Nckume: Thermodynamics Part 2: Practice Quiz

23 Questions | Total Attempts: 233

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Nckume: Thermodynamics Part 2: Practice Quiz - Quiz

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Questions and Answers
  • 1. 
    What is the entropy of a gas at T = 0K?
    • A. 

      S = 0 kJ/kg.K

    • B. 

      S > 0 kJ/kg.K

    • C. 

      It depends on the pressure.

    • D. 

      None of the above.

  • 2. 
    The ideal gas law may be written as: P*(specific volume) = R*T. What are the units of R in this case if the units of P are Pa?
    • A. 

      KJ/kg.K

    • B. 

      J/K.mol

    • C. 

      J/K

    • D. 

      None of these options are correct.

  • 3. 
    Two heat engines operate between the same high and low energy sources / sinks, as shown here. Both heat engines receive the same amount of energy QH. If one heat engine is reversible, and the other is not, what can we say about how much heat each engine rejects? (QL)
    • A. 

      According to the first law, they must be the same.

    • B. 

      There is not enough information - you didn't tell me what W is.

    • C. 

      QL from the reversible engine will be less than QL from the irreversible one.

    • D. 

      QL from the reversible engine will be more than QL from the irreversible one.

  • 4. 
    A car engine burns fuel at 2000K and rejects the used air from the car exhaust at around 400K. If the car engine is assumed completely ideal, what is the thermal efficiency of this engine?
    • A. 

      I don't have enough information - I require the compression ratio.

    • B. 

      I don't have enough information - I require something else.

    • C. 

      50%

    • D. 

      80%

  • 5. 
    A car engine takes atmospheric air in at 20 degrees C. It uses no fuel, but produces work and exhaust gas at -20 C. What does the first law saw about this?
    • A. 

      The first law is just a mathematical equation, and cannot say anything about anything - it can't speak.

    • B. 

      The 1st Law may be satisfied.

    • C. 

      The 1st Law is not satisfied, because there is no fuel.

    • D. 

      (DON'T PICK THIS ONE. IT'S ONE OF THE ABOVE OPTIONS)

  • 6. 
    A car engine takes atmospheric air in at 20 degrees C. It uses no fuel, but produces work and exhaust gas at -20 C. What does the 2nd law saw about this?
    • A. 

      The 2nd law is violated because we can't move energy from a hot place to a cold place while producing work.

    • B. 

      The 2nd law is violated because if the exhaust gas temperature is negative, the efficiency is larger than 1.

    • C. 

      The 2nd law may be satisfied.

    • D. 

      (Not this one. Try again. Pick one of the others)

  • 7. 
    A car engine burns 5 kg of fuel at 1500 K and rejects energy at 750 K through the radiator and exhaust. If the fuel provides 40,000 kJ/kg of energy, what is the theoretical maximum amount of work the engine may produce?PS: Assume a Carnot cycle is used here. ~ Prof. Smith
    • A. 

      This was a homework question. If I did the homework and got the answer right, but I don't get this answer right, Prof. Smith won't be suspicious at all.

    • B. 

      100,000 kJ

    • C. 

      100,000 kW

    • D. 

      20,000 kJ.

  • 8. 
    What are the units of k (the ratio of specific heats of a gas)?
    • A. 

      J/kg

    • B. 

      J/K

    • C. 

      K/kg

    • D. 

      None of these options are correct.

  • 9. 
    Water in a rigid box is originally 100 degrees C with a quality of 50%. The box is then heated to 110 C (increase of 10 degrees). What can we say about the quality x and entropy s?
    • A. 

      The quality will increase, and the entropy will increase.

    • B. 

      The quality will remain constant, and the entropy will increase.

    • C. 

      The quality will increase, but the entropy will remain constant.

    • D. 

      Both the entropy and quality remain the same.

  • 10. 
    A process 1-2 is performed on a gas in a piston-cylinder arrangement. What can you say about the heat transfer which occurs during this process?
    • A. 

      There is no heat transfer in this process.

    • B. 

      We are removing heat, which produces a decrease in temperature.

    • C. 

      We are adding heat, which produces a decrease in temperature because reasons.

    • D. 

      There is not enough information here.

  • 11. 
    Which one of these equations is the Clausius Inequality? 
    • A. 

      Option 1

    • B. 

      Option 2

    • C. 

      Option 3

    • D. 

      None of these are correct.

  • 12. 
    Two long bars of steel, one at 250 degrees C and the other at 25 degrees C, come into contact. They are then allowed to cool until they reach a constant temperature. Assuming the ends are insulated, find the final temperature and change in entropy in the process.(Assume C = 0.46 kJ/kg.K for steel)
    • A. 

      137.5 C, dS = 0.1794 kJ/K

    • B. 

      137.5 C, dS = 0.0359 kJ/kg.K

    • C. 

      137.5 C, dS = 0.5092 kJ/kg.K

    • D. 

      This is an engineering mathematics question, not a thermodynamics question. (DON'T PICK THIS)

  • 13. 
    A P-v and T-s diagram is shown for a reversible process involving steady air flowing in (i) and out (e) of a device. What can we say about the heat transfer involved in this process?
    • A. 

      There is no heat transfer as T is the same at the inlet and exit.

    • B. 

      We must have heat coming into the device.

    • C. 

      We must have heat leaving the device.

    • D. 

      None of these options are correct. (Really? Go on, pick this one)

  • 14. 
    Which of the below are possible units for specific entropy?
    • A. 

      KJ/kg.K

    • B. 

      KJ/kg

    • C. 

      KJ/K

    • D. 

      None of these options are possible.

  • 15. 
    An air compressor takes air from standard conditions (100 kPa, 300K). Air exits the compressor at 400 kPa, 480K. The compressor uses 100 kW of power. What is the minimum compressor work input (choose the closest answer).Hint: Assume ideal gas. ST0 (100kPa,300K) = 6.8692, ST0 (400kPa,480K)=7.3449.
    • A. 

      87.1 kW

    • B. 

      92.1 kW

    • C. 

      80.25 kW

    • D. 

      Another homework question - I can relax because I won't see these on the exam. (WRONG)

  • 16. 
    A refrigerator removes 1.5 kW from its cold food at -10 degrees C and rejects heat at room temperature (25 degrees C). If the fridge uses 750 W, calculate the irreversibility. 
    • A. 

      550 W

    • B. 

      500 W

    • C. 

      250 W

    • D. 

      None of these are correct. The units are all wrong.

  • 17. 
    The equation for the change in entropy of a pump is shown here. What does this equation assume?
    • A. 

      The temperature is constant during the pumping process.

    • B. 

      The stuff going through the pump is an incompressible fluid.

    • C. 

      The pump is adiabatic.

    • D. 

      All of these options are correct.

  • 18. 
    An engineer working in a steam power plant running a Rankine cycle wants to increase the pressure in the boiler to increase the quality of the steam exiting the turbine. The temperature in the boiler will be the same. Is he right or wrong, and why?
    • A. 

      He is wrong - the quality at the exit of the turbine is only a function of T, not P.

    • B. 

      He is wrong - the quality will be reduced because the gas leaving the boiler is less superheated.

    • C. 

      He is right - the higher pressure means higher energy, which means a higher x at the exit.

    • D. 

      He is right because he is the boss and the boss is always right.

  • 19. 
    Consider a Rankine cycle powered using a solar energy source. Saturated vapor leaves the solar panels at 1500 kPa, passes through the turbine and enters the condenser at 15 kPa. Compute the thermal efficiency of this cycle. (PS: You'll need steam tables to solve this problem.)
    • A. 

      0.24

    • B. 

      0.784

    • C. 

      0.56

    • D. 

      None of these answers are correct.

  • 20. 
    A steam power cycle has a high pressure of 3.0 MPa and a condenser exit temperature of 45 degrees C. The turbine used in this cycle operates at an efficiency of 85%. If the other components are ideal, and the highest temperature in the boiler is 800 degrees C, compute the overall thermal efficiency of the cycle.
    • A. 

      0.97

    • B. 

      0.65

    • C. 

      Not enough information to answer this question.

    • D. 

      0.347

  • 21. 
    Why is the efficiency of a Rankine cycle higher than the efficiency of a Brayton cycle?
    • A. 

      Because liquids require less energy to pump than gasses do.

    • B. 

      The Brayton and Rankine cycles have the same theoretical efficiency for the same values of TH and TL.

    • C. 

      Because water holds more energy per unit kg than air does.

    • D. 

      None of these answers are correct.

  • 22. 
    A Brayton cycle has a compression ratio of 15:1 with a high temperature of 1600 K with inlet conditions 290 K, 100 kPa. Assuming ideal gas conditions with constant Cp and Cv values, compute the net work output. (Choose the closest answer)
    • A. 

      525 kJ/kg

    • B. 

      475 kJ/kg

    • C. 

      575 kJ/kg

    • D. 

      None of these options are close enough to my answer, and my answer is correct. (NOT THIS ONE)

  • 23. 
    A diesel engine has an inlet at 95 kPa, 300 K and a compression ratio of 20:1. The combustion releases 1300 kJ/kg of energy per kg of air in the engine. Find the temperature after combustion is complete. Assume ideal gas properties and constant values of Cp and Cv.
    • A. 

      ~21,000 K

    • B. 

      ~ 2290 K

    • C. 

      ~2000 K

    • D. 

      I didn't get to see the example Prof. Smith provided, but I can relax - it won't be on the test.

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