This set of Thermodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Comments on Exergy”.
1. ____ is conserved but ____ is not conserved.
a) exergy, energy
b) energy, exergy
c) both exergy and energy are conserved
d) neither exergy nor energy is conserved
View Answer
Explanation: Energy is conserved but once the exergy is wasted, it can never be recovered.
2. Exergy is a composite property.
a) true
b) false
View Answer
Explanation: It depends on the state of the system and the surroundings.
3. A dead state
a) is in equilibrium with its surroundings
b) has zero exergy
c) both of the mentioned
d) none of the mentioned
View Answer
Explanation: This is the basic fact about dead state which we have seen earlier.
4. Which of the following statement is true?
a) KE is entirely available energy
b) PE is entirely exergy
c) The exergy of thermal energy of reservoirs is equivalent to the work output of a Carnot engine operating between the reservoir at temperature T and environment To.
d) all of the mentioned
View Answer
Explanation: W=Q(1-To/T).
5. Useful work is given by
a) difference between the actual work and the surrounding work
b) W – W for surroundings
c) W – p(V2-V1)
d) all of the mentioned
View Answer
Explanation: Some work is lost to the surroundings and hence the useful work is reduced.
6. The surrounding work is zero for
a) cyclic devices
b) steady flow devices
c) system with fixed boundaries
d) all of the mentioned
View Answer
Explanation: There is no work being done on surroundings in all these cases.
7. The maximum amount of useful work that can be obtained from a system as it undergoes a process between two specified states is called
a) adiabatic work
b) reversible work
c) irreversible work
d) none of the mentioned
View Answer
Explanation: Reversible work is the maximum work that we can get in a process.
8. If the final state of the system is the dead state, the ____ and the ____ become identical.
a) reversible work, exergy
b) irreversible work, exergy
c) reversible work, irreversible work
d) none of the mentioned
View Answer
Explanation: For a dead state, reversible work and exergy are same.
9. The difference between the reversible work and the useful work for a process is called irreversibility.
a) true
b) false
View Answer
Explanation: Irreversibility is given by the product of To and rate of entropy generation, where To is the environment temperature.
10. For a total reversible process,
a) W reversible = useful work
b) irreversibility = 0
c) both of the mentioned
d) none of the mentioned
View Answer
Explanation: I = W in reversible – useful work = To * rate of entropy generation.
11. Two engines having same thermal efficiency but supplied with heat from source at different temperatures,
a) will convert same fraction of heat they receive into work
b) from second law, one will perform better than other
c) this can be considered as a deficiency of the first law
d) all of the mentioned
View Answer
Explanation: This proves that we need second law efficiency.
12. Second law efficiency is defined as the ratio of actual thermal efficiency to the maximum possible thermal efficiency.
a) true
b) false
View Answer
Explanation: Second law efficiency = first law efficiency / efficiency of a reversible process.
13. Which of the following is true?
a) for work producing devices, second law efficiency = useful work / reversible work
b) for work absorbing devices, second law efficiency = reversible work / useful work
c) both of the mentioned
d) none of the mentioned
View Answer
Explanation: Both of the above statements are true and comes from the formula of second law efficiency.
14. In the context of first law efficiency and second law efficiency, which of the following statement is true?
a) first law efficiency = energy output / energy input and their difference is the energy loss
b) second law efficiency = exergy output / exergy input and their difference is irreversibility
c) by reducing energy loss, first law efficiency can be reduced and by reducing irreversibilities, second law efficiency can be reduced
d) all of the mentioned
View Answer
Explanation: All of these statements just give a summary of what we have learned in first law efficiency and second law efficiency.
Sanfoundry Global Education & Learning Series – Thermodynamics.
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