100 Important Basic MCQs (Level -1) 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 1) – 100 Basic MCQs with Answers (MCQs 1–100)

MCQ No. 1

Heat is defined as the energy transferred from one body to another due to a difference in:

a) Pressure

b) Volume

c) Temperature

d) Density

Correct Answer: c) Temperature

Explanation:
Heat is a form of energy that flows naturally from a body at higher temperature to a body at lower temperature because of the temperature difference between them.

MCQ No. 2

The SI unit of heat is:

a) Calorie

b) Joule

c) Watt

d) Kelvin

Correct Answer: b) Joule

Explanation:
Although heat was traditionally measured in calories, the SI unit of heat and all forms of energy is the joule (J).

MCQ No. 3

Heat always flows naturally from:

a) A colder body to a hotter body

b) A body with lower pressure to a body with higher pressure

c) A hotter body to a colder body

d) A body with greater mass to a body with smaller mass

Correct Answer: c) A hotter body to a colder body

Explanation:
According to the Second Law of Thermodynamics, heat flows spontaneously from a region of higher temperature to a region of lower temperature until thermal equilibrium is established.

MCQ No. 4

Two bodies are said to be in thermal equilibrium when they have:

a) Equal masses

b) Equal volumes

c) Equal temperatures

d) Equal densities

Correct Answer: c) Equal temperatures

Explanation:
When two bodies have the same temperature, there is no net transfer of heat between them. This condition is called thermal equilibrium.

MCQ No. 5

Which law of thermodynamics explains the concept of thermal equilibrium?

a) First Law

b) Second Law

c) Third Law

d) Zeroth Law

Correct Answer: d) Zeroth Law

Explanation:
The Zeroth Law of Thermodynamics states that if two bodies are separately in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.

MCQ No. 6

Thermodynamics is the branch of physics that deals with:

a) Motion of planets

b) Electricity and magnetism

c) Heat, work, and energy transformations

d) Sound waves only

Correct Answer: c) Heat, work, and energy transformations

Explanation:
Thermodynamics studies the relationships between heat, work, temperature, and different forms of energy.

MCQ No. 7

Internal energy of a substance is mainly due to the:

a) Gravitational potential energy of the body

b) Translational motion of the entire body

c) Random kinetic and potential energies of its molecules

d) Rotational motion of the Earth

Correct Answer: c) Random kinetic and potential energies of its molecules

Explanation:
Internal energy is the sum of the microscopic kinetic and potential energies of all the molecules in a substance.

MCQ No. 8

The mechanical equivalent of heat relates:

a) Force and acceleration

b) Pressure and volume

c) Heat and mechanical work

d) Mass and weight

Correct Answer: c) Heat and mechanical work

Explanation:
The mechanical equivalent of heat expresses the relationship between mechanical work and heat energy, showing that both are equivalent forms of energy.

MCQ No. 9

Who experimentally established the mechanical equivalent of heat?

a) Isaac Newton

b) James Prescott Joule

c) Albert Einstein

d) Lord Kelvin

Correct Answer: b) James Prescott Joule

Explanation:
James Prescott Joule demonstrated experimentally that mechanical work can be completely converted into heat, establishing the principle of energy conservation.

MCQ No. 10

The SI unit of temperature is:

a) Celsius

b) Fahrenheit

c) Kelvin

d) Joule

Correct Answer: c) Kelvin

Explanation:
The kelvin (K) is the SI base unit of thermodynamic temperature and is widely used in scientific calculations.

MCQ No. 11

The First Law of Thermodynamics is based on the law of conservation of:

a) Mass

b) Momentum

c) Energy

d) Charge

Correct Answer: c) Energy

Explanation:
The First Law of Thermodynamics states that energy can neither be created nor destroyed. It can only be transformed from one form into another.

MCQ No. 12

The mathematical expression of the First Law of Thermodynamics is:

a) ΔU = W − Q

b) ΔU = Q − W

c) Q = W

d) W = ΔU

Correct Answer: b) ΔU = Q − W

Explanation:
The change in internal energy of a system equals the heat supplied to the system minus the work done by the system.

MCQ No. 13

When heat is supplied to a gas at constant volume, the work done by the gas is:

a) Maximum

b) Equal to the heat supplied

c) Zero

d) Negative

Correct Answer: c) Zero

Explanation:
Since the volume remains constant (ΔV = 0), no expansion occurs and therefore no work is done by the gas.

MCQ No. 14

A thermodynamic system is best defined as:

a) The entire universe

b) The surroundings only

c) The part of the universe selected for study

d) A vacuum chamber only

Correct Answer: c) The part of the universe selected for study

Explanation:
A thermodynamic system is the specific portion of the universe under consideration, separated from its surroundings by a real or imaginary boundary.

MCQ No. 15

Everything outside a thermodynamic system is called the:

a) Environment

b) Reservoir

c) Surroundings

d) Medium

Correct Answer: c) Surroundings

Explanation:
The surroundings include everything external to the system that can exchange energy or matter with it.

MCQ No. 16

Which of the following is a state function?

a) Heat

b) Work

c) Temperature

d) Friction

Correct Answer: c) Temperature

Explanation:
Temperature depends only on the current state of the system, whereas heat and work depend on the path followed during a process.

MCQ No. 17

Which of the following is a path function?

a) Internal energy

b) Temperature

c) Pressure

d) Work

Correct Answer: d) Work

Explanation:
Work depends on the path taken during a thermodynamic process and is therefore called a path function.

MCQ No. 18

For an ideal gas, internal energy depends only on:

a) Pressure

b) Volume

c) Temperature

d) Density

Correct Answer: c) Temperature

Explanation:
The internal energy of an ideal gas is determined solely by its absolute temperature.

MCQ No. 19

During an expansion process, a gas does work on its surroundings because its:

a) Pressure becomes zero

b) Volume increases

c) Temperature becomes constant

d) Mass decreases

Correct Answer: b) Volume increases

Explanation:
When a gas expands against an external pressure, it performs work by increasing its volume.

MCQ No. 20

The work done by a gas at constant pressure is given by:

a) W = PΔV

b) W = VΔP

c) W = PV

d) W = QΔT

Correct Answer: a) W = PΔV

Explanation:
For an isobaric (constant-pressure) process, the work done is equal to the pressure multiplied by the change in volume.

MCQ No. 21

In an isothermal process, the temperature of an ideal gas remains:

a) Increasing

b) Decreasing

c) Constant

d) Zero

Correct Answer: c) Constant

Explanation:
An isothermal process occurs at constant temperature. For an ideal gas, the internal energy remains unchanged during this process.

MCQ No. 22

Which thermodynamic process takes place at constant volume?

a) Isothermal

b) Isobaric

c) Isochoric

d) Adiabatic

Correct Answer: c) Isochoric

Explanation:
An isochoric process occurs without any change in volume, so no mechanical work is performed.

MCQ No. 23

Which thermodynamic process takes place at constant pressure?

a) Isothermal

b) Isobaric

c) Isochoric

d) Adiabatic

Correct Answer: b) Isobaric

Explanation:
An isobaric process is one in which the pressure remains constant while the volume and temperature may change.

MCQ No. 24

In an adiabatic process, the heat exchanged between the system and its surroundings is:

a) Maximum

b) Equal to work done

c) Zero

d) Infinite

Correct Answer: c) Zero

Explanation:
In an adiabatic process, no heat enters or leaves the system (Q = 0). Any change in internal energy is due only to work done.

MCQ No. 25

Which thermodynamic process involves no heat transfer between the system and its surroundings?

a) Isothermal

b) Isochoric

c) Isobaric

d) Adiabatic

Correct Answer: d) Adiabatic

Explanation:
An adiabatic process is characterized by zero heat exchange. The temperature of the system changes because of work done during expansion or compression.

MCQ No. 26

Specific heat capacity is defined as the amount of heat required to raise the temperature of:

a) 1 gram of a substance by 1°C

b) 1 kilogram of a substance by 1 K

c) 1 mole of a substance by 1 K

d) Any quantity of a substance by 10°C

Correct Answer: b) 1 kilogram of a substance by 1 K

Explanation:
Specific heat capacity is the heat required to increase the temperature of 1 kg of a substance by 1 K (or 1°C).

MCQ No. 27

The SI unit of specific heat capacity is:

a) J kg⁻¹ K⁻¹

b) J mol⁻¹ K⁻¹

c) Cal g⁻¹ °C⁻¹

d) W m⁻¹ K⁻¹

Correct Answer: a) J kg⁻¹ K⁻¹

Explanation:
The SI unit of specific heat capacity is joule per kilogram per kelvin (J kg⁻¹ K⁻¹).

MCQ No. 28

The heat required to raise the temperature of one mole of a substance by one kelvin is called:

a) Specific heat capacity

b) Latent heat

c) Molar specific heat capacity

d) Heat capacity

Correct Answer: c) Molar specific heat capacity

Explanation:
Molar specific heat capacity is defined for one mole of a substance instead of one kilogram.

MCQ No. 29

The SI unit of molar specific heat capacity is:

a) J kg⁻¹ K⁻¹

b) J mol⁻¹ K⁻¹

c) J K⁻¹

d) Cal mol⁻¹

Correct Answer: b) J mol⁻¹ K⁻¹

Explanation:
Molar specific heat capacity is measured in joules per mole per kelvin.

MCQ No. 30

The specific heat capacity at constant volume is represented by:

a) Cp

b) Cv

c) R

d) γ

Correct Answer: b) Cv

Explanation:
Cv represents the molar specific heat capacity measured while keeping the volume constant.

MCQ No. 31

The specific heat capacity at constant pressure is represented by:

a) Cv

b) Cp

c) R

d) K

Correct Answer: b) Cp

Explanation:
Cp is the molar specific heat capacity measured under constant pressure.

MCQ No. 32

For an ideal gas, which quantity is always greater?

a) Cv

b) Cp

c) Both are equal

d) Both are zero

Correct Answer: b) Cp

Explanation:
At constant pressure, additional heat is required for expansion work, making Cp greater than Cv.

MCQ No. 33

The relation between Cp and Cv for an ideal gas is:

a) Cp + Cv = R

b) Cp − Cv = R

c) Cv − Cp = R

d) Cp × Cv = R

Correct Answer: b) Cp − Cv = R

Explanation:
This important equation is known as Mayer's relation for ideal gases.

MCQ No. 34

The ratio Cp/Cv is called the:

a) Gas constant

b) Mechanical equivalent of heat

c) Adiabatic index

d) Thermal conductivity

Correct Answer: c) Adiabatic index

Explanation:
The ratio γ = Cp/Cv is called the adiabatic index or ratio of specific heats.

MCQ No. 35

The value of the adiabatic index (γ) for a monoatomic ideal gas is approximately:

a) 1.20

b) 1.33

c) 1.40

d) 1.67

Correct Answer: d) 1.67

Explanation:
For monoatomic gases,
γ = 5/3 ≈ 1.67.

MCQ No. 36

Which thermodynamic process is represented by a horizontal line on a P–V diagram?

a) Isochoric

b) Isobaric

c) Isothermal

d) Adiabatic

Correct Answer: b) Isobaric

Explanation:
A horizontal line on a pressure-volume graph indicates constant pressure.

MCQ No. 37

Which thermodynamic process is represented by a vertical line on a P–V diagram?

a) Isothermal

b) Adiabatic

c) Isochoric

d) Isobaric

Correct Answer: c) Isochoric

Explanation:
A vertical line on a P–V diagram represents constant volume.

MCQ No. 38

During an isothermal expansion of an ideal gas, its internal energy:

a) Increases

b) Decreases

c) Remains constant

d) Becomes zero

Correct Answer: c) Remains constant

Explanation:
Since the temperature remains constant, the internal energy of an ideal gas also remains constant.

MCQ No. 39

During an adiabatic expansion of an ideal gas, its temperature:

a) Increases

b) Decreases

c) Remains constant

d) Becomes infinite

Correct Answer: b) Decreases

Explanation:
The gas performs work without receiving heat, so its internal energy and temperature decrease.

MCQ No. 40

During adiabatic compression, the temperature of a gas:

a) Decreases

b) Remains unchanged

c) Increases

d) Becomes zero

Correct Answer: c) Increases

Explanation:
When a gas is compressed adiabatically, work is done on it, increasing its internal energy and temperature.

MCQ No. 41

A heat engine converts:

a) Mechanical work into heat

b) Heat energy into mechanical work

c) Electrical energy into heat

d) Chemical energy into light

Correct Answer: b) Heat energy into mechanical work

Explanation:
A heat engine operates by converting a portion of absorbed heat into useful mechanical work.

MCQ No. 42

Every heat engine operates between:

a) One heat reservoir

b) Two reservoirs at the same temperature

c) Two reservoirs at different temperatures

d) Three heat reservoirs

Correct Answer: c) Two reservoirs at different temperatures

Explanation:
A heat engine absorbs heat from a hot reservoir and rejects some heat to a cold reservoir.

MCQ No. 43

The efficiency of a heat engine is always:

a) Greater than 100%

b) Equal to 100%

c) Less than 100%

d) Equal to zero

Correct Answer: c) Less than 100%

Explanation:
According to the Second Law of Thermodynamics, no heat engine can convert all absorbed heat into useful work.

MCQ No. 44

The most efficient theoretical heat engine is the:

a) Diesel engine

b) Petrol engine

c) Carnot engine

d) Steam engine

Correct Answer: c) Carnot engine

Explanation:
The Carnot engine is an ideal reversible engine with the maximum possible efficiency.

MCQ No. 45

The efficiency of a Carnot engine depends only on:

a) Type of working substance

b) Engine size

c) Temperatures of the heat reservoirs

d) Pressure of the gas

Correct Answer: c) Temperatures of the heat reservoirs

Explanation:
Carnot efficiency depends solely on the temperatures of the hot and cold reservoirs.

MCQ No. 46

A Carnot engine is an example of a:

a) Real irreversible engine

b) Ideal reversible engine

c) Diesel engine

d) Gas turbine

Correct Answer: b) Ideal reversible engine

Explanation:
The Carnot engine is a completely reversible engine that represents the highest possible efficiency.

MCQ No. 47

A refrigerator works by transferring heat from:

a) A hot body to a hotter body

b) A cold body to a hot body

c) A hot body to a cold body naturally

d) The surroundings to itself only

Correct Answer: b) A cold body to a hot body

Explanation:
A refrigerator uses external work to transfer heat from a colder region to a warmer region.

MCQ No. 48

The performance of a refrigerator is measured by its:

a) Efficiency

b) Power

c) Coefficient of Performance (COP)

d) Mechanical advantage

Correct Answer: c) Coefficient of Performance (COP)

Explanation:
Unlike heat engines, refrigerators are rated using the coefficient of performance (COP).

MCQ No. 49

The Second Law of Thermodynamics introduces the concept of:

a) Pressure

b) Volume

c) Entropy

d) Density

Correct Answer: c) Entropy

Explanation:
The Second Law introduces entropy, which measures the degree of disorder or randomness in a system.

MCQ No. 50

Entropy is a measure of the:

a) Pressure of a system

b) Volume of a gas

c) Degree of disorder in a system

d) Density of a substance

Correct Answer: c) Degree of disorder in a system

Explanation:
Entropy measures the randomness or disorder of a system. In natural processes, the entropy of an isolated system tends to increase.

MCQ No. 51

A reversible process is one that:

a) Occurs only at constant temperature

b) Can be reversed without leaving any change in the system and surroundings

c) Occurs only in liquids

d) Always takes place rapidly

Correct Answer: b) Can be reversed without leaving any change in the system and surroundings

Explanation:
A reversible process proceeds infinitely slowly and can be reversed so that both the system and surroundings return to their original states.

MCQ No. 52

An irreversible process is characterized by:

a) No entropy change

b) Complete equilibrium throughout

c) Finite temperature or pressure differences

d) Constant volume only

Correct Answer: c) Finite temperature or pressure differences

Explanation:
Irreversible processes occur with finite differences in pressure or temperature and cannot restore both the system and surroundings to their initial states.

MCQ No. 53

Which of the following is an example of an irreversible process?

a) Ideal Carnot cycle

b) Slow isothermal expansion

c) Free expansion of a gas into a vacuum

d) Infinitesimally slow compression

Correct Answer: c) Free expansion of a gas into a vacuum

Explanation:
Free expansion occurs spontaneously without external control and is an irreversible process.

MCQ No. 54

The Carnot cycle consists entirely of:

a) Irreversible processes

b) Reversible processes

c) Isobaric processes only

d) Isochoric processes only

Correct Answer: b) Reversible processes

Explanation:
The Carnot cycle is composed of two reversible isothermal and two reversible adiabatic processes.

MCQ No. 55

Entropy of an isolated system during a natural process generally:

a) Decreases

b) Remains constant

c) Increases

d) Becomes zero

Correct Answer: c) Increases

Explanation:
According to the Second Law of Thermodynamics, the entropy of an isolated system increases in spontaneous natural processes.

MCQ No. 56

Energy degradation means that energy becomes:

a) Destroyed completely

b) Converted into matter

c) Less available for useful work

d) Equal to mass

Correct Answer: c) Less available for useful work

Explanation:
Although energy is conserved, some of it becomes unavailable for performing useful work due to entropy increase.

MCQ No. 57

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

a) Zeroth Law

b) First Law

c) Second Law

d) Third Law

Correct Answer: c) Second Law

Explanation:
The Second Law of Thermodynamics states that no engine can convert all absorbed heat into useful work.

MCQ No. 58

A process that occurs naturally without external assistance is called a:

a) Cyclic process

b) Spontaneous process

c) Isothermal process

d) Mechanical process

Correct Answer: b) Spontaneous process

Explanation:
Spontaneous processes occur naturally in the direction that increases the entropy of the universe.

MCQ No. 59

Heat transfer between two bodies stops when they reach:

a) Equal pressure

b) Equal volume

c) Thermal equilibrium

d) Equal mass

Correct Answer: c) Thermal equilibrium

Explanation:
Once two bodies reach the same temperature, no net heat flows between them.

MCQ No. 60

Which thermodynamic law introduces the concept of thermal equilibrium?

a) First Law

b) Second Law

c) Third Law

d) Zeroth Law

Correct Answer: d) Zeroth Law

Explanation:
The Zeroth Law provides the basis for defining temperature and thermal equilibrium.

MCQ No. 61

A system that exchanges both matter and energy with its surroundings is called a(n):

a) Isolated system

b) Closed system

c) Open system

d) Adiabatic system

Correct Answer: c) Open system

Explanation:
An open system allows both matter and energy to cross its boundaries.

MCQ No. 62

A closed thermodynamic system can exchange:

a) Matter only

b) Neither matter nor energy

c) Energy but not matter

d) Matter and charge only

Correct Answer: c) Energy but not matter

Explanation:
A closed system permits the transfer of energy while preventing the transfer of matter.

MCQ No. 63

An isolated system exchanges with its surroundings:

a) Heat only

b) Work only

c) Matter only

d) Neither matter nor energy

Correct Answer: d) Neither matter nor energy

Explanation:
An isolated system is completely separated from its surroundings.

MCQ No. 64

The boundary separating a thermodynamic system from its surroundings may be:

a) Real only

b) Imaginary only

c) Real or imaginary

d) Metallic only

Correct Answer: c) Real or imaginary

Explanation:
The boundary of a system may be physical or simply an imaginary surface chosen for analysis.

MCQ No. 65

A thermometer works on the principle of the:

a) First Law of Thermodynamics

b) Second Law of Thermodynamics

c) Zeroth Law of Thermodynamics

d) Third Law of Thermodynamics

Correct Answer: c) Zeroth Law of Thermodynamics

Explanation:
A thermometer reaches thermal equilibrium with the object whose temperature is being measured.

MCQ No. 66

The efficiency of a real heat engine is always ______ the efficiency of a Carnot engine operating between the same temperatures.

a) Greater than

b) Equal to

c) Less than

d) Independent of

Correct Answer: c) Less than

Explanation:
The Carnot engine represents the maximum theoretical efficiency; real engines always have lower efficiency.

MCQ No. 67

Which of the following cannot have 100% efficiency?

a) Electric heater

b) Carnot engine

c) Heat engine

d) Electric motor

Correct Answer: c) Heat engine

Explanation:
No heat engine can convert all absorbed heat into useful work because some heat must always be rejected.

MCQ No. 68

The coefficient of performance (COP) of a refrigerator is generally:

a) Always less than 1

b) Equal to zero

c) Greater than or less than 1 depending on operating conditions

d) Always exactly 1

Correct Answer: c) Greater than or less than 1 depending on operating conditions

Explanation:
Unlike efficiency, the COP of a refrigerator may be greater than one because it compares heat removed to work supplied.

MCQ No. 69

The SI unit of entropy is:

a) J

b) J K⁻¹

c) J kg⁻¹

d) W K⁻¹

Correct Answer: b) J K⁻¹

Explanation:
Entropy is measured in joules per kelvin (J K⁻¹).

MCQ No. 70

During a reversible process, the entropy of the universe:

a) Increases

b) Decreases

c) Remains constant

d) Becomes maximum

Correct Answer: c) Remains constant

Explanation:
In an ideal reversible process, the total entropy change of the universe is zero.

MCQ No. 71

Which of the following is a spontaneous natural process?

a) Heat flowing from cold water to boiling water without work

b) Water flowing uphill by itself

c) Heat flowing from a hot object to a cold object

d) Gas compressing itself into half its volume

Correct Answer: c) Heat flowing from a hot object to a cold object

Explanation:
Heat naturally flows from higher to lower temperature without external assistance.

MCQ No. 72

The main purpose of a heat engine is to produce:

a) Cooling

b) Mechanical work

c) Electricity only

d) Light energy

Correct Answer: b) Mechanical work

Explanation:
Heat engines convert thermal energy into useful mechanical work.

MCQ No. 73

A refrigerator requires external work because heat is transferred:

a) From hot to cold

b) Between bodies of equal temperature

c) From cold to hot

d) Without any temperature difference

Correct Answer: c) From cold to hot

Explanation:
Moving heat against its natural direction requires an external energy input.

MCQ No. 74

The efficiency of a Carnot engine increases when:

a) The temperature difference between the reservoirs increases

b) The temperatures become equal

c) The cold reservoir temperature increases

d) The hot reservoir temperature decreases

Correct Answer: a) The temperature difference between the reservoirs increases

Explanation:
A larger temperature difference between the hot and cold reservoirs results in higher Carnot efficiency.

MCQ No. 75

Which statement best summarizes the Second Law of Thermodynamics?

a) Energy can neither be created nor destroyed.

b) Heat naturally flows from a hotter body to a colder body, and no heat engine is perfectly efficient.

c) Pressure is directly proportional to volume.

d) Internal energy always remains constant.

Correct Answer: b) Heat naturally flows from a hotter body to a colder body, and no heat engine is perfectly efficient.

Explanation:
The Second Law determines the natural direction of heat flow and sets the fundamental limit on the efficiency of heat engines.

MCQ No. 76

The primary function of a thermometer is to measure:

a) Heat

b) Temperature

c) Internal energy

d) Pressure

Correct Answer: b) Temperature

Explanation:
A thermometer measures the temperature of a body by attaining thermal equilibrium with it.

MCQ No. 77

Which of the following is a thermodynamic state variable?

a) Heat

b) Work

c) Pressure

d) Path length

Correct Answer: c) Pressure

Explanation:
Pressure is a state variable because it depends only on the current state of the system, not on how the state was reached.

MCQ No. 78

When a gas expands against an external pressure, it performs:

a) Electrical work

b) Mechanical work

c) Chemical work

d) Nuclear work

Correct Answer: b) Mechanical work

Explanation:
As a gas expands, it pushes the surroundings and performs mechanical work.

MCQ No. 79

The area enclosed by a closed cycle on a P–V diagram represents the:

a) Pressure

b) Temperature

c) Net work done during the cycle

d) Internal energy

Correct Answer: c) Net work done during the cycle

Explanation:
The area enclosed by a cyclic process on a pressure-volume diagram equals the net work done by the system.

MCQ No. 80

If no work is done by a system, the change in its volume is:

a) Positive

b) Negative

c) Zero

d) Infinite

Correct Answer: c) Zero

Explanation:
Mechanical work in gases is associated with volume change. If there is no change in volume, no work is done.

MCQ No. 81

The efficiency of a heat engine is the ratio of:

a) Heat rejected to heat absorbed

b) Work done to heat absorbed

c) Heat absorbed to work done

d) Work done to heat rejected

Correct Answer: b) Work done to heat absorbed

Explanation:
Efficiency measures how much of the absorbed heat is converted into useful work.

MCQ No. 82

Which quantity must always be rejected by a heat engine to the cold reservoir?

a) Pressure

b) Temperature

c) Some heat

d) Internal energy

Correct Answer: c) Some heat

Explanation:
According to the Second Law of Thermodynamics, a heat engine must reject some heat to a cold reservoir.

MCQ No. 83

The working substance in a Carnot engine is usually considered to be:

a) A liquid

b) An ideal gas

c) A solid

d) Steam only

Correct Answer: b) An ideal gas

Explanation:
The Carnot cycle is generally analyzed assuming the working substance behaves as an ideal gas.

MCQ No. 84

The Carnot cycle consists of:

a) Two isothermal and two adiabatic processes

b) Four isothermal processes

c) Four adiabatic processes

d) Two isobaric and two isochoric processes

Correct Answer: a) Two isothermal and two adiabatic processes

Explanation:
The Carnot cycle comprises two reversible isothermal and two reversible adiabatic processes.

MCQ No. 85

Which device transfers heat from a low-temperature region to a high-temperature region?

a) Heat engine

b) Refrigerator

c) Electric heater

d) Solar cooker

Correct Answer: b) Refrigerator

Explanation:
A refrigerator uses external work to move heat from a colder body to a warmer one.

MCQ No. 86

The coefficient of performance (COP) is used to evaluate the performance of a:

a) Steam engine

b) Petrol engine

c) Refrigerator

d) Diesel engine

Correct Answer: c) Refrigerator

Explanation:
COP indicates the effectiveness of a refrigerator or heat pump.

MCQ No. 87

Which quantity measures the degree of randomness in a thermodynamic system?

a) Enthalpy

b) Entropy

c) Pressure

d) Density

Correct Answer: b) Entropy

Explanation:
Entropy is a measure of the disorder or randomness of a thermodynamic system.

MCQ No. 88

Energy degradation occurs because:

a) Energy is destroyed

b) Useful energy is converted into less useful forms

c) Mass is converted into energy

d) Pressure becomes zero

Correct Answer: b) Useful energy is converted into less useful forms

Explanation:
Although total energy remains constant, its ability to perform useful work decreases due to entropy.

MCQ No. 89

Which law of thermodynamics determines the direction of spontaneous processes?

a) Zeroth Law

b) First Law

c) Second Law

d) Third Law

Correct Answer: c) Second Law

Explanation:
The Second Law predicts the natural direction of physical processes through the concept of entropy.

MCQ No. 90

The entropy of an isolated system during a reversible process:

a) Increases

b) Decreases

c) Remains constant

d) Becomes zero

Correct Answer: c) Remains constant

Explanation:
For a reversible process, the total entropy change of an isolated system is zero.

MCQ No. 91

The First Law of Thermodynamics is primarily concerned with the conservation of:

a) Mass

b) Momentum

c) Energy

d) Charge

Correct Answer: c) Energy

Explanation:
The First Law states that energy can neither be created nor destroyed but only transformed.

MCQ No. 92

Which of the following processes occurs without any exchange of heat with the surroundings?

a) Isothermal

b) Isobaric

c) Isochoric

d) Adiabatic

Correct Answer: d) Adiabatic

Explanation:
In an adiabatic process, heat transfer is zero, and changes occur due to work done.

MCQ No. 93

The SI unit of internal energy is:

a) Joule

b) Watt

c) Pascal

d) Kelvin

Correct Answer: a) Joule

Explanation:
Internal energy is a form of energy and is measured in joules.

MCQ No. 94

Which scientist is most closely associated with the experimental determination of the mechanical equivalent of heat?

a) Newton

b) Joule

c) Boyle

d) Charles

Correct Answer: b) Joule

Explanation:
James Prescott Joule established the equivalence between mechanical work and heat.

MCQ No. 95

A spontaneous process proceeds in the direction of:

a) Decreasing entropy of the universe

b) Increasing entropy of the universe

c) Constant pressure only

d) Constant volume only

Correct Answer: b) Increasing entropy of the universe

Explanation:
Natural processes proceed in the direction that increases the total entropy of the universe.

MCQ No. 96

The ratio $\gamma = \frac{C_p}{C_v}$ is also known as the:

a) Universal gas constant

b) Adiabatic index

c) Heat capacity

d) Thermal coefficient

Correct Answer: b) Adiabatic index

Explanation:
The ratio of molar specific heats is called the adiabatic index or ratio of specific heats.

MCQ No. 97

No heat engine can have an efficiency of 100% because of the:

a) Zeroth Law of Thermodynamics

b) First Law of Thermodynamics

c) Second Law of Thermodynamics

d) Boyle's Law

Correct Answer: c) Second Law of Thermodynamics

Explanation:
The Second Law requires that some heat must always be rejected to the cold reservoir.

MCQ No. 98

The Kelvin temperature scale begins at:

a) 100°C

b) −100°C

c) Absolute zero

d) Freezing point of water

Correct Answer: c) Absolute zero

Explanation:
The Kelvin scale starts at absolute zero (0 K), the lowest possible temperature.

MCQ No. 99

Heat, work, and internal energy are all measured in the SI unit of:

a) Watt

b) Joule

c) Pascal

d) Kelvin

Correct Answer: b) Joule

Explanation:
Heat, work, and internal energy are all forms of energy and are measured in joules.

MCQ No. 100

The study of heat, work, temperature, and energy transformations is called:

a) Mechanics

b) Electromagnetism

c) Thermodynamics

d) Optics

Correct Answer: c) Thermodynamics

Explanation:
Thermodynamics is the branch of physics that studies heat, temperature, work, internal energy, and their interrelationships.

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