Deck 19: Thermodynamics and the Second Law: Efficiency, Entropy, and Exergy

Question 1

According to Carnot's theorem, all heat engines operating between a given constant temperature source and sink, none has a higher efficiency than a reversible engine.

Answer 1

True

Question 2

The efficiency of all reversible heat engines operating between the same heat reservoirs is

A)same
B)independent of the nature of working substance
C)independent of the amount of working substance
D)all of the mentioned

Answer 2

all of the mentioned

Question 3

For a reversible heat pump, COP is given by

A)t2/(t1-t2)
B)t1/(t1-t2)
C)t2/(t2-t1)
D)t1/(t2-t1)

Answer 3

t1/(t1-t2)

Question 4

The entropy of an isolated system can never___________

A)increase
B)decrease
C)be zero
D)none of the mentioned

Question 5

According to entropy principle, the entropy of an isolated system can never decrease and remains constant only when the process is reversible.

Question 6

Entropy may decrease locally at some region within the isolated system. How can this statement be justified?

A)this cannot be possible
B)this is possible because entropy of an isolated system can decrease.
C)it must be compensated by a greater increase of entropy somewhere within the system.
D)none of the mentioned

Question 7

The final temperatures of two bodies, initially at T1 and T2 can range from

A)(t1-t2)/2 to sqrt(t1*t2)
B)(t1+t2)/2 to sqrt(t1*t2)
C)(t1+t2)/2 to (t1*t2)
D)(t1-t2)/2 to (t1*t2)

Question 8

The part of___________available for conversion is referred to___________

A)high grade energy, available energy
B)low grade energy, available energy
C)low grade energy, unavailable energy
D)high grade energy, unavailable energy

Question 9

The___________obtainable from a certain heat input in a cyclic heat engine is called___________

A)minimum work output, available energy
B)maximum work output, available energy
C)minimum work input, unavailable energy
D)none of the mentioned

Question 10

The available energy is known as___________and the unavailable energy is known as___________

A)energy, exergy
B)exergy, energy
C)both are called exergy
D)both are called energy

Question 11

Whenever heat is transferred through a finite temperature difference, there is always a decrease in the availability of energy so transferred.

Question 12

Any thermodynamic process is accompanied by entropy generation.

Question 13

The rate of entropy increase of the control volume___________or___________the net rate of entropy transfer to it.

A)exceeds or is less than
B)exceeds, is equal to
C)is less than, or equal to
D)none of the mentioned

Question 14

The rate at which entropy is transferred out must___________the rate at which entropy enters the control volume.

A)be less than
B)equal to
C)exceed
D)none of the mentioned

Question 15

The first law efficiency is defined as the ratio of the output energy to the input energy.

Question 16

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

A)to analyse means of minimizing the consumption of available energy to perform a given process
B)to ensure most efficient possible conversion of energy
C)both of the mentioned
D)none of the mentioned

Question 17

If work is involved, Amin=___________and if heat is involved, Amin=___________

A)w, q(1+to/t)
B)w, q(1-to/t)
C)q(1+to/t), w
D)q(1-to/t), w

Question 18

If solar energy Qr is available at a reservoir storage temperature of Tr and if quantity of heat Qa is transferred by the solar collector at temperature Ta, then which of the following is true?

A)first law efficiency = qa/qr
B)second law efficiency = exergy output / exergy input
C)second law efficiency = (first law efficiency)*(1-to/ta)/(1-to/tr)
D)all of the mentioned

Question 19

For proper utilization of exergy, it is desirable to make first law efficiency___________ and the source and use temperatures should

A)as close to unity, be different
B)as close to unity, match
C)as close to zero, match
D)as close to zero, be different

Question 20

The slopes of sublimation and vaporization curves for all substances are

A)negative
B)positive
C)zero
D)none of the mentioned

Question 21

When does a vapour become superheated?

A)when the temperature of vapour is less than the saturation temperature at given pressure
B)when the temperature of vapour is more than the saturation temperature at given pressure
C)when the temperature of vapour is equal to the saturation temperature at given pressure
D)none of the mentioned

Question 22

The properties of liquid___________  with pressure.

A)do not vary
B)vary largely
C)vary little
D)none of the mentioned

Question 23

What is the effect of reheat pressure on mean temperature of heat addition Tml?

A)reheat pressure is directly proportional to tml
B)reheat pressure is inversely proportional to tml
C)reheat pressure is equal to tml
D)tml is independent of reheat pressure