Drossman/Veirs EV112

Block 5, 2002

1. Describe the energy flows and exchanges that underly changes in bonding. Describe in some detail three systems that go from strong binding to weak binding and three systems that gofrom weak binding to strong binding. Make all these examples as diverse as you can.

2. Look up the total energy radiated by our Sun in one second (the rate of energy emission in Joules/sec is termed a watt) and estimate the mass of hydrogen that must be fusing per second to produce this much energy. The mass of the sun is about 2 10³⁰ kg. How long do you think the sun may last?

(a) What is the rate at which one square meter of the sun's surface radiates energy?  Give your answer in watts per meter squared.

(b)  Based on your answer to (a), what is the total rate at which the entire sun radiates energy?

(c)  What is the intensity (W/m²) of solar energy above the earth's atmosphere.  That is, how much solar energy falls upon a square meter solar collector in one second if the collector is up in orbit around the earth and facing the sun?  This number is called the "solar constant".

1.   Suppose that all of the solar energy that is not reflected from the earth for one year is absorbed in the top meter of the earth's oceans.  Oceans cover about 70% of the surface area of the earth.  Show that after about two weeks of this heating, this one meter thick layer on the top of the oceans would be boiling!  (Refer to your text or the glossary for the specific heat and density of water.)

2. Compare the total energy that is absorbed from the sun each year with the current human consumption of energy per year (~400 10⁺¹⁸ J/yr, or 400 ExaJoules).  Discuss your result.

3. (Rubin 12.7) Suppose the earth’s current albedo were to decrease by just 1 percent due to a decrease in surface ice cover caused by warmer temperatures.

(a)What would be the resulting increase in radiative forcing?
(b)How does this increase compare to the estimated radiative forcing from CO₂ since preindustrial times?

4. (rubin 12.14) Suppose all of the world’s proven reserves of fossil fuels (see Rubin Table 12.8) were extracted and burned to supply the world’s energy needs.

(a)What is the approximate total mass of carbon that would be emitted to the atmosphere?
(b)How much of this would come from each major fuel type (oil,gas,coal)?
(c)If roughly half of this additional carbon remained in the atmosphere,what would be the percentage increase in the current atmospheric mass of carbon,which is estimated to be 750 GtC?
(d)What would be the resulting atmospheric concentration of CO 2 ?
(e)Compare your answer from part (d) to the IPCC scenario results in Figure 12.17. Would the magnitude of potential global warming be greater or smaller relative to the IPCC results for 2100?

5. Build your own model of a global warming greenhouse effect

1. Currently there are about 6 billion people in the world. If the current growth rate of 1.6% per yr continues, how many people would be in the world at the turn of the next century?

2. What is meant by the "Exponential Expiration Time", EET? How is it determined if you know the current total amount, the current usage rate and the growth rate.

3. Prove that if a resource would last 300 years at present rates of consumption, then it would last 49 years if the rate of consumption grew 6% per year.

4. Why is it likely that production will drop long before the EET is reached?

Project 1: Personal Footprint Analysis

1. (CB 2-26) Complete the following table for H₂O:

2. (CB 2-30) Complete the following table for H₂O:

3. (CB 2-39) Water is to be boiled at sea level in a 30-cm-diameter stainless steel pan placed on top of a 3-kW electric burner. If 60 percent of the heat generated by the burner is transferred to the water during boiling, determine the rate of evaporation of water.

4. (CB 2-70) The pressure in an automobile tire depends on the temperature of the air in the tire. When the air temperature is 25°C, the pressure gage reads 210 kPa. If the volume of the tire is 0.025 m³, determine the pressure rise in the tire when the air temperature in the tire rises to 50°C. Also, determine the amount of air that must be bled off to restore pressure to its original value at this temperature. Assume the atmospheric pressure to be 100 kPa.

5. (CB 2-112) During a hot summer day when the air temperature is 35°C and the relative humidity is 70 percent, you buy a supposedly “cold’’ canned drink from a store. The store owner claims that the temperature of the drink is below 10°C. Yet the drink does not feel so cold and you are skeptical since you notice no condensation forming outside the can. Can the store owner be telling the truth?

6. (CB 2-104) Determine the enthalpy change Dh of nitrogen, in kJ/kg, as it is heated from 600 to 1000 K, using (a) the Cp value at the average temperature (1.099 kJ/kg-K), and (b) the Cp value at room temperature (1.005 kJ/kg-K).

7. (CB 1-64) Consider a 70-kg woman who has a total foot imprint area of 400 cm2. She wishes to walk on the snow, but the snow cannot withstand pressures greater than 0.5 kPa. Determine the minimum size of the snowshoes needed (imprint area per shoe) to enable her to walk on the snow without sinking.

8. (CB 1-68) The barometer of a mountain hiker reads 930 mbars at the beginning of a hiking trip and 780 mbars at the end. Neglecting the effect of altitude on local gravitational acceleration, determine the vertical distance climbed. Assume an average air density of 1.20 kg/m3.

9. (CB 1-106) Consider a classroom that is losing heat to the outdoors at a rate of 20,000 kJ/h. If there are 30 students in class, each dissipating sensible heat at a rate of 100 W, determine if it is necessary to turn the heater in the classroom on to prevent the room temperature from dropping.  

10. (CB 1-115) A 12-oz serving of a regular beer contains 13 g of alcohol and 13 g of carbohydrates, and thus 150 Calories. A12-oz serving of a light beer contains 11 g of alcohol and 5 g of carbohydrates, and thus 100 Calories. An average person burns 700 Calories per hour while exercising on a treadmill. Determine how long it will take to burn the calories from a 12-oz can of (a) regular beer and (b) light beer on a treadmill.

11. How can you explain the difference between temperature and heat to a friend? Use some practical, everyday examples to help your friend understand. Assume your friend has not taken a chemistry or physics course.

2. Suppose 1000 kg of water is initially at rest.  Then forces act on this mass and the kinetic energy of the mass of water increases by the same amount as when random thermal energy was added in HW-II:5.  What will the speed of the mass of water be after it acquires the specified amount of kinetic energy?

3. Calculate the number of Joules of energy converted on one day to supply an average person's 100 watts of power throughout that day.  Also, express your answer in calories and in kilocalories.  Compare this with your average dietary input of energy.

4. Calculate how much your typical breakfast would cost if you could buy its energy at the rates charged by electric utility companies.  State your assumptions.

5. (CB 3-15) A frictionless piston-cylinder device initially contains 200 L of saturated liquid refrigerant-134a.The piston is free to move,and its mass is such that it maintains a pressure of 800 kPa on the refrigerant.The refrigerant is now heated until its temperature rises to 50 °C.Calculate the work done during this process.

6. (CB 3 –31) A piston-cylinder device with a set of stops contains 10 kg of refrigerant-134a.Initially,8 kg of the refrigerant is in the liquid form,and the temperature is -8 °C. Now heat is transferred slowly to the refrigerant until the piston hits the stops,at which point the volume is 400 L.Determine (a )the temperature when the piston first hits the stops and (b )the work done during this expansion process.Also,show the process on a P-V diagram.

Properties of refrigerant-134A from CB Tables:

7. (CB 3 –40) Determine the work required to deflect a linear spring with a spring constant of 70 kN/m by 20 cm from its rest position.

8. (CB3 –42) A ski lift has a one-way length of 1 km and a vertical rise of 200 m.The chairs are spaced 20 m apart,and each chair can seat three people.The lift is operating at a steady speed of 10 km/h.Neglecting friction and air drag and assuming that the average mass of each loaded chair is 250 kg,determine the power required to operate this ski lift. Also estimate the power required to accelerate this ski lift in 5 s to its operating speed when it is first turned on.

1. (CB 3 –20) A gas is compressed from an initial volume of 0.42 m³ to a final volume of 0.12 m³ .During the quasi-equilibrium process,the pressure changes with volume according to the relation P= aV+ b ,where a =1200 kPa/m³ and b =600 kPa.Calculate the work done during this process (a )by plotting the process on a P-V diagram and finding the area under the process curve and (b )by performing the necessary integrations.

2. (CB 4 –10) A university campus has 200 classrooms and 400 faculty offices.The classrooms are equipped with 12 fluorescent tubes,each consuming 110 W,including the electricity used by the ballasts.The faculty offices,on average, have half as many tubes. The campus is open 240 days a year.The classrooms and faculty offices are not occupied an average of four hours a day, but the lights are kept on.If the unit cost of electricity is $0.082/kWh, determine how much the campus will save a year if the lights in the classrooms and faculty offices are turned off during unoccupied periods.

3. (CB 4 –16) An insulated tank is divided into two parts by a partition.One part of the tank contains 2.5 kg of compressed liquid water at 60 °C and 600 kPa while the other part is evacuated. The partition is now removed,and the water expands to fill the entire tank.Determine the final temperature of the water and the volume of the tank for a final pressure of 10 kPa.

4. (CB 4 –160) An insulated 0.08-m 3 tank contains helium at 2 MPa and 80 °C.A valve is now opened,allowing some helium to escape.The valve is closed when one-half of the initial mass has escaped.Determine the final temperature and pressure in
the tank.

2. Engineers desire to make the temperature of the steam in a power plant as high as possible to increase efficiency but are limited by the diminishing strength of metals as temperatures rise.  In a nuclear power plant, the maximum allowable temperature is less than in a fossil fuel plant.  Why is this the case?

3. A nuclear power plant is sited near a coal-fired power plant.  Both plants each produce 2000 MW (mega watts) of electric power.  The temperature of the steam in the nuclear plant is 450 ^oC while the temperature of the steam in the coal plant is 550 ^oC.  How much waste heat does each power plant produce each second and what is the ratio of these two waste heat flows? 

4. For the two power plants in the above question, how many kilograms of water must be flow through the cooling system of each power plant each second if the maximum allowable temperature rise of the cooling water is 10 ^oC?

_{5. (CB 5 –59) A house that was heated by electric resistance heaters consumed 1200 kWh of electric energy in a winter month.If this house were heated instead by a heat pump that has an average COP of 2.4,determine how much money the homeowner would have saved that month.Assume a price of 8.5 ¢/kWh for electricity.}

_{6. (CB 5 –106E) A heat pump is to be used for heating a house in winter.The house is to be maintained at 78 °F at all times. When the temperature outdoors drops to 25 °F,the heat losses from the house are estimated to be 55,000 Btu/h. Determine the
minimum power required to run this heat pump if heat is extracted from (a ) the outdoor air at 25 °F and (b )the well water at 50 °F.}

_{7. (R 5.7) Improved engineering design and higher steam temperatures raised the steam cycle efficiency of a fossil fuel power plant from 42 percent to 44 percent.Additional design changes increased the boiler efficiency from 87 percent to 89 percent.What was the resulting improvement in overall plant efficiency?Express your answer two ways:
(a)As an absolute value (in percentage points), referring to the difference between final and initial efficiency.(b) As a relative value referring to the percentage improvement over the original efficiency.}

1. (CB 5 –27) Wind energy has been used since 4000 BC to power sailboats,grind grain,pump water for farms,and,more recently,generate electricity.In the United States alone,more than 6 million small windmills,most of them under 5 hp, have been used since the 1850s to pump water.Small windmills have been used to generate electricity since 1900,but the development of modern wind turbines occurred only recently in response to the energy crises in the early 1970s.The cost of wind power has dropped an order of magnitude from about $0.50/kWh in the early 1980s to about $0.05/kWh in the mid 1990s,which is about the price of electricity generated at coalfired power plants.Areas with an average wind speed of 6 m/s (or 14 mph)are potential sites for economical wind power generation.Commercial wind turbines generate from 100 kW to 3.2 MW of electric power each at peak design conditions.The blade span (or rotor)diameter of the 3.2 MWwind turbine built by Boeing Engineering is 320 ft (97.5 m).The rotation speed
of rotors of wind turbines is usually under 40 rpm (under 20 rpm for large turbines). Altamont Pass in California is the world ’s largest windfarm with 15,000 modern wind turbines. This farm and two others in California produced 2.8 billion kWh of electricity in 1991,which is enough power to meet the electricity needs of San Francisco. In 1999, over 3600 MW of new wind energy generating capacity were installed worldwide, bringing the world ’s total wind energy capacity to 13,400 MW.The United States,Ger-
many,Denmark,and Spain account for over 70 percent of current wind energy generating capacity worldwide. Denmark uses wind turbines to supply 10 percent of its national
electricity. Many wind turbines currently in operation have just two blades. This is because at tip speeds of 100 to 200 mph, the efficiency of the two bladed turbine approaches the theoretical maximum,and the increase in the efficiency by adding a third
or fourth blade is so little that they do not justify the added cost and weight.
Consider a wind turbine with an 80-m-diameter rotor that is rotating at 20 rpm under steady winds at an average velocity of 30 km/h.Assuming the turbine has an efficiency of 35 percent (i.e.,it converts 35 percent of the kinetic energy of the wind to electricity), determine (a) the power produced,in kW; (b )the tip speed of the blade,in km/h;and (c) the revenue generated by the wind turbine per year if the electric power produced is
sold to the utility at $0.06/kWh.Take the density of air to be 1.20 kg/m³.
2. (CB 7-15) 3. (7-27) 4. (CB 7-60) 5. (CB 7-140)

6. (R 5.16) A hydroelectric plant has been proposed to meet future electricity demand in a newly industrialized country in lieu of a fossil fuel plant.The projected facility size is 800
MW.The overall efficiency of the plant is estimated to be 87 percent,and the operating head is 70 m. (a)What is the water flow rate required for this facility? (b)If the reservoir serving this facility were designed to hold a three-month supply of water based on full plant operation,how much surface area would be needed based on a reservoir depth of 50 m?

Worksheets:

Combustion

Electric Power

Auto Pollution

Pollution

Gibbs Free Energy

Equilibrium

Entropy

LeChatlier

Equilibrium Calculations

Reaction Quotient

Fuel Cell