302 ENGINEERING THERMODYNAMICS
dharm
/M-therm/th5-4.pm5
- Which of the following is the correct statement?
(a) All the reversible engines have the same efficiency
(b) All the reversible and irreversible engines have the same efficiency
(c) Irreversible engines have maximum efficiency
(d) All engines are designed as reversible in order to obtain maximum efficiency.
Answers - (d) 2. (a) 3. (a) 4. (b) 5. (e) 6. (e) 7. (b)
- (b) 9. (d) 10. (a) 11. (c) 12. (d) 13. (d) 14. (b)
- (e) 16. (c) 17. (a) 18. (a) 19. (d) 20. (e) 21. (a)
- (d) 23. (d) 24. (a).
Theoretical Questions
- State the limitations of first law of thermodynamics.
- What is the difference between a heat engine and a reversed heat engine?
- Enumerate the conditions which must be fulfilled by a reversible process. Give some examples of ideal
reversible processes. - What is an irreversible process? Give some examples of irreversible processes.
- Give the following statements of second law of thermodynamics.
(i) Clausius statement
(ii) Kelvin-Planck statement. - Define heat engine, refrigerator and heat pump.
- What is the perpetual motion machine of the second kind?
- What do you mean by ‘Thermodynamic temperature’?
- What do you mean by ‘Clausius inequality’?
- Describe the working of a Carnot cycle.
- Derive an expression for the efficiency of the reversible heat engine.
- What do you mean by the term ‘Entropy’?
- What are the characteristics of entropy?
- Prove that entropy is a property of a system.
- Derive an expression for the change in entropy of the universe.
- What is a temperature-entropy diagram?
- Derive expressions for entropy changes for a closed system in the following cases :
(i) General case for change of entropy of a gas
(ii) Heating a gas at constant volume
(iii) Heating a gas at constant pressure
(iv) Polytropic process. - Give an expression for entropy changes for an open system.
Unsolved Examples
Heat Engines and Reversible Heat Engines
- A cyclic heat engine operates between a source temperature of 800°C and a sink temperature of 30°C.
What is the least rate of heat rejection per kW net output of the engine? [Ans. 0.392 kW]