50 Higher-Order Thinking Skills (HOTS) MCQs (Level 3) on Heat and Thermodynamics, Physics (Unit-Wise MCQs Practice):

👉Physics MCQs Hub – Unit wise Physics MCQs Practice (Main-Page)👈

Whether you are preparing for board examinations, chapter tests, college assessments, or competitive entrance exams (MDCAT, ECAT, NUST, PIEAS, GIKI, UET, FAST, and other engineering or medical admission tests), this comprehensive Heat and Thermodynamics MCQ Collection is designed to help you master one of the most important and widely tested chapters in physics. The questions are arranged progressively—from fundamental concepts to advanced numerical problems and higher-order thinking—ensuring complete and systematic preparation for every type of examination.

This chapter-wise MCQ collection includes:

100 Basic MCQs Level-1 (1–100) – Covering the fundamental concepts of heat and temperature, thermal equilibrium, thermometers, thermal expansion, specific heat capacity, latent heat, calorimetry, heat transfer, kinetic theory of gases, gas laws, and introductory thermodynamics.

100 Advanced & Numerical MCQs (101–200) – Focusing on heat calculations, calorimetry, thermal expansion, gas equations, the First and Second Laws of Thermodynamics, work done by gases, internal energy, Carnot engine, refrigerator, coefficient of performance (COP), entropy, and exam-oriented numerical problems.

50 Higher-Order Thinking Skills (HOTS) MCQs (201–250) – Designed to strengthen analytical reasoning, conceptual understanding, case-based learning, assertion–reason questions, thermodynamic cycle analysis, Carnot engine applications, entropy concepts, and advanced problem-solving skills.

50 Challenging MCQs Quiz with Answers – A carefully selected collection of the most important conceptual, numerical, and HOTS questions with detailed explanations for quick revision, self-assessment, and complete exam preparation.

This MCQ collection covers:

Heat, temperature, and thermal equilibrium
Thermometers and temperature scales (Celsius, Fahrenheit, Kelvin)
Thermal expansion of solids, liquids, and gases
Specific heat capacity and calorimetry
Change of state and latent heat
Heat transfer by conduction, convection, and radiation
Kinetic theory of gases and molecular interpretation of heat
Boyle's Law, Charles's Law, Gay-Lussac's Law, and the Ideal Gas Equation
Internal energy and mechanical equivalent of heat
First Law of Thermodynamics and conservation of energy
Isothermal, adiabatic, isobaric, and isochoric processes
Heat engines, Carnot engine, and Carnot cycle
Refrigerators, heat pumps, and coefficient of performance (COP)
Second and Third Laws of Thermodynamics
Entropy, reversible and irreversible processes
Pressure-volume (P–V) diagrams and thermodynamic cycles
Practical engineering applications of thermodynamics
Real-life applications of heat transfer and thermal systems

Every MCQ includes the correct answer along with a clear, concept-based explanation to strengthen understanding, improve problem-solving skills, and reinforce key physics concepts.

This question bank helps students to:

Build a strong conceptual foundation in heat and thermodynamics
Master thermal expansion, calorimetry, and gas laws
Develop confidence in solving numerical and analytical problems
Understand thermodynamic processes and energy transformations
Learn the working principles of heat engines, refrigerators, and heat pumps
Strengthen concepts of entropy, Carnot cycle, and thermodynamic laws
Avoid common examination mistakes through concept-based practice
Increase speed, accuracy, and confidence in objective-type questions
Prepare effectively for both board examinations and competitive entrance tests

With 250 carefully selected MCQs arranged into 100 Basic, 100 Advanced & Numerical, and 50 Higher-Order Thinking Skills (HOTS) questions, along with a Top 50 Challenging MCQs Quiz, this all-in-one MCQ bank provides complete preparation for Heat and Thermodynamics. It is an excellent study resource for strengthening conceptual understanding, improving numerical problem-solving skills, enhancing exam performance, and achieving success in both school examinations and highly competitive engineering and medical entrance tests.

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Heat and Thermodynamics MCQs (Level 3)– 50 Higher-Order Thinking Skills (HOTS) MCQs with Answers (MCQs 101–200)

MCQ No. 201

Two identical ideal heat engines operate between the same hot and cold reservoirs. Engine A is reversible, while Engine B is irreversible. Which statement is correct?

a) Engine B has higher efficiency than Engine A.

b) Both engines have equal efficiency.

c) Engine A has higher efficiency than Engine B.

d) Efficiency depends only on the working substance.

Correct Answer: c) Engine A has higher efficiency than Engine B.

Explanation:
According to Carnot's theorem, no irreversible engine operating between the same two temperatures can be more efficient than a reversible engine. The reversible (Carnot) engine always has the maximum possible efficiency.

MCQ No. 202

Assertion (A): During an isothermal expansion of an ideal gas, heat supplied to the gas is equal to the work done by the gas.

Reason (R): The internal energy of an ideal gas depends only on its temperature.

a) Both A and R are true, and R is the correct explanation of A.

b) Both A and R are true, but R is not the correct explanation of A.

c) A is true, but R is false.

d) A is false, but R is true.

Correct Answer: a) Both A and R are true, and R is the correct explanation of A.

Explanation:
For an ideal gas, internal energy depends only on temperature. During an isothermal process, the temperature remains constant, so ΔU = 0. From the First Law,

Q=W

Thus, the reason correctly explains the assertion.

MCQ No. 203

A gas undergoes the cyclic process shown on a P–V diagram. The enclosed area increases while the temperatures of the reservoirs remain unchanged. This indicates that the engine:

a) Produces less work per cycle.

b) Produces more work per cycle.

c) Has zero efficiency.

d) Violates the Second Law of Thermodynamics.

Correct Answer: b) Produces more work per cycle.

Explanation:
The area enclosed by a P–V cycle represents the net work done. A larger enclosed area means greater work output for each cycle.

MCQ No. 204

A student claims that an engine operating between 600 K and 300 K has an efficiency of 70%. What is the correct conclusion?

a) The claim is correct.

b) The engine is a Carnot engine.

c) The claim violates the Second Law of Thermodynamics.

d) The engine must be reversible.

Correct Answer: c) The claim violates the Second Law of Thermodynamics.

Explanation:
The maximum possible efficiency is

\eta=1-\frac{300}{600}=50\%

Any efficiency greater than 50% is impossible for these temperatures.

MCQ No. 205

A gas expands adiabatically. Which combination of changes is correct?

Quantity	Change
Heat	?
Temperature	?

a) Heat increases; Temperature increases

b) Heat = 0; Temperature decreases

c) Heat = 0; Temperature remains constant

d) Heat increases; Temperature decreases

Correct Answer: b) Heat = 0; Temperature decreases

Explanation:
During adiabatic expansion, no heat is exchanged. The gas performs work using its internal energy, causing its temperature to decrease.

MCQ No. 206

Which of the following modifications will always increase the efficiency of a Carnot engine?

a) Increasing both reservoir temperatures equally.

b) Increasing only the hot reservoir temperature.

c) Increasing only the cold reservoir temperature.

d) Decreasing both temperatures equally.

Correct Answer: b) Increasing only the hot reservoir temperature.

Explanation:
Since

\eta=1-\frac{T_C}{T_H},

raising only $T_H$ increases the efficiency.

MCQ No. 207

A refrigerator and a heat engine operate between the same two temperature reservoirs. Which statement is correct?

a) Both have the same efficiency.

b) The refrigerator transfers heat from hot to cold naturally.

c) The refrigerator requires external work, whereas the heat engine produces work.

d) Both violate the Second Law.

Correct Answer: c) The refrigerator requires external work, whereas the heat engine produces work.

Explanation:
A refrigerator uses external work to transfer heat from a colder body to a hotter body, while a heat engine converts part of the absorbed heat into mechanical work.

MCQ No. 208

An inventor claims to have designed a heat engine that converts all absorbed heat into useful work without rejecting heat to the surroundings. According to thermodynamics, the invention:

a) Is theoretically possible.

b) Obeys the First Law only.

c) Violates the Second Law of Thermodynamics.

d) Can operate only in space.

Correct Answer: c) Violates the Second Law of Thermodynamics.

Explanation:
No heat engine can have 100% efficiency because some heat must always be rejected to a colder reservoir.

MCQ No. 209

Assertion (A): Entropy of an isolated system never decreases during a spontaneous process.

Reason (R): Entropy is a measure of disorder.

a) Both A and R are true, and R correctly explains A.

b) Both A and R are true, but R does not completely explain A.

c) A is true, but R is false.

d) A is false, but R is true.

Correct Answer: b) Both A and R are true, but R does not completely explain A.

Explanation:
Entropy is indeed a measure of disorder, but the statement that entropy never decreases in an isolated spontaneous process follows specifically from the Second Law of Thermodynamics, not merely from the definition of entropy.

MCQ No. 210

A gas completes one thermodynamic cycle. Which quantity must return to its original value?

a) Heat transferred

b) Work done

c) Internal energy

d) Entropy of the surroundings

Correct Answer: c) Internal energy

Explanation:
Internal energy is a state function and depends only on the state of the system. After one complete cycle, the system returns to its initial state, so the change in internal energy is zero.

MCQ No. 211

A Carnot engine operates between two reservoirs at temperatures $T_H$ and $T_C$ . If both temperatures are doubled while maintaining the same ratio $\frac{T_C}{T_H}$ , the efficiency of the engine will:

a) Double

b) Become half

c) Remain unchanged

d) Become zero

Correct Answer: c) Remain unchanged

Explanation:

The efficiency of a Carnot engine is

\eta = 1 - \frac{T_C}{T_H}

Since doubling both temperatures does not change the ratio $T_C/T_H$ , the efficiency remains unchanged.

MCQ No. 212

Assertion (A): The First Law of Thermodynamics alone cannot determine whether a process is spontaneous.

Reason (R): The First Law deals only with conservation of energy and not with the direction of energy transfer.

a) Both A and R are true, and R is the correct explanation of A.

b) Both A and R are true, but R is not the correct explanation of A.

c) A is true, but R is false.

d) A is false, but R is true.

Correct Answer: a) Both A and R are true, and R is the correct explanation of A.

Explanation:

The First Law ensures conservation of energy but gives no information about whether a process can occur naturally. The direction of spontaneous processes is determined by the Second Law.

MCQ No. 213

A perfectly insulated cylinder contains an ideal gas. The piston is suddenly pulled outward. Compared with a slow reversible expansion, the sudden expansion will:

a) Produce more work.

b) Produce less work.

c) Produce the same work.

d) Produce no temperature change.

Correct Answer: b) Produce less work.

Explanation:

During a sudden (irreversible) expansion, the gas remains far from equilibrium and performs less useful work than during a slow reversible expansion.

MCQ No. 214

A student says, "Since energy is conserved, every heat engine should eventually reach 100% efficiency." Which statement best evaluates this claim?

a) Correct because energy cannot be destroyed.

b) Incorrect because the Second Law limits efficiency.

c) Correct only for reversible engines.

d) Correct only for ideal gases.

Correct Answer: b) Incorrect because the Second Law limits efficiency.

Explanation:

The First Law ensures conservation of energy, but the Second Law states that some absorbed heat must always be rejected to a colder reservoir.

MCQ No. 215

Which process produces the greatest amount of useful work between the same initial and final equilibrium states?

a) Free expansion

b) Sudden expansion

c) Reversible expansion

d) Rapid compression

Correct Answer: c) Reversible expansion

Explanation:

A reversible process produces the maximum possible work because the system remains in thermodynamic equilibrium throughout the process.

MCQ No. 216

An engine absorbs 3000 J of heat and rejects 1800 J. Another engine absorbs 6000 J and rejects 3600 J. Which statement is correct?

a) The first engine is more efficient.

b) The second engine is more efficient.

c) Both have the same efficiency.

d) Efficiency cannot be determined.

Correct Answer: c) Both have the same efficiency.

Explanation:

For Engine 1:

\eta=\frac{3000-1800}{3000}=0.40

For Engine 2:

\eta=\frac{6000-3600}{6000}=0.40

Both engines have an efficiency of 40%.

MCQ No. 217

A refrigerator has a very high coefficient of performance (COP). This means that it:

a) Produces more work than it consumes.

b) Removes a large amount of heat for each unit of work supplied.

c) Violates the Second Law.

d) Converts all electrical energy into cooling.

Correct Answer: b) Removes a large amount of heat for each unit of work supplied.

Explanation:

A high COP indicates an efficient refrigerator that transfers a large amount of heat from the cold reservoir for a relatively small amount of work.

MCQ No. 218

Which statement best explains why entropy increases during the spontaneous mixing of two gases?

a) Total energy increases.

b) Molecular disorder increases.

c) Pressure becomes zero.

d) Internal energy always increases.

Correct Answer: b) Molecular disorder increases.

Explanation:

When two gases mix spontaneously, the number of possible molecular arrangements increases, leading to greater disorder and higher entropy.

MCQ No. 219

An ideal gas undergoes the following sequence:

Isothermal expansion
Adiabatic expansion
Isothermal compression
Adiabatic compression

This sequence represents:

a) Otto cycle

b) Diesel cycle

c) Carnot cycle

d) Stirling cycle

Correct Answer: c) Carnot cycle

Explanation:

The Carnot cycle consists of two reversible isothermal processes and two reversible adiabatic processes.

MCQ No. 220

A scientist proposes a machine that continuously transfers heat from a colder body to a hotter body without consuming any external work. This machine would:

a) Obey both the First and Second Laws.

b) Obey only the First Law.

c) Violate the Second Law of Thermodynamics.

d) Work only at absolute zero.

Correct Answer: c) Violate the Second Law of Thermodynamics.

Explanation:

According to the Clausius statement of the Second Law, heat cannot flow spontaneously from a colder body to a hotter body without external work. Such a machine is impossible.

MCQ No. 221

A Carnot engine operates between 900 K and 300 K. Another Carnot engine operates between 600 K and 200 K. Which statement is correct?

a) The first engine is more efficient.

b) The second engine is more efficient.

c) Both engines have the same efficiency.

d) Efficiency cannot be compared.

Correct Answer: c) Both engines have the same efficiency.

Explanation:

For Engine 1,

\eta=1-\frac{300}{900}=\frac{2}{3}

For Engine 2,

\eta=1-\frac{200}{600}=\frac{2}{3}

Since the temperature ratio is the same, both engines have the same efficiency.

MCQ No. 222

Assertion (A): Entropy remains constant during a reversible adiabatic process.

Reason (R): No heat is exchanged between the system and the surroundings during a reversible adiabatic process.

a) Both A and R are true, and R correctly explains A.

b) Both A and R are true, but R does not explain A.

c) A is true, but R is false.

d) A is false, but R is true.

Correct Answer: a) Both A and R are true, and R correctly explains A.

Explanation:

For a reversible process,

dS=\frac{\delta Q_{rev}}{T}

Since

\delta Q_{rev}=0,

the entropy change is zero.

MCQ No. 223

A gas expands freely into a vacuum inside a perfectly insulated container. Which statement is correct?

a) Work is done by the gas.

b) Heat enters the gas.

c) Both heat transfer and work are zero.

d) Temperature must increase.

Correct Answer: c) Both heat transfer and work are zero.

Explanation:

In free expansion,

No external pressure exists, so no work is done.
The container is insulated, so no heat is transferred.

Thus,

Q=0,\qquad W=0

MCQ No. 224

A student states that increasing the pressure of the hot reservoir will always increase the efficiency of a Carnot engine. Which evaluation is correct?

a) Correct

b) Incorrect, because efficiency depends only on reservoir temperatures.

c) Correct for ideal gases only.

d) Correct if volume remains constant.

Correct Answer: b) Incorrect, because efficiency depends only on reservoir temperatures.

Explanation:

Carnot efficiency depends only on absolute temperatures:

\eta=1-\frac{T_C}{T_H}

Pressure has no direct effect.

MCQ No. 225

Which thermodynamic process is represented by the steepest curve on a P–V diagram for an ideal gas?

a) Isothermal process

b) Isobaric process

c) Adiabatic process

d) Isochoric process

Correct Answer: c) Adiabatic process

Explanation:

An adiabatic curve is steeper than an isothermal curve because pressure decreases more rapidly during adiabatic expansion.

MCQ No. 226

A heat engine receives 10,000 J of heat from a source and converts 4,000 J into useful work. Another engine receives 5,000 J and converts 2,000 J into work. Which engine is more efficient?

a) First engine

b) Second engine

c) Both have equal efficiency

d) Cannot be determined

Correct Answer: c) Both have equal efficiency

Explanation:

Engine 1:

\eta=\frac{4000}{10000}=40\%

Engine 2:

\eta=\frac{2000}{5000}=40\%

MCQ No. 227

Which modification would decrease the efficiency of a heat engine?

a) Increasing the hot reservoir temperature

b) Decreasing the cold reservoir temperature

c) Increasing the cold reservoir temperature

d) Improving insulation

Correct Answer: c) Increasing the cold reservoir temperature

Explanation:

Since

\eta=1-\frac{T_C}{T_H},

increasing $T_C$ decreases efficiency.

MCQ No. 228

During the compression stroke of a diesel engine, the air temperature rises mainly because:

a) Heat enters from the surroundings.

b) Work is done on the gas.

c) Entropy decreases.

d) Pressure remains constant.

Correct Answer: b) Work is done on the gas.

Explanation:

Compression is nearly adiabatic. Work done on the gas increases its internal energy and temperature.

MCQ No. 229

A reversible engine and an irreversible engine absorb the same amount of heat from identical reservoirs. Which statement is always true?

a) Both produce equal work.

b) The irreversible engine produces more work.

c) The reversible engine produces more work.

d) Both reject the same amount of heat.

Correct Answer: c) The reversible engine produces more work.

Explanation:

Since the reversible engine has the maximum efficiency, it converts more of the absorbed heat into useful work.

MCQ No. 230

A thermodynamic cycle encloses zero area on a P–V diagram. This implies that:

a) No heat is supplied.

b) Net work done during the cycle is zero.

c) Internal energy continuously increases.

d) The process violates the First Law.

Correct Answer: b) Net work done during the cycle is zero.

Explanation:

The area enclosed by a cyclic P–V diagram equals the net work done. If the enclosed area is zero, the net work is zero.

MCQ No. 231

A heat engine operates between 800 K and 400 K. An engineer lowers the cold reservoir temperature to 300 K while keeping the hot reservoir temperature constant. What will happen to the engine?

a) Efficiency decreases.

b) Efficiency remains unchanged.

c) Efficiency increases.

d) The engine stops working.

Correct Answer: c) Efficiency increases.

Explanation:

The efficiency of a Carnot engine is

\eta = 1-\frac{T_C}{T_H}

Initially,

\eta=1-\frac{400}{800}=50\%

After lowering the cold reservoir,

\eta=1-\frac{300}{800}=62.5\%

Hence, the efficiency increases.

MCQ No. 232

Assertion (A): No cyclic heat engine can convert all the heat absorbed into work.

Reason (R): Every heat engine must reject some heat to a colder reservoir.

a) Both A and R are true, and R correctly explains A.

b) Both A and R are true, but R does not explain A.

c) A is true, but R is false.

d) A is false, but R is true.

Correct Answer: a) Both A and R are true, and R correctly explains A.

Explanation:

This is the Kelvin–Planck statement of the Second Law of Thermodynamics.

MCQ No. 233

A scientist develops an engine with an efficiency equal to that of a Carnot engine operating between the same reservoirs. Which statement must be true?

a) The engine is irreversible.

b) The engine is reversible.

c) The engine has zero work output.

d) The engine violates the First Law.

Correct Answer: b) The engine is reversible.

Explanation:

Only a reversible engine can achieve Carnot efficiency.

MCQ No. 234

A gas undergoes a complete thermodynamic cycle. During the cycle, 1200 J of heat is supplied and 900 J is rejected. The net work done by the gas is:

a) 1200 J

b) 900 J

c) 300 J

d) Zero

Correct Answer: c) 300 J

Explanation:

For a complete cycle,

\Delta U=0

Therefore,

W=Q_H-Q_C

=1200-900 =300\text{ J}

MCQ No. 235

Which statement correctly compares entropy in reversible and irreversible processes?

a) Entropy increases equally in both.

b) Entropy change is always zero.

c) Entropy generation is greater in irreversible processes.

d) Entropy decreases during irreversible processes.

Correct Answer: c) Entropy generation is greater in irreversible processes.

Explanation:

Irreversible processes generate entropy, whereas reversible processes occur with no entropy generation.

MCQ No. 236

During rapid compression of an ideal gas in a perfectly insulated cylinder, the gas temperature increases because:

a) Heat enters the cylinder.

b) Molecules gain internal energy through external work.

c) Pressure becomes constant.

d) Entropy decreases to zero.

Correct Answer: b) Molecules gain internal energy through external work.

Explanation:

In adiabatic compression,

Q=0

The increase in temperature results from work done on the gas.

MCQ No. 237

Which one of the following situations best represents an approximately reversible process?

a) Explosion inside an engine cylinder

b) Sudden expansion into a vacuum

c) Very slow compression without friction

d) Rapid cooling in ice water

Correct Answer: c) Very slow compression without friction

Explanation:

A reversible process proceeds infinitely slowly and without dissipative effects such as friction.

MCQ No. 238

An ideal refrigerator and an ideal heat pump operate between the same two temperature reservoirs. Compared with the refrigerator, the heat pump has:

a) A smaller coefficient of performance.

b) The same coefficient of performance.

c) A larger coefficient of performance.

d) Zero coefficient of performance.

Correct Answer: c) A larger coefficient of performance.

Explanation:

For ideal devices,

COP_{\text{Heat Pump}}=COP_{\text{Refrigerator}}+1

Hence, the heat pump always has the larger COP.

MCQ No. 239

A Carnot engine and a real engine operate between the same temperatures and absorb equal amounts of heat. Which quantity will definitely be greater for the Carnot engine?

a) Heat rejected

b) Useful work produced

c) Entropy generated

d) Fuel consumed

Correct Answer: b) Useful work produced

Explanation:

Since the Carnot engine has the maximum possible efficiency, it converts a larger fraction of absorbed heat into useful work.

MCQ No. 240

A student says that entropy is simply another name for energy. This statement is:

a) Correct

b) Incorrect because entropy measures energy conservation.

c) Incorrect because entropy measures the degree of energy dispersal or disorder, not energy itself.

d) Correct only for ideal gases.

Correct Answer: c) Incorrect because entropy measures the degree of energy dispersal or disorder, not energy itself.

Explanation:

Entropy is a thermodynamic property that describes how energy is distributed or dispersed within a system. It is not a form of energy.

MCQ No. 241

Two identical metal blocks at different temperatures are placed in thermal contact inside an insulated container. After sufficient time, the total entropy of the system will:

a) Decrease

b) Remain constant

c) Increase

d) Become zero

Correct Answer: c) Increase

Explanation:

Heat flows spontaneously from the hotter block to the colder block until thermal equilibrium is established, increasing the total entropy of the isolated system.

MCQ No. 242

Which statement best explains why perpetual motion machines of the second kind are impossible?

a) They violate Newton's laws.

b) They violate the First Law of Thermodynamics.

c) They violate the Second Law of Thermodynamics.

d) They violate Boyle's law.

Correct Answer: c) They violate the Second Law of Thermodynamics.

Explanation:

A perpetual motion machine of the second kind would convert all absorbed heat into work without rejecting heat, contradicting the Second Law.

MCQ No. 243

An engine operating between fixed temperature limits becomes less efficient after prolonged use. The most likely reason is:

a) The Second Law has changed.

b) Increased friction and irreversible losses.

c) The gas constant has decreased.

d) The First Law no longer applies.

Correct Answer: b) Increased friction and irreversible losses.

Explanation:

Mechanical friction, heat losses, and other irreversible effects reduce the efficiency of practical engines over time.

MCQ No. 244

Which thermodynamic law introduces the concept of entropy?

a) Zeroth Law

b) First Law

c) Second Law

d) Third Law

Correct Answer: c) Second Law

Explanation:

The Second Law of Thermodynamics introduces entropy and explains the direction of spontaneous processes.

MCQ No. 245

If all irreversible processes could somehow be made reversible, the efficiency of practical heat engines would:

a) Decrease

b) Remain unchanged

c) Increase toward the Carnot limit

d) Become infinite

Correct Answer: c) Increase toward the Carnot limit

Explanation:

Removing irreversibilities such as friction and finite temperature differences increases efficiency, but it can never exceed the Carnot limit.

MCQ No. 246

A reversible engine and a real engine absorb the same amount of heat from identical reservoirs. The real engine rejects more heat because it:

a) Has greater entropy generation.

b) Has lower pressure.

c) Uses less fuel.

d) Operates at constant volume.

Correct Answer: a) Has greater entropy generation.

Explanation:

Irreversibilities generate entropy, forcing the real engine to reject more heat and produce less useful work.

MCQ No. 247

According to the Third Law of Thermodynamics, as the temperature of a perfect crystal approaches absolute zero, its entropy approaches:

a) Infinity

b) One

c) Zero

d) The gas constant

Correct Answer: c) Zero

Explanation:

The Third Law states that the entropy of a perfect crystal approaches zero as its temperature approaches absolute zero (0 K).

MCQ No. 248

Which of the following engineering devices operates primarily on the principles of adiabatic compression and expansion?

a) Electric heater

b) Petrol or diesel internal combustion engine

c) Electric fan

d) Water purifier

Correct Answer: b) Petrol or diesel internal combustion engine

Explanation:

Internal combustion engines rely heavily on rapid adiabatic compression and expansion during their operating cycles.

MCQ No. 249

A thermodynamic process is represented by a closed loop on a P–V diagram traversed in the clockwise direction. This indicates that:

a) Net work is done on the system.

b) Net work is done by the system.

c) No work is exchanged.

d) Internal energy continuously increases.

Correct Answer: b) Net work is done by the system.

Explanation:

A clockwise cycle on a P–V diagram represents a heat engine producing positive net work equal to the enclosed area.

MCQ No. 250

Which statement best summarizes the significance of the laws of thermodynamics in engineering and science?

a) They apply only to ideal gases.

b) They explain only heat transfer.

c) They establish the principles governing energy conservation, heat transfer, work, entropy, and the performance limits of all thermal systems.

d) They describe only reversible processes.

Correct Answer: c) They establish the principles governing energy conservation, heat transfer, work, entropy, and the performance limits of all thermal systems.

Explanation:

The laws of thermodynamics form the foundation of thermal science and engineering. They explain how energy is conserved, how heat and work are related, the direction of natural processes through entropy, and the maximum possible efficiency of engines, refrigerators, power plants, and countless other thermal systems.