Thermodynamics With Answers

TECHNOLOGICAL INSTITUTE OF THE PHILIPPINES QUEZON CITY ELECTRONICS AND COMMUNICATIONS ENGINEERING

THERMODYNAMICS 1. What do you call a system in which there is a flow of matter through the boundary? This system usually encloses the device that involves mass flow, such as; compressor, turbine, or nozzle. A. Closed system C. Isolated system B. Open system D. All of these

11. An automobile tire is inflated to 220.6 kPa gage pressure 16ºC. After the car has been driven the temperature rises to 24ºC. Assuming that the volume remains constant. What is the final gage pressure? A. 230 kPa gage C. 331 kPa gage B. 433 kPa gage D. 322 kPa gage

2. In an experiment to determine the specific heat of copper, a piece of copper weighing 50 g is first heated to 100ºC in steam. It is then immersed into water at 27ºC. The water in the calorimeter weighs 100 g and the inner aluminum cap weighs 50 g. If the final temperature is 30ºC, what is the specific heat of copper, specific heat of aluminum is 0.22 Cal/g-Cº. A. 0.095 Cal/g-Cº C. 0.88 Cal/g-Cº B. 0.077 Cal/g-Cº D. 0.66 Cal/g-Cº

12. Air is considered to be an ideal gas with a value of R= 0.287 kJ/kg-K. If there are 2 kg of air in a pistoncylinder at a temperature of 280 K, a volume of 0.2 m3, find the pressure. A. 80.36 kPa C. 36.08 kPa B. 803.6 kPa D. 360.8 kPa

3. What refers to the heat needed to change the temperature of the substance without changing its phase? A. Latent heat C. Specific heat B. Sensible heat D. Entropy 4. How much heat is required to change 30 kg of 20ºC water into 100ºC steam at one atmosphere? A. 88,858 kJ C. 55,556 kJ B. 77,860 kJ D. 44,454 kJ 5. Heat is supplied to 20 lbm of ice (Cp = 0.5 BTU/lbmFº) at 0ºF at the rate of 160 BTU/sec. How long would it take to convert the ice to steam (Cp = 0.5 BTU/lbmFº) at 213ºF? A. 173.78 sec C. 183.78 sec B. 163.78 sec D. 153.78 sec 6. 500 kcal of heat is added to 2kg of water at 80ºC. How much steam is produce? (Take latent heat of vaporization as 540 kcal/kg) A. 0.58 kg C. 0.68 kg B. 0.78 kg D. 0.85 kg 7. A tank of 1 m 3 contains air at 19.6 N/cm 2 and 15ºC. Air is pumped into the tank until the pressure is 98 N/cm2 and the temperature is 40ºC. Find the mass of air pumped into the tank. A. 8.538 kg C. 5.883 kg B. 3.85 kg D. 8.853 kg 8. A 287.5 cm3 of vapor at 100ºC and 100kPa has a mass of 0.725 g. What is the molecular weight of the vapor? A. 77.94 g/mole C. 55.94 g/mole B. 66.94 g/mole D. 88.94 g/mole 9. At STP the density of chlorine is 3.22 kg/m 3. What is the weight of this gas if contained in a flask of 100 cc at 24ºC and 100kPa? A. 0.292 g C. 0.563 g B. 0.420 g D. 0.321 g 10. A tank whose capacity is 0.1 m 3 contains helium at an absolute pressure of 10 atm and a temperature of 20ºC. A rubber balloon is inflated with this helium. The gas cools as it expands and when the pressure of the helium in the balloons is 1 atm, its temperature is -40ºC. Find the volume of the balloon A. 0.8 m3 C. 0.7 m3 3 B. 0.1 m D. 1.0 m3

13. When the temperature of an ideal gas is doubled, while the absolute pressure is halved, the volume is A. quadrupled C. doubled B. quartered D. halved 14. A sealed tank contains 27ºC at a pressure of 2 atm. If the temperature increases to 100ºC the pressure inside the tank will be… A. 1.49 Pa C. 3.49 atm B. 2.49 atm D. 5.49 Pa 15. Air flow steadily through an engine at constant temperature, 128°C. Find the work per kg, if the exit pressure is ½ the initial pressure and the initial pressure is equals to 220 kPa. A. 88 C. 79.8 B. 75.9 D. 83.5 16. An ideal gas occupies a volume of 0.8 m 3 at a temperature of 38°C and a given pressure of 200 kPa. The gas undergoes a constant pressure process until the temperature decreases to 6°C. Determine the work done in kJ. A. 18.1 C. 16.4 B. 20.3 D. 10.7 17. A piston cylinder containing 0.25 kg helium of 278 K receives heat at constant temperature until the pressure is one-half its initial value. Find the work done. A. W = 120 kJ C. W = 100 kJ B. W = 153 kJ D. W = 167 kJ 18. In terms of QH (heat from high temperature source) and QL (heat to low temperature sink), the network of the Carnot cycle is: A. W = QH – QL C. W = QH + QL B. W = QH / QL D. W = QL – QH 19. A 3 HP refrigerator or heat pump operates between 0ºF to 100ºF. The maximum theoretical heat that can be transferred from the cold reservoir is nearest to: A. 7,620 BTU/hr. C. 13,540 BTU/hr. B. 23,830 BTU/hr. D. 35,120 BTU/hr. 20. The maximum thermal efficiency that can be obtained in an ideal reversible heat engine operating between 1540ºF and 340ºF is closest to: A. 100% C. 60% B. 78% D. 40%

TECHNOLOGICAL INSTITUTE OF THE PHILIPPINES QUEZON CITY ELECTRONICS AND COMMUNICATIONS ENGINEERING

SUPPLEMENTARY PROBLEMS 21. A Carnot engine absorbs 1000 kJ of heat from reservoir at 300ºC and exhausts heat to a reservoir at 150ºC. Find the work it does. A. 162 kJ C. 26.2 kJ B. 262 kJ D. 16.2 kJ 22. Propane gas has a constant R = 0.1886 kJ/kg-K and a specific heat ratio k = 1.127. Determine the value of Cp and Cv of the gas. A. 2.13, 1.67 C. 1.48, 1.88 B. 1.67, 1.48 D. 1.12, 2.13 23. A heat engine is operated between temperature limits of 1370ºC and 260ºC. Engine is supplied with 14,142 kJ/kWh. Find the Carnot cycle efficiency in percent. A. 67.56 C. 70.10 B. 65.05 D. 69.32 24. For a gas with k = 1.4, what compression ratio must an Otto cycle have to achieve an ideal efficiency of 48.2%. A. 1.48 C. 5.18 B. 2.34 D. 0.85 25. What system in which neither mass nor energy cross the boundaries and it is not influenced by the surroundings? A. Closed system C. Isolated system B. Open system D. All of these 26. An ideal gas is 45 psig and 80ºF is heated in a closed container to 130ºF. What is the final pressure? A. 65 psia C. 55 psia B. 45 psia D. 75 psia 27. What is the area under the curve on a temperature – entropy diagram? A. Heat C. Entropy B. Work D. Volume 28. Two pounds of air initially at 60 psia and 600ºF expands isentropically until the temperature is 200ºF. Compute the work done by the gas. A. 137 BTU C. 173 BTU B. 127 BTU D. 147 BTU 29. Determine the amount of air in a room whose dimensions are 4m x 5m x 6m ay 100 kPa and 25ºC. A. 140.3 kg C. 130.4 kg B. 190.7 kg D. 240 kg 30. Find the change in internal energy of 5 lbm of oxygen gas when the temperature changes from 100ºF to 120ºF. Cv = 0.157 BTU/lbm-R. A. 147 BTU C. 15.7 BTU B. 14.7 BTU D. 157 BTU 31. One kilogram of water (Cv = 4.2 kJ/kg-K) is heated by 300 BTU of energy, what is the change in temperature in K? A. 17.9 K C. 71.4 K B. 75.4 K D. 125.2 K 32. An automobile tire is inflated at 35 psig pressure at 75°F. After being driven, the temperature rise of 25°F was determined. Find the gage pressure in psig assuming the volume remains constant. A. 21.9 C. 37.3 B. 67.4 D. 23.1

33. A gas is enclosed in a cylinder with weighted piston as the top boundary. The gas is heated and expands from a volume of 0.04 m 3 to 0.10 m3 at a constant pressure of 200 kPa. Find the work done on the system. A. 12 kJ C. 14 kJ B. 10 kJ D. 8 kJ 34. A closed vessel contains air at a pressure of 160 kN/m2 gauge and temperature of 30ºC. The air is heated at a constant volume to 60ºC with atmospheric pressure of 759 mm Hg. What is the final gauge pressure? A. 186 kPa C. 174kPa B. 167 kPa D. 172 kPa 35. What is the horse power required to isothermally compressed 800ft3 of air per minute from 14.7 psia to 120 psia? A. 108 HP C. 190 HP B. 180 HP D. 203 HP 36. A piston cylinder containing 0.35 kg methane at 5°C receives heat at constant temperature, until the pressure is 1/3 the initial value. Find the heat added. (R = 0.5183 kJ/kg-K) A. 55.4 kJ C. 45.5 kJ B. 56.4 kJ D. 54.5 kJ 37. When the expansion or compression of gas takes place without transfer of heat to or from the gas, the process is called: A. Isometric process C. Isobaric process B. Isothermal process D. Adiabatic process 38. What is the efficiency of an Otto cycle with a compression ratio of 6:1? (The gas is air) A. 51.2% C. 86.7% B. 45.6% D. 76.5% 39. What is the measure of the energy that is no longer available to perform useful work within the current environment? It is also known as the measure of the randomness of the molecules. A. Entropy C. Internal energy B. Enthalpy D. Flow work 40. The first law of thermodynamics is based on which of the following principles? A. Conservation of mass B. Conservation of energy C. Action and reaction D. The entropy – temperature relationship