439 Thermodynamic MCQ For All Mechanical Exams

126)The available energy is known as ____ and the unavailable energy is known as ____

1. a) energy, exergy
2. b) exergy, energy
3. c) both are called exergy
4. d) both are called energy

Ans-B

127)For an infinitesimal reversible process at constant pressure,

1. a) dS=m*dT/T
2. b) dS=Cp*dT/T
3. c) dS=m*dT/T
4. d) dS=m*Cp*dT/T

Ans-D

128)The ___ in entropy in an irreversible change is a measure of the extent to which energy ___ in that

change.

3. c) increase, increases
4. d) decrease, increases

Ans-B

129)For a process from state 1 to state 2, heat transfer in an irreversible process is given by

1. a) Q for irreversible=(To)*(S1-S2)
2. b) Q for irreversible>(To)*(S1-S2)
3. c) Q for irreversible<(To)*(S1-S2)
4. d) none of the mentioned

Ans-C

130)In an open system, for maximum work, the process must be entirely

1. a) irreversible
2. b) reversible
4. d) none of the mentioned

Ans-B

131)Useful work is given by

1. a) actual work + p(V2-V1)
2. b) actual work – p(V2-V1)
3. c) actual work + p(V2+V1)
4. d) none of the mentioned

Ans-B

132)Availability function for a closed system is given by

1. a) U-pV+TS
2. b) U+pV+TS
3. c) U-pV-TS
4. d) U+pV-TS

Ans-D

133)When a system exchanges heat with a thermal energy reservoir at temperature Tr in addition to the atmosphere, the maximum useful work increases by

1. a) dQ(1+Tr/To)
2. b) dQ(1-Tr/To)
3. c) dQ(1+To/Tr)
4. d) dQ(1-To/Tr)

Ans-D

134)When a system changes its state towards that of surroundings, the opportunity to produce more work

1. a) increases
2. b) decreases
3. c) remains constant
4. d) none of the mentioned

Ans-B

135)Availability function for a closed system is given by

1. a) u-pv-Ts
2. b) u+pv+Ts
3. c) u-pv+Ts
4. d) u+pv-Ts

Ans-D

136)The Helmholtz function F is given by

1. a) U-TS
2. b) U+TS
3. c) -U-TS
4. d) -U+TS

Ans-A

137)The actual work done by a system is always ____ than the reversible work, and the difference between the two is called ____ of the process.

1. a) more, irreversibility
2. b) less, irreversibility
3. c) more, reversibility
4. d) less, reversible

Ans-B

138)For a non-flow process between equilibrium states, when the system exchanges heat only with the environment

1. a) I=0
2. b) I>0
3. c) I<0
4. d) I>=0

Ans-D

139)A thermodynamically efficient process would involve ____ exergy loss with ____ rate of entropy generation.

1. a) minimum, minimum
2. b) maximum, maximum
3. c) minimum, maximum
4. d) maximum, minimum

Ans-B

140)Energy is ____ conserved and exergy is ____ conserved.

1. a) always, generally
2. b) always, not generally
3. c) not always, always
4. d) always, always

Ans-B

141)When the closed system is allowed to undergo a spontaneous change from a given state to a dead state, its exergy is ____ destroyed ____ producing useful work.

1. a) not completely, though
2. b) not completely, without
3. c) completely, though
4. d) completely, without

Ans-D

142)The exergy of an isolated system can ____

1. a) increase
2. b) decrease
3. c) never increase
4. d) never decrease

Ans-C

143)Since irreversibility > 0, the only processes allowed by the second law are those for which the exergy of the isolated system

1. a) increases
2. b) decreases
3. c) remains constant
4. d) none of the mentioned

Ans-B

144)For a closed system, availability or exergy transfer occurs through

1. a) heat interactions
2. b) work interactions
3. c) both of the mentioned
4. d) none of the mentioned

Ans-C

145)For an isolated system, the exergy balance gives

1. a) ΔA=-I
2. b) ΔA=I
3. c) ΔA=0
4. d) none of the mentioned

Ans-A

146)Which law is used for exergy balance?

1. a) first law
2. b) second law
3. c) first law and second law
4. d) third law

Ans-C

147)With the concept of exergy available, which of the following is possible?

1. a) to analyse means of minimizing the consumption of available energy to perform a given process
2. b) to ensure most efficient possible conversion of energy
3. c) both of the mentioned
4. d) none of the mentioned

Ans-C

148)Second law efficiency is defined as

1. a) actual exergy intake / minimum exergy intake
2. b) minimum exergy intake / actual exergy intake
3. c) actual exergy intake / maximum exergy intake
4. d) maximum exergy intake / minimum exergy intake

Ans-B

149)Second law efficiency can also be given as

1. a) 1 / ( first law efficiency * Carnot efficiency)
2. b) Carnot efficiency * first law efficiency
3. c) Carnot efficiency / first law efficiency
4. d) first law efficiency / Carnot efficiency

Ans-D

150)In case of a heat pump, second law efficiency is given as

1. a) (first law efficiency)*(1-Ta/To)
2. b) (first law efficiency)*(1+To/Ta)
3. c) (first law efficiency)*(1-To/Ta)
4. d) none of the mentioned

Ans-C

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