Thermodynamics and Its Four Laws
Many a time students get scared merely by hearing the name of the topic of thermodynamics itself as if it is some scary mammoth. Are you one of them? Do not worry, Vedantu is here to help you.
Thermodynamics is nothing but the study of the relations between heat, work, temperature, and energy. It is a very practical subject whose examples can be observed in daily life itself like when you boil water to make tea or when you pour that tea into a thermos.
The Foundation of Thermodynamics is laid over its four laws which are
Zeroth law
First law
Second law
Third law
Zeroth of Thermodynamics
It states that if two systems are in thermodynamic equilibrium with a third system, then the two original systems are also in thermal equilibrium with each other. If A is in thermodynamic equilibrium with C and B is also in thermodynamic equilibrium with C, then A and B are also in thermodynamic equilibrium.
The First Law of thermodynamics
The first law of thermodynamics is nothing but the law of conservation of energy. It states that energy can neither be created nor it can be destroyed. It can only be converted from one form to another.
The Second Law of Thermodynamics
The second law of thermodynamics states that any isolated system's entropy always increases. Isolated systems evolve spontaneously towards thermal equilibrium— the system's state of maximum entropy. In simple terms, Universe entropy (the ultimate isolated system) only increases and never decreases.
A straightforward way of thinking about the second law of thermodynamics is that if it is not cleaned and tidy, a room will eventually get messier and messier over time –no matter how careful one is to keep it clean.
The Third Law of Thermodynamics
The third thermodynamic law states that the entropy of a system approaches a constant value as it reaches absolute zero. The entropy of a system at absolute zero usually is zero and is determined in every case only by the number of different ground states it has. Entropy for a pure crystalline material at absolute zero temperature (ideal order) is 0. This statement holds if only one minimum energy condition exists for the perfect crystal.
MCQs of 2nd and 3rd Law of Thermodynamics
1. A refrigerator has a performance coefficient of 5. Calculate the ambient heat discharged if the temperature inside the freezer is -20oC
11oC
41oC
21oC
31oC
Ans: D
2. Which of the following factors affects the heat of reaction based on with Kirchhoff Equation
Molecularity
Temperature
Pressure
Volume
Ans: B
3. Chemical Dissociation of all reactions is
Exothermic
Reversible
Endothermic
Reversible and Endothermic
Ans: D
4. Select the largest unit of Energy
Electron volt
Joule
Calorie
Erg
Ans: C
5. What is the unique characteristic feature of a perfect Black Body
A good absorber only
A good radiator
A good absorber and a good radiator
Neither a radiator nor an absorber
Ans: C
6. Which Thermodynamic process where heat is not exchanged with the surroundings is
Isothermal
Adiabatic
Isobaric
Isotropic
Ans: B
Entropy
In physics and chemistry, entropy is an important concept, and it can be extended to other sciences, including cosmology and economy. It's part of thermodynamics in physics. It is a core concept in physical chemistry.
Solved MCQs on Entropy
This series of Multiple-Choice Questions & Answers (MCQs) in Thermodynamics primarily focuses on "Entropy Theory and its Applications."
1. Which of the following is correct?
For an isolated system, dS>=0
For a reversible process, dS=0
For an irreversible process, dS>0
All of the mentioned
Ans: D
Explanation: For an isolated system with no exchange of energy with environment Q=0 and also dS>=dQ / T.
2. According to the entropy theorem, the entropy of an isolated system can never decrease and will remain constant only when the process is reversible.
True
False
Ans: B
Explanation: This is the declaration for the principle of increase of entropy.
3. Entropy may decline locally somewhere within the isolated system. How can one clarify this statement?
This cannot be possible
This is possible because it can decrease the entropy of an isolated system.
This must be balanced by increased entropy somewhere within the system.
None of the mentioned
Ans: C
Explanation: The net effect of an irreversible process is an increase in entropy of the entire system.
4. Clausius summed up the first and second laws concerning thermodynamics as
The energy of the world is constant
The entropy tends towards a maximum
Both of the mentioned
None of the mentioned
Ans: C
Explanation: Clausius gave these two statements.
5. The entropy of an isolated system continuously ____ and becomes a ____ at the state of equilibrium.
decreases, minimum
increases, maximum
increases, minimum
decreases, maximum
Ans: B
Explanation: If an isolated system’s entropy differs from some parameter, then that parameter has a certain value that maximizes the entropy.
6. The entropy principle is the quantitative statement of the second law of thermodynamics.
True
False
Ans: a
Explanation: This is an overall fact about the entropy principle.
7. Which of the following may be regarded as applying the principle of entropy?
Transfer of heat through a finite temperature difference
Mixing of two fluids
Maximum temperature attainable from two finite bodies
All of the mentioned
Ans: D
Explanation: These are some general applications of the entropy principle.
8. The final temperatures of two bodies, initially at T1 and T2 can range from
(T1-T2)/2 to √(T1*T2)
(T1+T2)/2 to √(T1*T2)
(T1+T2)/2 to (T1*T2)
(T1-T2)/2 to (T1*T2)
Ans: B
Explanation: (T1+T2)/2 is when no work is done, and sqrt(T1*T2) is the temperature with maximum work distribution.
9. Which of the following processes exhibit external mechanical Irreversibility?
Isothermal dissipation of work
Adiabatic dissipation of work
Both of the mentioned
None of the mentioned
Ans: C
Explanation: These processes reveal external mechanical irreversibility.
10. Which of the following laws was expressed by Nernst?
The first law of thermodynamics
The second law of thermodynamics
Third law of thermodynamics
None of the above
Ans: C
Explanation: Third law of thermodynamics was expressed by Nernst and it states that entropy of a system at absolute zero remains constant.
Importance of MCQs
MCQs play a very important role in gauging a student’s understanding of a specific topic.
It helps in evaluating one’s objective understanding of a topic.
All the entrance exams conducted after Class 12 for getting admission into various professional degree courses consist of MCQ type of questions only
These questions are a very good way for you to test your level of preparation
These MCQs can act as your mock test for various exams
All of these are available for free on Vedantu’s website
You can access them anytime from anywhere.
FAQs on MCQ’s on 2nd and 3rd Law of Thermodynamics and Entropy
1. What is the Second Law of Thermodynamics in simple terms as per the CBSE syllabus?
The Second Law of Thermodynamics defines the direction of spontaneous processes. It can be stated in two main ways:
- Kelvin-Planck Statement: It is impossible to construct an engine that, operating in a cycle, will produce no other effect than extracting heat from a single reservoir and performing an equivalent amount of work. This essentially means no heat engine can be 100% efficient.
- Clausius Statement: It is impossible to design a self-acting machine, unaided by any external agency, which would transfer heat from a body at a lower temperature to another at a higher temperature. This explains why heat spontaneously flows from hot to cold.
2. What are some real-world examples that illustrate the Second Law of Thermodynamics?
The Second Law of Thermodynamics is visible in many everyday phenomena that show a natural progression towards disorder or equilibrium. Key examples include:
- A cup of hot tea left on a table will gradually cool down to room temperature, never spontaneously heat up.
- Ice cubes in a drink will always melt, absorbing heat from the warmer liquid, rather than the drink freezing further.
- A room, if left unattended, will naturally become messier and more disorganized over time, not neater.
- Air flowing from a punctured tyre into the atmosphere, a process that is not naturally reversible.
3. What is the fundamental concept of the Third Law of Thermodynamics?
The Third Law of Thermodynamics provides a definitive baseline for the property of entropy. It states that the entropy of a perfect crystalline substance is zero at the absolute zero of temperature (0 Kelvin). At this temperature, the system is in its state of minimum possible energy, representing the highest degree of order. This law is crucial because it allows us to determine the absolute entropy of a substance at other temperatures.
4. How does the concept of entropy connect the Second and Third Laws of Thermodynamics?
Entropy is the central concept that links these two laws. The Second Law is about the change in entropy; it states that for any spontaneous process in an isolated system, the total entropy always increases (ΔS > 0). It tells us the direction of natural change is towards greater disorder. The Third Law, on the other hand, provides a reference point by defining the absolute value of entropy. It states that the entropy of a perfectly ordered crystal is zero at absolute zero (0 K). Therefore, the Second Law describes *how* entropy behaves, while the Third Law provides a fundamental starting point from which entropy can be calculated.
5. Why does the entropy of an isolated system always increase? Can it ever decrease in a specific area?
The entropy of an isolated system (one that cannot exchange energy or matter with its surroundings) always increases because systems naturally evolve towards their most probable state, which is the state of maximum disorder or randomness. However, the entropy of a part of a system, or a non-isolated system, can indeed decrease. A classic example is a refrigerator. It decreases the entropy inside by making it colder, but to do so, it must perform work and release a larger amount of heat into the warmer room, thereby increasing the overall entropy of the kitchen (the surroundings). The net entropy change of the universe (system + surroundings) is always positive, in accordance with the Second Law.
6. What is a key difference between reversible and irreversible processes according to the Second Law?
The key difference lies in the change in the total entropy of the universe.
- For a reversible process, which is an ideal, infinitesimally slow process, the total change in entropy of the universe (system + surroundings) is zero (ΔS_universe = 0). The system is always in equilibrium.
- For an irreversible process, which includes all real-world, spontaneous processes, the total change in entropy of the universe is always positive (ΔS_universe > 0). This entropy increase is what drives the process in a single direction.
This distinction is a fundamental concept tested in MCQs.
7. Why is it impossible to reach absolute zero (0 K), as implied by the Third Law of Thermodynamics?
Reaching absolute zero is impossible due to the principles of both the Second and Third Laws. To cool an object, you need to extract heat from it and transfer that heat to a colder reservoir. According to the Second Law, this process can never be 100% efficient. As a system's temperature approaches absolute zero, the amount of work required to extract a small amount of heat becomes infinitely large. You would need a reservoir colder than absolute zero to dump the final bit of heat into, which is a contradiction. Therefore, while we can get incredibly close, reaching the absolute zero point is a theoretical and practical impossibility.
8. How does mastering the concepts of the 2nd and 3rd laws of thermodynamics help in solving physics problems?
A strong conceptual understanding is crucial for problem-solving. These laws allow you to:
- Predict Feasibility: Determine if a process or reaction can occur spontaneously under given conditions by analysing the change in entropy.
- Calculate Efficiency: For problems involving heat engines and refrigerators, the Second Law sets the theoretical upper limit on their performance (Carnot efficiency).
- Determine Entropy Changes: Use the principles to calculate the change in entropy for various thermodynamic processes like isothermal or adiabatic expansions, which is a common type of numerical problem.
- Understand Equilibrium: Recognise that maximum entropy in an isolated system corresponds to a state of thermal equilibrium.
