Second Law of Thermodynamics MCQ – Objective Questions with Answers
Mechanical Engineering › Thermodynamics | Free practice MCQs with detailed explanations
Last Updated: June 2026
📌 About this MCQ Set
The Second Law of Thermodynamics introduces the direction of natural processes and the concept of entropy. It limits the efficiency of heat engines and explains why heat flows spontaneously from hot to cold.
These MCQs cover the Kelvin-Planck and Clausius statements, entropy, reversibility, and Carnot’s theorem.
8 questions • every answer comes with a worked explanation. Click Show Answer to check yourself.
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Second Law of Thermodynamics MCQs
Q1. The Second Law of Thermodynamics deals primarily with the ____ of a process.
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Answer: B. Direction and feasibility
While the First Law gives energy balance, the Second Law tells which direction a process can naturally proceed and whether it is feasible.
Q2. The Kelvin-Planck statement concerns the impossibility of:
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Answer: B. Converting all heat from a single reservoir into work
No device can operate in a cycle taking heat from a single reservoir and producing an equal amount of work; some heat must be rejected.
Q3. The Clausius statement says heat cannot flow from a cold to a hot body:
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Answer: B. Without external work input
Heat will not flow from low to high temperature spontaneously; it requires external work (as in a refrigerator).
Q4. A perpetual motion machine of the second kind (PMM-2) violates the:
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Answer: B. Second Law
A PMM-2 would convert all heat from one reservoir into work with 100% efficiency, violating the Second Law.
Q5. For any reversible process, the change in entropy of the universe is:
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Answer: C. Zero
Reversible processes are ideal; total entropy of the universe stays constant (ΔS_universe = 0).
Q6. For a real (irreversible) process, the entropy of the universe always:
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Answer: C. Increases
The principle of increase of entropy: all real processes increase the total entropy of the universe.
Q7. Carnot’s theorem states that no engine between two reservoirs can be more efficient than:
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Answer: B. A reversible (Carnot) engine
No engine operating between two given temperatures can exceed the efficiency of a reversible engine working between the same temperatures.
Q8. The efficiency of a Carnot engine depends only on:
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Answer: B. The two reservoir temperatures
η = 1 − T_L/T_H; it depends only on the absolute temperatures of the source and sink, not the working substance.
Frequently Asked Questions
What does the Second Law of Thermodynamics state?
It states that natural processes have a preferred direction, that entropy of an isolated system never decreases, and that no heat engine can be 100% efficient.
What is the difference between the First and Second Laws?
The First Law is about energy conservation (quantity); the Second Law is about the direction and quality of energy and the role of entropy.
What is a PMM-2?
A perpetual motion machine of the second kind — a hypothetical engine that converts all heat from a single reservoir into work. It is impossible by the Second Law.