb book to a height of 8ft. 21. How much work do you expend to move a 450-lb refrigerator 6ft across the floor if you are exerting a force of 90lb? 23. To what height would a 2-kg object have to be raised such that its gravitational potential energy would equal the kinetic energy it would possess if it were moving at 3m/s? 25. What is the potential energy of the water in a lake of surface area 10 square miles, average depth 40ft, and elevation above the electrical generator of 400ft? (Note that 1 square mile is about 28 million square feet and that 1ft 3 of water weighs 62lb.) Calculate the answer in foot-pounds, not Joules: it s much easier. 27. An engine performs 4000J of work in 10s. What is its power output in kilowatts and in horsepower? page 1 of 6

2 Monday, Sept. 16: HW#4: Chapter 2 & 3 Chapter 2 Special Topic PROBLEMS, page 70: 40. A net force of 10N is applied to a 3-kg block at rest on a smooth, level surface. Find the block s velocity after 9 seconds. (Hint: Find the acceleration first.) 41. A 1300-kg car experiences a net force of 3900N. After 100 meters, what is the car s kinetic energy and velocity? Chapter 3 QUESTIONS 1. Because energy is a conserved quantity, what happens to the kinetic energy of a car after you take your foot off the accelerator? 3. The pendulum of a clock swings back and forth. At what position will its kinetic energy be the greatest? At what position will its kinetic energy equal its potential energy? 7. Table 3.5 examines the conversion process in an internal combustion engine (ICE) and an electric vehicle. For both power systems, calculate the overall efficiencies. (See the example in the first half of the table and then complete the second half of the table by filling in the dashes.) ICE (step efficiency) Electrical Car (step efficiency) Raw fuel production 83% 96% Generation of electricity 33% Transmission of electricity 90% Battery 80% Engine 25% 90% Mechanical 70% Transmission system 70% 90% 8. If the efficiency of a coal-fired electrical generating plant is 35%, then what do we mean when we say that energy is a conserved quantity? Chapter A skateboard with a mass of 3kg is moving at a speed of 5m/s on a level surface. It encounters a hill and rolls up until it stops. To what vertical height does it rise? (Ignore friction.) (Hint: See equations in Chapter 2.) 21. Suppose you left a 100W light bulb on continuously for one month. If the electricity generation and transmission efficiency is 30%, how much chemical energy (in joules) was wasted at the power plant for this oversight? If the fuel consumption for one meal in Cambodia using a kerosene wick stove is 6MJ (1MJ=1,000,000 joules), how many equivalent meals could be obtained with this wasted energy? Waste = Total Useful Friday, Sept. 27: HW#5: Chapter 4 QUESTIONS 4. Give an example when heat is released from water as it undergoes a change of phase. 7. Illustrate all three methods of heat transfer by a wood burning stove. Sketch these methods. 13. How would you explain to a group of citizens why waste heat has to be released in the operation of a steamgenerating plant? Why can t there be zero waste heat? 18. What illustrations can you give of the second law of thermodynamics in your everyday life? 19. Can you think of any examples in which the entropy of a system decreases? What is the change of entropy of the surroundings? 23. How much electrical energy (in kwh) is needed to heat the water in a well-insulated electric hot water heater of capacity 40 gal from 20ºC to 50ºC (68ºF to 122ºF)? Use units of Btu/lb/ F. 24. How long will it take to heat 40 gal of water from 70 F to 120 F with a 20-kW immersion heater? Use units of Btu/lb/ F. 30. A simple heat engine might make use of the warm air around New York City. Energy could be taken as heat from the atmosphere (assume 30 C) and rejected as heat to the Hudson River (10 C). What is the maximum efficiency of such an engine for the conversion of thermal energy into mechanical energy? page 2 of 6

3 31. A coal-fired electrical generating plant has an efficiency of 38%. The temperature of the steam leaving the boiler is 550 C. What percentage of the maximum possible efficiency does this plant obtain? (Assume the temperature of the environment is 20 C.) Friday, Oct. 4: HW #6: CHAPTER 5 2. A hot briquette is taken from the fire and placed on a metal cookie sheet; discuss the types of heat transfer that take place as the briquette cools. 10. What is the average number of annual heating degree-days or cooling degree-days in your location? (Check the Web.) Cite your URL. 15. A wall is made up of four elements, as follows: ½ wood siding (lapped) [ R1 = 0.81 ft 2 h F/Btu ] ½ plywood sheathing [ R2 = 0.62 ft 2 h F/Btu ] 3½ fiberglass [ R3 = 10.9 ft 2 h F/Btu ] ½ Sheetrock [ R4 = 0.45 ft 2 h F/Btu ] Using the R-values of Table 5.2, how many Btu per hour per square foot will be lost through the wall when the outside temperature is 50 F colder than the inside? 16. Heat loss through windows is substantial. What percentage savings will be gained by covering a double-pane window with 2-in. sheet of rigid polystyrene board? 21. A room air conditioner has a capacity of 6000 Btu/h. Would this be sufficient to maintain the temperature of a small hut at 70ºF when the outside temperature is 95ºF? Assume the hut is 10ft 10ft 6ft and the exterior surfaces are made of 1-in. softwood. Wednesday, Oct. 9: HW #7: CHAPTER 6 1. Give several ways that you could increase the amount of solar energy delivered to a square foot of material. 11. What are the disadvantages and advantages of locating solar collectors on a vertical, south-facing wall? 15. What are the advantages to using thermal mass on a direct-gain passive solar system? PROBLEMS 24. Solar energy can be attractive not only environmentally but also economically. If an electric clothes dryer has a power rating of 5000 watts and is used for 1h per day, how much money can be saved in a month by using the experimental backyard clothes dryer of Figure 6.37, assuming electricity costs 9 cents per kwh? 25. Suppose the solar radiation is 850W/m 2 and you can collect 20% of the energy that falls on the reflecting surface of a solar hot dog cooker. If you need 240W for the cooker, what is the minimum collector area required? 29. What maximum percentage of the 40,000 Btu/h heating needs of a house in Minnesota in January can be met with a flat plate collector of area 700ft 2? Assume that the collector is tilted at an angle equal to the latitude, and the system efficiency is 50%. Use tables in Appendix D. Wednesday, Oct. 23: HW #8: CHAPTER 7 (p. 224) 13. If all of our [US] needs for oil were to be provided by that available from the estimated reserves located in the Arctic National Wildlife Reserve, how long would that supply last, assuming no growth in demand? 14. A gas-fired turbine generator for producing electricity has an efficiency of about 50%. How many cubic feet would be needed to produce 1000 MWe for 1 year? (Note conversion factors in Appendix B. DO NOT use the Fuel Requirements data: they assume a different efficiency.) 16. For a gasification project being planned, 26,000 tons of coal per day will be used to produce 250,000,000ft 3 /d of natural gas. If the coal is rated at 8700 Btu/lb and the gas at 950 Btu/ft 3, what will be the efficiency of this plant? (Metric ton = 1000kg=2200lb. British ton = 2000lb. Use the metric ton.) page 3 of 6

4 Monday, Oct. 28: HW #9: CHAPTER 8 1. How can you increase the pressure exerted by this book on a tabletop without increasing its weight (or pushing on it)? 4. The deepest well from which water can be pumped, by creating a vacuum on the top end of the pipe, is 10m (34ft). In drilling for oil, wells of a mile or more in depth have to be made. From this depth, how do you think the oil is recovered? 5. What is the buoyant force on a 2-ton ship that is floating in water? In alcohol? If this ship gets a hole in its side and sinks in the water, how and why does the buoyant force change? 23. Why might you expect indoor air pollution to be more of a factor for a person living in a house built in 1995 than for one built in 1895? Give two distinct answers. 27. Calculate the pressure exerted on your finger if you hold up a 15-lb box of apples. Assume 3" long finger of 0.5" width. 28. What is the pressure exerted by your finger if you hold back water leaking through a hole in a dike, as shown in Figure 8.29? (1atm = 29.9" of Hg = 33.9ft of H 2 O, if that helps.) 30. A swimming pool measures 10m 7m 2m. Calculate the total force exerted by the water against the bottom. What is the pressure at the bottom due to water? 33. A 1000-MWe plant uses coal with 3% sulfur content. How many tons of SO 2 will be emitted into the air during the operation of the plant for a day? (Use the list of conversions and equivalences located on the inside back cover.) 34. What is the rate of emission (in kg/h) of particulates from a power plant with a thermal output of 3000 MW that burns coal with an ash content of 2%? How much is emitted if an electrostatic precipitator of 95% efficiency is used? Wednesday, Nov. 6: HW #10: CHAPTER 9 1. Why does deforestation contribute to global warming? 3. If the temperature of the earth rises enough so that the polar ice caps begin to melt, would this melting cause the global temperature to remain the same, drop, or rise higher? Why? (Consider feedback mechanisms.) 4. What strategies are available to reduce global warming? What are the detrimental consequences of such actions? 13. How much CO 2 is produced by driving 100mi, drying a load of clothes, and watching 5 hours of television? (See Tables 9.4 and 10.3.) 16. Gasoline is made up of hydrocarbons. Suppose 80% of the mass of gasoline is carbon, and all of this becomes CO 2 in combustion in an automobile. If the density of gasoline is 6.7 lb/gal, then how much CO 2 is emitted per gallon of gasoline burned? (Compare with Table 9.4.) 18. If the efficiency of a geothermal plant is half that of a fossil-fuel plant, then how much more waste heat will be discharged to the environment from a geothermal facility than from a fossil-fuel plant with the same electrical output? 20. If the temperature of the water leaving the condenser in a steam turbine cycle were raised from 20 C to 30 C, what would be the decrease in the Carnot efficiency? Assume a steam temperature of 500 C. (See Chapter 4.) page 4 of 6

5 Monday, Nov. 11: HW #11: CHAPTER Why are some people killed by 120V while others are only shocked by the same voltage? 6. If you want a particular circuit to carry a large current, is it better to use a large-diameter or small-diameter wire? 9. If you plug a stereo system into one wall socket and a lamp into another socket in that room, are you connecting these loads in series or parallel? What limits the number of devices that can be plugged into the outlets of the same house circuit? 10. What is the disadvantage of using an inexpensive extension cord (with a small-diameter wire) to run an air conditioner or a refrigerator? 13. In the following sketch of a three-way switch (Fig ) used to control a light from two different places, trace the circuit and conclude whether the light is on or off. (The blackened parts in the switch are metal conductors.) 26. A light bulb draws a current of 2 amps when connected to a wall outlet (120V). What is the resistance and wattage of the bulb? 27. What is the cost of running a 1500-W hair dryer for 30 minutes, when the cost of electricity is $0.12/kWh? 30. With a 1200-W toaster, how much electrical energy is needed to make a slice of toast (cooking time = 1 minute)? At $0.08/kWh, how much does this cost? 37. You might challenge the assertion that batteries are inexpensive energy converters. Calculate the price per kilwatt-hour for a 12-V automobile battery with a 50 amp-hour capacity that sells for $40. If its weight is 45 lb, what is the energy density in watt-hours per pound? 39. Show that a 60 amp-hour battery stores the equivalent of 2% the energy available in 1 gal of gasoline. Use Table 3.4 for conversion factors (1 gal gasoline = 125,000 Btu). Convert both to Btu? 41. Calculate the price of electricity (per kwh) in 1925 using the information found in the Edison Mazda ad in Focus On 10.3, p EDISON MAZDA LAMPS, CIRCA 1925 From an ad in the November 1926 Ladies Home Journal: In the days of Gov. Bradford, light was so expensive that the frugal Puritan family extinguished its single candle during prayer. The early settlers had to learn to make candles themselves. Your light comes at a finger s touch -- and it s more than 100 times cheaper than candle light. So use light freely. A 75-W Edison Mazda Lamp will give more than twice as much light as a 40-W Edison Mazda Lamp -- but will average only a third of a cent more per hour for current. Photo: pictorial/default4.asp page 5 of 6

6 Friday, Nov. 15: HW #12: CHAPTER 11 and 13 CHAPTER 11 5 Why do utilities transmit electricity at high voltages? 6 A transformer is used to step down the voltage to 240V for residential use. Does this mean that the electrical energy is also reduced? Explain. CHAPTER 13 1 What does the atomic number of an element represent? 9 From the following nuclear reaction with heavy ions, find the mass and atomic number of the argon nucleus: 30 A 6 C+ 14Si Z Ar He Complete the following nuclear reaction: U n Mo? 2n CHAPTER 11 PROBLEMS 16. A small transformer used for a doorbell steps down the voltage from 120V and 0.5A to 12V. What is the current flow to the doorbell? 17. Ten megawatts of electricity are to be transmitted over a power line of resistance 4 ohms. a) If the electricity is generated at 10,000V and is to be transmitted at 130,000V, what should be the ratio of turns of the transformer? b) At what current will the power be transmitted? c) What percentage of the original power will be lost in the line because of the resistive heating? Treat the power line as a resistor carrying the current in part (b). It does not have the entire 130kV drop across it: the customers still get some voltage, although it is less than the original 130kV. Friday, Dec. 6: HW #13: CHAPTER 14, 16 CHAPTER Why doesn t uranium undergo spontaneous chain reactions in nature? 2. What are the two main roles of water in the operation of a reactor? (At least one of them should be different than its role in a coal-fired plant.) 5. If the temperature of the steam in a PWR is 315 C (600 F), what is the maximum efficiency obtainable from the power plant? 7. Why is a conventional nuclear reactor not able to explode as a bomb? 16. If the probability of being killed in an airplane crash is many times greater than that of being killed as a result of a nuclear reactor accident, then why aren t all planes grounded until their safety is improved drastically? Discuss. CHAPTER Why is fusion such a difficult task to achieve? Why is our sun so successful in providing us energy via fusion reactions? 3. In words, explain the Lawson criterion and its significance. 9. What power output does one achieve with a 10,000-J laser that provides a pulse of duration one billionth of a second? The answer should be comparable to the world-wide average electrical power usage. PROBLEM SET TO BE ANNOUNCED. Wednesday, Dec. 8: HW #14: page 6 of 6