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.

Accepted Answer

Carnot's theorem states that no engine operating between two heat reservoirs can be more efficient than a Carnot engine, which is a theoretical reversible engine operating between the same two reservoirs.

Question 2

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

Accepted Answer

The efficiency of all reversible heat engines operating between the same two heat reservoirs is the same, independent of the nature or amount of the working substance, according to the Carnot theorem. This principle is a fundamental concept in thermodynamics, illustrating the maximum efficiency limit for any heat engine.

Question 3

For a reversible heat pump, COP is given by

Accepted Answer

The Coefficient of Performance (COP) for a heat pump is given by the formula COP = Qc/W, where Qc is the heat delivered to the hot reservoir and W is the work input. For a reversible heat pump operating between two reservoirs with temperatures T1 (hot side) and T2 (cold side), the COP can also be expressed in terms of these temperatures. Since the temperatures in thermodynamics are usually given in Kelvin, the correct formula in terms of temperature difference is COP = T1 / (T1 - T2), where T1 > T2. This corresponds to option B.

Question 4

The entropy of an isolated system can never___________

Accepted Answer

The second law of thermodynamics states that the entropy of an isolated system can never decrease; it can only increase or remain constant.

Question 5

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

Accepted Answer

The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time, and is constant if and only if all processes are reversible. If a process is irreversible, the total entropy of the system and its surroundings increases.

Question 6

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

Accepted Answer

Entropy can decrease in a local region of an isolated system as long as there is a compensating increase in entropy elsewhere within the system, ensuring that the total entropy of the system does not decrease, in accordance with the second law of thermodynamics.

Question 7

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

Accepted Answer

The final temperature when two bodies of the same material and mass come into thermal equilibrium without any loss of heat to the surroundings is given by the average of their initial temperatures, which is (T1+T2)/2. The square root of the product of two temperatures, sqrt(T1*T2), is related to the geometric mean, which can also represent a possible intermediate temperature under certain conditions, but it's not a general final temperature for two mixing bodies. The correct range is from the arithmetic mean to potentially the geometric mean, depending on specific conditions, making option B correct.

Question 8

The part of___________available for conversion is referred to___________

Accepted Answer

The answer of The part of___________available for conversion is referred...

Question 9

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

Accepted Answer

The maximum work output obtainable from a certain heat input in a cyclic heat engine is referred to as available energy. This concept is fundamental in thermodynamics, particularly in the study of heat engine efficiency and the second law of thermodynamics.

Question 10

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

Accepted Answer

Exergy is the term used for available energy, while energy itself can include both available and unavailable energy.

Question 11

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

Accepted Answer

This statement is a reflection of the Second Law of Thermodynamics, which implies that when heat is transferred across a finite temperature difference, the process is irreversible, leading to a decrease in the availability of energy for doing work. This is because some of the energy is dissipated as waste heat, reducing the overall efficiency of energy conversion.

Question 12

Any thermodynamic process is accompanied by entropy generation.

Accepted Answer

In any real thermodynamic process, entropy is generated due to irreversibilities such as friction, heat transfer, and mixing, which increase the total entropy of the system and its surroundings.

Question 13

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

Accepted Answer

The rate of entropy increase of the control volume either exceeds or is equal to the net rate of entropy transfer to it, according to the second law of thermodynamics. This accounts for the principle that entropy of an isolated system can never decrease over time.

Question 14

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

Accepted Answer

The answer of The rate at which entropy is transferred...

Question 15

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

Accepted Answer

The first law efficiency, also known as the energy efficiency, is indeed defined as the ratio of the output energy (useful energy or work output) to the input energy, reflecting how well a system converts input energy into useful output.

Question 16

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

Accepted Answer

Exergy analysis allows for the assessment of the quality of energy and its potential to perform work, enabling the optimization of energy use and ensuring the most efficient conversion of energy in processes.

Question 17

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

Accepted Answer

When work is involved, Amin is equal to w. When heat is involved, Amin is equal to q(1-to/t), where to is the temperature of the surroundings and t is the temperature of the system.

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?

Accepted Answer

All the statements are true regarding the efficiencies of solar collectors. The first law efficiency is indeed the ratio of the heat transferred by the solar collector (Qa) to the solar energy available (Qr). The second law efficiency is defined as the ratio of exergy output to exergy input, which accounts for the quality of energy being transferred. The formula given for the second law efficiency in terms of the first law efficiency and temperatures is a way to relate these efficiencies considering the ambient temperature (To) and the temperatures at which the heat is absorbed (Ta) and available (Tr).

Question 19

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

Accepted Answer

For proper utilization of exergy, the first law efficiency should be as close to unity as possible, and the source and use temperatures should match to minimize irreversibility and maximize efficiency.

Question 20

The slopes of sublimation and vaporization curves for all substances are

Accepted Answer

The slopes of sublimation and vaporization curves are positive because as temperature increases, the pressure required for sublimation or vaporization also increases.

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