ANSWER ALL QUESTIONS. BE SURE TO ANSWER

2 -1 C What is the difference between the macroscopic and microscopic forms of energy? 2-2C What is total energy? Identify the different forms of energy that constitute the total energy. 2-3C How are heat, internal energy, and thermal energy related to each other? 2-4C What is mechanical energy? How does it differ from thermal energy? What are the forms of mechanical energy of a fluid stream? 2-5C Natural gas, which is mostly methane CH4, is a fuel and a major energy source. Can we say the same about hydrogen gas, H2? 2-6E Calculate the total kinetic energy, in Btu, of an object with a mass of 15 lbm when its velocity is 100 ft/s. Answer: 3 .0 Btu

2 -7 Calculate the total kinetic energy, in kJ, of an object whose mass is 100 kg and whose velocity is 20 m/s. 2-8E The specific potential energy of an object with respect to some datum level is given by gz where g is the local gravitational acceleration and z is the elevation of the object above the datum. Determine the specific potential energy, in Btu/lbm, of an object elevated 100 ft above a datum at a location where g = 32.1 ft/s2. 2-9E Calculate the total potential energy, in Btu, of an object with a mass of 200 lbm when it is 10 ft above a datum level at a location where standard gravitational acceleration exists.

2-10 Calculate the total potential energy, in kJ, of an object whose mass is 20 kg when it is located 20 m below a datum level in a location where g = 9.5 m/s2. 2-11 A person gets into an elevator at the lobby level of a hotel together with his 30-kg suitcase, and gets out at the 10th floor 35 m above. Determine the amount of energy consumed by the motor of the elevator that is now stored in the suitcase. * Problems designated by a " C" are concept questions, and students are encouraged to answer them all. Problems designated by an " E" are in English units, and the SI users can ignore them . Problems w ith the @ icon are solved using EES, and com plete solutions together w ith param etric studies are included on the enclosed DVD. Problems w ith the E3 icon are comprehensive in nature, and are intended to be solved w ith a com puter, preferably using the EES software th a t accompanies this text.

2-12 Electric power is to be generated by installing a hydraulic turbine-generator at a site 160 m below the free surface of a large water reservoir that can supply water at a rate of 3500 kg/s steadily. Determine the power generation potential. 2-13 At a certain location, wind is blowing steadily at 10 m/s. Determine the mechanical energy of air per unit mass and the power generation potential of a wind turbine with 60-m-diameter blades at that location. Take the air density to be 1.25 kg/m3. 2-14 A water jet that leaves a nozzle at 60 m/s at a flow rate of 120 kg/s is to be used to generate power by striking the buckets located on the perimeter of a wheel. Determine the power generation potential of this water jet. 2-15 Two sites are being considered for wind power generation. In the first site, the wind blows steadily at 7 m/s for 3000 hours per year, whereas in the second site the wind blows at 10 m/s for 2000 hours per year. Assuming the wind velocity is negligible at other times for simplicity, determine which is a better site for wind power generation. Hint: Note that the mass flow rate of air is proportional to wind velocity.

2-16 A river flowing steadily at a rate of 175 m3/s is considered for hydroelectric power generation. It is determined that a dam can be built to collect water and release it from an elevation difference of 80 m to generate power. Determine how much power can be generated from this river water after the dam is filled. 2-17 Consider a river flowing toward a lake at an average velocity of 3 m/s at a rate of 500 m3/s at a location 90 m above the lake surface. Determine the total mechanical energy of the river water per unit mass and the power generation potential of the entire river at that location. 3 m/s FIGURE P 2 -1 7 Energy Transfer by Heat and Work 2-18C In what forms can energy cross the boundaries of a closed system? 2-19C When is the energy crossing the boundaries of a closed system heat and when is it work? 2-20C W hat is an adiabatic process? W hat is an adiabatic system? 2-21C W hat are point and path functions? Give some examples. 99 CHAPTER 2 2-22C Consider an automobile traveling at a constant speed along a road. Determine the direction of the heat and work interactions, taking the following as the system: (a) the car radiator, (b) the car engine, (c) the car wheels, (d) the road, and (e) the air surrounding the car.

2-23C The length of a spring can be changed by (a) applying a force to it or (b) changing its temperature (i.e., thermal expansion). What type of energy interaction between the system (spring) and surroundings is required to change the length of the spring in these two ways? 2-24C Consider an electric refrigerator located in a room. Determine the direction of the work and heat interactions (in or out) when the following are taken as the system: (a) the contents of the refrigerator, (b) all parts of the refrigerator including the contents, and (c) everything contained within the room during a winter day. FIGURE P 2 -2 4 C 2-25C A personal computer is to be examined from a thermodynamic perspective. Determine the direction of the work and heat transfers (in or out) when the (a) keyboard, (b) monitor, (c) processing unit, and id) all of these are taken as the system. 2-26 A small electrical motor produces 5 W of mechanical power. What is this power in (a) N, m, and s units; and (b) kg, m, and s units? Answers: (a) 5 N-m/s, (>) 5 kg-m2/s3 2-27E A model aircraft internal-combustion engine produces 10 W of power. How much power is this in (a) lbf-ft/s and (b) hp? Forms of Work

2-28C A car is accelerated from rest to 85 km/h in 10 s. Would the energy transferred to the car be different if it were accelerated to the same speed in 5 s? 2-29 Determine the energy required to accelerate an 800-kg car from rest to 100 km/h on a level road. Answer: 309 kJ 2-30E A construction crane lifts a prestressed concrete beam weighing 3 tons from the ground to the top of piers that are 24 ft above the ground. Determine the amount of work done considering (a) the beam and (b) the crane as the system. Express your answers in both lbf ft and Btu. 2-31E A man weighing 180 lbf is pushing a cart that weighs 100 lbf with its contents up a ramp that is inclined at an angle of 10 from the horizontal. Determine the work needed to move along this ramp a distance of 100 ft considering (a) the man and (b) the cart and its contents as the system. Express your answers in both lbf ft and Btu. FIGURE P 2 -3 1 The McGraw-Hill Companies, Inc./Lars A. Niki, photographer.

2-32E The force F required to compress a spring a distance x is given by F F0 = kx where k is the spring constant and F0 is the preload. Determine the work required to compress a spring whose spring constant is k = 200 lbf/in a distance of one inch starting from its free length where F0 = 0 lbf. Express your answer in both lbf-ft and Btu. 2-33E A spherical soap bubble with a surface-tension of 0.005 lbf/ft is expanded from a diameter of 0.5 in to 2.0 in. How much work, in Btu, is required to expand this bubble? Answer: 2.11 x 10~6 Btu 2-34E A 0.5-in diameter, 12-in-long steel rod with a Young's modulus of 30,000 lbf/in2 is stretched 0.125 in. How much work does this require, in Btu? The strain work is given by w = (ej - a?) where l/0 is the original volume of the solid, E is Young's modulus and e is the strain at the beginning and ending of the process. 100 ENERGY, ENERGY TRANSFER 2-35E A spring whose spring constant is 200 lbf/in has an initial force of 100 lbf acting on it. Determine the work, in Btu, required to compress it another 1 inch. 2-36 How much work, in kJ, can a spring whose spring constant is 3 kN/cm produce after it has been compressed 3 cm from its unloaded length? 2-37 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. 2-38 Determine the power required for a 1150-kg car to climb a 100-m-long uphill road with a slope of 30 (from horizontal) in 12 s (a) at a constant velocity, (b) from rest to a final velocity of 30 m/s, and (c) from 35 m/s to a final velocity of 5 m/s. Disregard friction, air drag, and rolling resistance. Answers: (a) 4 7 .0 kW, (>) 90.1 kW, (c) - 1 0 .5 kW

2-39 A damaged 1200-kg car is being towed by a truck. Neglecting the friction, air drag, and rolling resistance, determine the extra power required (a) for constant velocity on a level road, (b) for constant velocity of 50 km/h on a 30 (from horizontal) uphill road, and (c) to accelerate on a level road from stop to 90 km/h in 12 s. Answers: (a) 0, (b) 8 1 .7 kW, (c) 3 1 .3 kW The First Law of Thermodynamics 2-40C For a cycle, is the net work necessarily zero? For what kind of systems will this be the case? 2-41C What are the different mechanisms for transferring energy to or from a control volume? 2-^t2C On a hot summer day, a student turns his fan on when he leaves his room in the morning. When he returns in the evening, will the room be warmer or cooler than the neighboring rooms? Why? Assume all the doors and windows are kept closed. 2-43E One way to improve the fuel efficiency of a car is to use tires that have a lower rolling resistance tires that roll with less resistance, and highway tests at 65 mph showed that tires with the lowest rolling resistance can improve the fuel efficiency by nearly 2 mpg (miles per gallon). Consider a car that gets 35 mpg on high rolling resistance tires and is driven 15,000 miles per year. For a fuel cost of $2.20/gal, determine how much money will be saved per year by switching to low rolling resistance tires. 2-44 An adiabatic closed system is accelerated from 0 m/s to 30 m/s. Determine the specific energy change of this system, in kJ/kg. 2-45 An adiabatic closed system is raised 100 m at a location where the gravitational acceleration is 9.8 m/s2. Determine the energy change of this system, in kJ/kg. 2-46E A water pump increases the water pressure from 10 psia to 50 psia. Determine the power input required, in hp, to pump 1.2 ft3/s of water. Does the water temperature at the inlet have any significant effect on the required flow power? Answer: 12.6 hp

2-47 A classroom that normally contains 40 people is to be air-conditioned with window air-conditioning units of 5-kW cooling capacity. A person at rest may be assumed to dissipate heat at a rate of about 360 kJ/h. There are 10 lightbulbs in the room, each with a rating of 100 W. The rate of heat transfer to the classroom through the walls and the windows is estimated to be 15,000 kJ/h. If the room air is to be maintained at a constant temperature of 21C, determine the number of window air-conditioning units required. Answer: 2 units 2-48 The lighting needs of a storage room are being met by 6 fluorescent light fixtures, each fixture containing four lamps rated at 60 W each. All the lamps are on during operating hours of the facility, which are 6 a m to 6 p m 365 days a year. The storage room is actually used for an average of 3 h a day. If the price of electricity is $0.08/kWh, determine the amount of energy and money that will be saved as a result of installing motion sensors. Also, determine the simple payback period if the purchase price of the sensor is $32 and it takes 1 hour to install it at a cost of $40.

2-49 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 4 h 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. 2-50 Consider a room that is initially at the outdoor temperature of 20C. The room contains a 100-W lightbulb, a 110-W TV set, a 200-W refrigerator, and a 1000-W iron. Assuming no heat transfer through the walls, determine the rate of increase of the energy content of the room when all of these electric devices are on. 101 CHAPTER 2 2-51 A fan is to accelerate quiescent air to a velocity of 8 m/s at a rate of 9 mVs. Determine the minimum power that must be supplied to the fan. Take the density of air to be 1.18 kg/m3. Answer: 340 W

2-52E Consider a fan located in a 3 ft X 3 ft square duct. Velocities at various points at the outlet are measured, and the average flow velocity is determined to be 22 ft/s. Taking the air density to 0.075 lbm/ft3, estimate the minimum electric power consumption of the fan motor. 2-53 The driving force for fluid flow is the pressure V