# Thermal Engineering Questions and Answers – Availability and Irreversibility

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This set of Thermal Engineering Questions and Answers for Aptitude test focuses on “Availability and Irreversibility”.

1. Availability function (∅) is given by ______
a) U-p0V-T0S
b) U+p0V-T0S
c) –U+p0V-T0S
d) p0V-T0S

Explanation: Wmax = ∅1-∅0 is known as availability of closed system at state 1. Where ∅1 availability function at state 1 and ∅0 is the availability function of dead state. The availability function is defined as ∅ = U+p0V-T0S

2. What is the lowest temperature at which heat rejection can takes place?
a) 100°C
b) 0K
c) 273K
d) Temperature equal to surrounding

Explanation: The lowest temperature at which heat rejection can takes place is equal to the temperature of surrounding. Efficiency will be maximum if heat rejection takes place at temperature of surrounding.

3. Gibbs function is given by _____
a) G = H – TS
b) G = pV – TS
c) G = U – pV
d) G = U + pV

Explanation: Gibbs function is given by G = H – TS. Also it is given by G =U +pV -TS

4. What is the part of heat energy which is converted into mechanical work which reduce system in state of equilibrium?
a) Available energy
b) Electrical energy
c) Unavailable energy
d) Hydraulic energy

Explanation: Available energy is the part of heat energy which is converted into mechanical work. It is converted by ideal processes which reduce system in state of equilibrium.

5. The _________ obtainable from a certain heat input in a cyclic heat engine is called _________
a) Maximum work, unavailable energy
b) Maximum work, available energy
c) Minimum work, available energy
d) Minimum work, unavailable energy

Explanation: The maximum work obtainable from a certain heat input in a cyclic heat engine is called available energy. Available energy is the part of heat energy which is converted into mechanical work.

6. What is the part of heat energy which cannot be converted into mechanical work which reduce system in state of equilibrium?
a) Available energy
b) Electrical energy
c) Unavailable energy
d) Hydraulic energy

Explanation: Unavailable energy is the part of heat energy which cannot be converted into mechanical work. It cannot be converted even by ideal processes which reduce system in state of equilibrium.

7. The _________ has to be rejected by any cyclic heat engine is called _________
a) Maximum energy, unavailable energy
b) Maximum energy, available energy
c) Minimum energy, available energy
d) Minimum energy, unavailable energy

Explanation: The minimum energy that has to be rejected by any cyclic heat engine is called unavailable energy. Unavailable energy is the part of heat energy which cannot be converted into mechanical work.

8. The difference between idealized reversible work and actual work done by the system is called _____
a) Reversibility
b) Irreversibility
c) Gain in available energy
d) Decrease in unavailable energy

Explanation: The difference between idealized reversible work and actual work done by the system is called irreversibility of the process. Irreversibility = loss in available energy = increase in unavailable energy.

9. Availability is also known as __________
a) Exergy
b) Ronthalpy
c) Enthalpy
d) Entropy

Explanation: The maximum work obtainable from a certain heat input in a cyclic heat engine is called available energy. Availability is also known as exergy.

10. Which of the following statements applicable to a perfect gas will also be true for an irreversible process?
a) dQ = dU + pdV
b) dQ = du – pdV
c) Tds = dU + pdV
d) Tds = du – pdV

Explanation: Tds = dU + pdV
All are properties thus it is applicable for reversible as well as irreversible process.

11. Which of the following statement is correct?
a) Available energy is generally conserved
b) Available energy is maximum theoretical work obtainable
c) Available energy can either be negative or positive
d) Available energy is minimum theoretical work obtainable

Explanation: The maximum work obtainable from a certain heat input in a cyclic heat engine is called available energy. Available energy is the part of heat energy which is converted into mechanical work.

12. When system undergoes a process such that ∫dQ/T = 0 and ∆S>0, the process is ______
c) Isothermal
d) Isobaric 