Heat and Temperature, Thermal Expansion & Laws of Thermodynamics – Complete Physics Notes with MCQs

Why does a metal railway track have small gaps between its sections? Why does a mercury thermometer rise when placed in hot water? Why does water behave differently when cooled below 4°C? And why can a pressure cooker cook food faster than an ordinary vessel? These everyday observations are explained by the concepts of heat, temperature, thermal expansion and the laws of thermodynamics – some of the most important topics in Physics.

Heat and temperature, thermal expansion and thermodynamics are frequently tested in SSC CGL, CHSL, MTS, GD, RRB NTPC, Group D, ALP, JE, WBCS, UPSC, Police, Defence and State PSC examinations. Questions are commonly asked about the difference between heat and temperature, Celsius-Fahrenheit-Kelvin conversion, specific heat capacity, latent heat, anomalous expansion of water, coefficients of thermal expansion, thermal equilibrium, the first and second laws of thermodynamics, entropy and absolute zero. This complete guide explains these concepts in simple language with formulas, comparison tables, practical examples, solved numerical problems, memory tricks and 20 MCQs with answers.

➤ Heat: Energy transferred from a hotter body to a colder body due to a temperature difference. SI unit: joule (J).
➤ Temperature: A physical quantity that indicates the hotness or coldness of a body and determines the direction of heat transfer.
➤ Temperature conversion: K = °C + 273.15; °F = (9/5 × °C) + 32.
➤ Specific heat capacity: Q = mcΔT; SI unit: J kg⁻¹ K⁻¹.
➤ Latent heat: Q = mL; heat absorbed or released during a change of state without a temperature change.
➤ Thermal expansion: Increase in the dimensions of a substance due to a rise in temperature.
➤ Expansion of solids: Linear, superficial and cubical expansion.
➤ Anomalous expansion of water: Water has maximum density at 4°C.
➤ Zeroth law: Establishes the concept of thermal equilibrium and temperature.
➤ First law: Conservation of energy; ΔQ = ΔU + W.
➤ Second law: The entropy of an isolated system never decreases.
➤ Third law: Entropy of a perfect crystal approaches zero as temperature approaches absolute zero (0 K).

What is Heat?

Heat is a form of energy that flows from a body at a higher temperature to a body at a lower temperature because of the temperature difference between them. Heat transfer continues until thermal equilibrium is reached, provided no other energy transfer or external influence changes the situation.

For example, when a cold metal spoon is placed in a cup of hot tea, heat flows from the tea to the spoon. The spoon becomes warmer while the tea loses some thermal energy. Heat does not flow naturally from the colder spoon to the hotter tea.

Heat is a form of energy in transit, not a substance stored inside an object. The internal energy of a body may change when it receives or releases heat.

SI unit: Joule (J)
CGS unit: Calorie (cal)
Other unit: Kilocalorie (kcal)

Important conversions:
1 calorie = 4.186 joules (approximately)
1 kilocalorie = 1,000 calories
1 kilocalorie = 4,186 joules (approximately)

Sources of Heat

Source	How heat is produced	Example
Sun	Nuclear fusion reactions	Solar energy reaching Earth
Combustion	Chemical energy changes into thermal energy	Burning wood, coal and LPG
Electricity	Electrical energy converts into heat	Electric iron, heater and geyser
Friction	Mechanical energy converts into thermal energy	Rubbing hands together
Geothermal energy	Heat from Earth's interior	Hot springs and geothermal power plants
Nuclear reactions	Energy released during nuclear reactions	Nuclear power plants

What is Temperature?

Temperature is the physical quantity that indicates how hot or cold a body is. It determines the direction in which heat will naturally flow when two bodies at different temperatures are brought into thermal contact.

At the microscopic level, temperature is related to the average translational kinetic energy of particles in an ideal gas. More generally, temperature is a thermodynamic property that determines thermal equilibrium.

A thermometer is used to measure temperature. Common temperature scales are Celsius, Fahrenheit and Kelvin.

Temperature Scales and Conversion Formulas

Scale	Freezing point of water	Boiling point of water	Absolute zero
Celsius (°C)	0°C	100°C	−273.15°C
Fahrenheit (°F)	32°F	212°F	−459.67°F
Kelvin (K)	273.15 K	373.15 K	0 K

These reference points assume pure water at standard atmospheric pressure (1 atm).

Important temperature conversion formulas:

F = (9/5)C + 32

C = (5/9)(F − 32)

K = C + 273.15

C = K − 273.15

The combined relationship is:

C/5 = (F − 32)/9 = (K − 273.15)/5

Exam shortcut: In many school-level and competitive-exam calculations, 273 is used instead of 273.15 for easy conversion.

Example 1: Convert 40°C into Fahrenheit and Kelvin.

F = (9/5 × 40) + 32 = 72 + 32 = 104°F.

K = 40 + 273.15 = 313.15 K.

Example 2: Convert 98.6°F into Celsius.

C = (5/9) × (98.6 − 32)
= (5/9) × 66.6
= 37°C.

Types of Thermometers

Thermometer	Working principle	Common use
Mercury thermometer	Thermal expansion of mercury	Traditional laboratory temperature measurement
Alcohol thermometer	Expansion of coloured alcohol	Low-temperature measurement
Digital thermometer	Change in electrical properties of a sensor	Body temperature measurement
Clinical thermometer	Designed for measuring human body temperature	Medical use
Laboratory thermometer	Thermal expansion of a liquid or another sensing mechanism	Laboratory experiments
Infrared thermometer	Detects infrared radiation emitted by an object	Contactless temperature measurement
Thermocouple thermometer	Seebeck effect: temperature difference produces an electrical voltage	Industrial temperature measurement

Important fact: Mercury was traditionally used in thermometers because it expands fairly uniformly, is easily visible and does not wet glass. However, mercury is toxic, and many modern thermometers use digital sensors or other liquids.

Difference Between Heat and Temperature

Heat and temperature are closely related, but they are not the same physical quantity. This is one of the most frequently asked conceptual questions in competitive examinations.

Heat	Temperature
Energy transferred due to a temperature difference	Indicates the thermal state of a body
SI unit is joule (J)	SI unit is kelvin (K)
Depends on the amount of energy transferred	Does not depend on the amount of substance in the same way as total heat capacity
Measured using a calorimeter in experiments	Measured using a thermometer
Flows from higher temperature to lower temperature naturally	Determines the direction of natural heat transfer
It is energy in transit	It is a thermodynamic state property

Remember: A large bucket of warm water may contain more internal energy than a small cup of hotter water, even though the cup has a higher temperature. Therefore, a higher temperature does not always mean a greater quantity of thermal energy.

Thermal Equilibrium

Thermal equilibrium is the condition in which two or more bodies in thermal contact have the same temperature and there is no net heat transfer between them.

For example, when a thermometer is placed under the tongue, heat is exchanged between the body and the thermometer. After sufficient time, both reach thermal equilibrium, and the thermometer gives a stable reading.

Thermal equilibrium is the foundation of the Zeroth Law of Thermodynamics, which provides the scientific basis for temperature measurement.

Specific Heat Capacity

Specific heat capacity is the amount of heat required to raise the temperature of 1 kg of a substance by 1 K (or 1°C), without a change of state.

Formula:

Q = mcΔT

Where:

• Q = heat supplied or removed (joule)
• m = mass of the substance (kg)
• c = specific heat capacity (J kg⁻¹ K⁻¹)
• ΔT = change in temperature (K or °C)

SI unit: J kg⁻¹ K⁻¹.

Example: Calculate the heat required to raise the temperature of 2 kg of water from 20°C to 30°C. Take the specific heat capacity of water as 4,200 J kg⁻¹ K⁻¹.

Q = mcΔT
= 2 × 4,200 × (30 − 20)
= 2 × 4,200 × 10
= 84,000 J.

Therefore, 84,000 joules of heat are required.

Specific Heat Capacity of Important Substances

Substance	Approximate specific heat capacity (J kg⁻¹ K⁻¹)
Water	4,200
Ice	2,100
Aluminium	900
Iron	450
Copper	385
Silver	235
Lead	130

Note: These are approximate values. Specific heat capacity may vary with temperature and physical conditions.

Why does water have a high specific heat capacity? Water requires a relatively large amount of energy to increase its temperature. This helps moderate the climate of coastal regions, regulates the temperature of living organisms and makes water useful as a coolant in many systems.

Latent Heat and Change of State

Latent heat is the heat absorbed or released by a substance during a change of physical state without a change in temperature, at a specified pressure.

For example, when ice melts at 0°C under standard atmospheric pressure, it absorbs heat but remains at 0°C until the melting process is complete.

Formula:

Q = mL

Where:

• Q = heat absorbed or released (J)
• m = mass (kg)
• L = specific latent heat (J/kg)

Types of Latent Heat

Type	Meaning	Example
Latent heat of fusion	Heat needed to convert a solid into a liquid at its melting point	Ice changing into water
Latent heat of vaporisation	Heat needed to convert a liquid into vapour at its boiling point	Water changing into steam
Latent heat of sublimation	Heat involved in changing a solid directly into gas	Dry ice changing into carbon dioxide gas

Approximate important values for water at standard atmospheric pressure:

• Latent heat of fusion of ice = 3.34 × 10⁵ J/kg.
• Latent heat of vaporisation of water = 2.26 × 10⁶ J/kg.

Example: How much heat is required to melt 0.5 kg of ice at 0°C? Take latent heat of fusion as 3.34 × 10⁵ J/kg.

Q = mL
= 0.5 × 3.34 × 10⁵
= 1.67 × 10⁵ J.

Why is steam more dangerous than boiling water at 100°C? Steam at 100°C contains additional latent heat of vaporisation. When it condenses on the skin, it releases this energy, causing more severe burns than an equal mass of boiling water at the same temperature.

Heating and Cooling During Change of State

When heat is supplied to ice, its temperature does not rise continuously. At the melting point, the temperature remains constant until all the ice melts. Similarly, during boiling, the temperature remains constant until the liquid changes completely into vapour at a given pressure.

These constant-temperature portions of a heating curve represent latent heat absorption. During freezing and condensation, the same energy is released.

Modes of Heat Transfer

Heat can be transferred from one place to another through three main methods: conduction, convection and radiation. Understanding their differences is important for both conceptual and objective questions.

1. Conduction

Conduction is the transfer of heat through a substance from a hotter region to a colder region without the overall movement of the substance as a whole.

It is most commonly observed in solids, particularly metals. When one end of an iron rod is heated, energy passes from particle to particle and through mobile electrons, making the other end warmer.

Examples:

• A metal spoon becomes hot when kept in hot tea.
• The handle of a metal cooking vessel becomes hot.
• Heat travels through a metal rod when one end is heated.

Good conductors: Silver, copper, aluminium and iron.

Poor conductors (insulators): Wood, rubber, plastic, glass and air.

2. Convection

Convection is the transfer of heat through the actual movement of particles in a fluid (liquid or gas).

When water is heated from below, the warmer and less dense water rises while cooler and denser water sinks. This circulation creates convection currents.

Examples:

• Boiling water in a cooking vessel.
• Sea breeze and land breeze.
• Warm air rising from a room heater.
• Ventilation through openings near the ceiling.

Important fact: Convection takes place in liquids and gases, not through the bulk movement of particles in ordinary solids.

3. Radiation

Radiation is the transfer of heat through electromagnetic waves, mainly infrared radiation for objects at ordinary temperatures. It does not require a material medium.

This explains how heat from the Sun reaches the Earth through the vacuum of space.

Examples:

• Heat received from the Sun.
• Feeling warmth near a fire without touching it.
• Heat emitted by a hot electric heater.

Important fact: All objects above absolute zero emit thermal radiation. A good absorber is generally also a good emitter of radiation under the same conditions.

Difference Between Conduction, Convection and Radiation

Basis	Conduction	Convection	Radiation
Transfer mechanism	Energy transfer through microscopic interactions	Bulk movement of fluid	Electromagnetic waves
Medium	Most common in solids	Liquids and gases	Not required
Vacuum	Not possible	Not possible	Possible
Example	Heating a metal rod	Boiling water	Sunlight reaching Earth

What is Thermal Expansion?

Thermal expansion is the tendency of a substance to increase its dimensions when its temperature rises. Most substances expand on heating and contract on cooling because the average separation between their particles changes.

Thermal expansion is an important property of solids, liquids and gases. The amount of expansion depends on the material, its original dimensions and the temperature change.

For example, railway tracks are provided with expansion arrangements to accommodate changes in length during seasonal temperature variations. Similarly, bridges use expansion joints to prevent excessive thermal stress.

Types of Thermal Expansion in Solids

Thermal expansion of solids is generally classified into three types:

1. Linear Expansion

Linear expansion is the increase in the length of a solid when its temperature rises.

Formula:

ΔL = αL₀ΔT

Where:

• ΔL = increase in length
• α = coefficient of linear expansion
• L₀ = original length
• ΔT = change in temperature

The final length is:

L = L₀(1 + αΔT)

2. Superficial or Area Expansion

Superficial expansion is the increase in the surface area of a solid due to a rise in temperature.

Formula:

ΔA = βA₀ΔT

Where β is the coefficient of superficial expansion and A₀ is the original area.

For an isotropic solid, with small temperature changes:

β ≈ 2α

3. Cubical or Volume Expansion

Cubical expansion is the increase in the volume of a substance when heated.

Formula:

ΔV = γV₀ΔT

Where γ is the coefficient of cubical expansion and V₀ is the original volume.

For an isotropic solid:

γ ≈ 3α

Therefore, for small temperature changes in an isotropic solid:

α : β : γ = 1 : 2 : 3

Important: These relationships are approximate and apply to isotropic solids under ordinary small-expansion conditions.

Coefficients of Thermal Expansion

Coefficient	Meaning	Formula	SI unit
Linear (α)	Change in length per unit original length per degree of temperature change	α = ΔL/(L₀ΔT)	K⁻¹
Superficial (β)	Change in area per unit original area per degree of temperature change	β = ΔA/(A₀ΔT)	K⁻¹
Cubical (γ)	Change in volume per unit original volume per degree of temperature change	γ = ΔV/(V₀ΔT)	K⁻¹

Example: A metal rod is 2 m long at 20°C. Its coefficient of linear expansion is 12 × 10⁻⁶ K⁻¹. Calculate its increase in length when heated to 120°C.

ΔL = αL₀ΔT
= 12 × 10⁻⁶ × 2 × (120 − 20)
= 12 × 10⁻⁶ × 200
= 0.0024 m
= 2.4 mm.

Thermal Expansion in Liquids

Liquids generally expand when heated and contract when cooled. Since liquids do not have a fixed shape, their thermal expansion is measured mainly in terms of volume.

When a liquid is heated in a container, both the liquid and the container may expand. The expansion observed in the liquid relative to the container is called apparent expansion.

Real expansion = Apparent expansion + Expansion of the container (when expressed using the same initial volume and temperature interval).

Example: When a bottle filled with liquid is heated, the liquid level may rise because the liquid expands. The observed rise is influenced by the expansion of the bottle itself.

Thermal Expansion in Gases

Gases generally expand considerably when heated, provided their pressure remains constant. Their volume is also strongly affected by pressure.

According to Charles's law, for a fixed mass of gas at constant pressure, its volume is directly proportional to its absolute temperature.

V₁/T₁ = V₂/T₂

Temperatures must be expressed in kelvin.

Example: A gas occupies 2 litres at 300 K. If its pressure remains constant and the temperature increases to 450 K:

V₂ = V₁ × T₂/T₁
= 2 × 450/300
= 3 litres.

Everyday Applications of Thermal Expansion

Application	Scientific explanation
Railway tracks	Expansion arrangements accommodate changes in track length due to temperature variations.
Bridge expansion joints	Allow bridge sections to expand and contract without developing excessive stress.
Electric overhead wires	Become longer and sag more in hot weather; contract in cold weather.
Bimetallic strip	Two bonded metals with different expansion coefficients bend when heated.
Liquid thermometers	Use the expansion of a liquid to indicate temperature changes.
Metal lids on glass jars	Gentle heating can expand the metal lid and make it easier to open.
Riveting	Hot rivets contract on cooling and help hold metal plates tightly together.

Bimetallic Strip

A bimetallic strip consists of two different metals firmly joined together. The metals have different coefficients of thermal expansion. When heated, the metal with the higher expansion coefficient expands more, causing the strip to bend towards the metal with the lower expansion coefficient.

Applications:

• Thermostats in electric irons.
• Temperature-control devices.
• Thermal switches and circuit protection devices.
• Some traditional temperature-indicating instruments.

Anomalous Expansion of Water

Water behaves differently from most liquids between 0°C and 4°C. This unusual behaviour is called anomalous expansion of water.

When water is heated from 0°C to 4°C, it contracts rather than expands. Its density increases and reaches a maximum at 4°C. When water is heated above 4°C, it expands normally. Similarly, when water is cooled from 4°C to 0°C, it expands.

Important facts:

• Maximum density of pure water occurs at approximately 4°C.
• At 4°C, water has minimum volume for a fixed mass in the ordinary pressure conditions considered here.
• Water expands when cooled from 4°C to 0°C.
• Ice is less dense than liquid water and therefore floats on water.

Why is anomalous expansion important for aquatic life?

During winter, the surface water of lakes and ponds cools. Water at around 4°C becomes denser and sinks. Water cooled below 4°C becomes less dense and stays near the surface, where it can eventually freeze. Ice floats and forms an insulating layer, allowing liquid water to remain beneath it. This helps aquatic organisms survive in cold conditions.

Thermal Stress and Thermal Strain

When a material is heated, it normally expands. If its expansion is completely prevented, internal forces develop within the material. This is called thermal stress.

For a uniform, isotropic material under idealised conditions, the magnitude of thermal stress is:

σ = YαΔT

Where:

• σ = magnitude of thermal stress (Pa)
• Y = Young's modulus (Pa)
• α = coefficient of linear expansion (K⁻¹)
• ΔT = temperature change (K)

This expression assumes complete restraint, linear elasticity and uniform temperature change.

Examples: Thermal stress may develop in railway tracks, concrete structures, metal pipelines and industrial equipment when expansion or contraction is restricted.

Laws of Thermodynamics

Thermodynamics is the branch of Physics that studies heat, temperature, work, internal energy and the conversion of energy from one form to another.

The laws of thermodynamics describe the fundamental rules governing energy transfer and the direction of natural processes. They are important in understanding engines, refrigerators, power plants, chemical reactions and many other physical systems.

There are four fundamental laws of thermodynamics:

• Zeroth Law of Thermodynamics – Thermal equilibrium.
• First Law of Thermodynamics – Conservation of energy.
• Second Law of Thermodynamics – Direction of heat transfer and entropy.
• Third Law of Thermodynamics – Behaviour of entropy near absolute zero.

1. Zeroth Law of Thermodynamics

Statement: If two systems are each in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other.

This law establishes the concept of temperature and provides the foundation for temperature measurement.

Understanding with an example:

Suppose body A is in thermal equilibrium with a thermometer C. Body B is also in thermal equilibrium with the same thermometer C. According to the Zeroth Law, bodies A and B must be in thermal equilibrium with each other.

In simple terms, if two objects have the same temperature as a third object, they have the same temperature as each other.

Applications of the Zeroth Law:

• Working principle of thermometers.
• Comparison of temperatures of different bodies.
• Establishing a consistent temperature scale.
• Temperature measurement in laboratories and industries.

Important exam point: The Zeroth Law is called the foundation of thermometry because it makes temperature comparison possible.

2. First Law of Thermodynamics

Statement: Energy can neither be created nor destroyed. It can only be transferred or transformed from one form into another. The total energy of an isolated system remains constant.

The First Law of Thermodynamics is the application of the law of conservation of energy to thermodynamic systems.

Mathematical expression:

Q = ΔU + W

Where:

• Q = heat supplied to the system.
• ΔU = change in internal energy of the system.
• W = work done by the system on its surroundings.

This sign convention means that heat supplied to the system is positive and work done by the system is positive.

Another form is:

ΔU = Q − W

Example: A gas receives 500 J of heat and performs 200 J of work. Calculate the change in its internal energy.

ΔU = Q − W
= 500 − 200
= 300 J.

Therefore, the internal energy of the gas increases by 300 J.

Important Thermodynamic Processes

Process	Condition	Important result
Isothermal	Temperature remains constant	ΔT = 0; for an ideal gas, ΔU = 0
Adiabatic	No heat exchange with surroundings	Q = 0; ΔU = −W
Isobaric	Pressure remains constant	Constant pressure; work = PΔV for a quasi-static process
Isochoric (Isovolumetric)	Volume remains constant	ΔV = 0; work done by the gas = 0
Cyclic	System returns to its initial state	Net change in internal energy = 0

Important: For an ideal gas in an isothermal process, internal energy remains constant because the internal energy of an ideal gas depends only on temperature.

Applications of the First Law

• Explains the energy balance in heat engines.
• Describes the relationship between heat, work and internal energy.
• Explains the energy changes in gases during compression and expansion.
• Forms the basis of energy calculations in refrigerators and power plants.

3. Second Law of Thermodynamics

Statement: The entropy of an isolated system never decreases in a spontaneous process. It remains constant for an ideal reversible process and increases for an irreversible process.

The Second Law explains why natural processes have a preferred direction. Although energy is conserved, not all energy can be converted completely into useful work.

For example, heat naturally flows from a hot object to a cold object. It does not flow spontaneously from a cold object to a hotter object without external work.

Two Important Statements of the Second Law

1. Kelvin-Planck Statement:

It is impossible to construct a cyclic heat engine whose only effect is to absorb heat from a single reservoir and convert it completely into work.

In simple words, a heat engine cannot convert all the heat it receives into useful work during a cycle. Some heat must be rejected to a colder reservoir.

2. Clausius Statement:

It is impossible to construct a cyclic device whose only effect is to transfer heat from a colder body to a hotter body without external work.

This explains why refrigerators require electrical energy to move heat from their cold interiors to the warmer surroundings.

Entropy

Entropy is a thermodynamic state function associated with the dispersal of energy and the number of microscopic arrangements available to a system.

For a reversible process:

dS = δQ_rev/T

Where:

• dS = infinitesimal change in entropy.
• δQ_rev = heat transferred reversibly.
• T = absolute temperature in kelvin.

SI unit: J/K.

Important points about entropy:

• Entropy is a state function.
• Entropy is an extensive property.
• The entropy of an isolated system never decreases.
• Entropy remains constant in an ideal reversible process involving an isolated system.
• Entropy increases in an irreversible spontaneous process involving an isolated system.

Example: When ice melts naturally in a warm room, the process occurs in the direction of increasing total entropy of the system and its surroundings.

Heat Engine and Refrigerator

Heat Engine: A heat engine absorbs heat from a hot reservoir, converts part of it into useful work and rejects the remaining heat to a cold reservoir.

Thermal efficiency:

η = W/Q_H = 1 − Q_C/Q_H

Where Q_H is heat absorbed from the hot reservoir and Q_C is heat rejected to the cold reservoir.

Refrigerator: A refrigerator uses external work to transfer heat from a colder region to a hotter region.

Coefficient of Performance (COP):

COP = Q_C/W

Where Q_C is heat removed from the cold space and W is the work supplied.

Exam point: A refrigerator does not create cold. It removes heat from its interior and releases that heat into the surrounding environment.

4. Third Law of Thermodynamics

Statement: The entropy of a perfect crystal approaches zero as its temperature approaches absolute zero (0 K).

At absolute zero, a perfect crystal in its unique ground state has zero entropy according to the Third Law of Thermodynamics.

Absolute zero is the lowest limiting temperature:

0 K = −273.15°C = −459.67°F

At temperatures approaching absolute zero, the thermal motion and thermal excitation of particles become extremely small. However, quantum mechanics means that particles can still possess zero-point energy.

Important points:

• Absolute zero is 0 kelvin.
• The entropy of a perfect crystal approaches zero at 0 K.
• Absolute zero cannot be reached through a finite sequence of ordinary physical processes.
• The Third Law provides a reference for determining absolute entropy.

Important exam fact: The Third Law of Thermodynamics is also associated with the impossibility of reaching absolute zero in a finite number of thermodynamic operations.

Difference Between the Four Laws of Thermodynamics

Law	Main concept	Key idea	Application
Zeroth Law	Thermal equilibrium	Establishes the concept of temperature	Thermometers
First Law	Conservation of energy	Energy cannot be created or destroyed	Heat engines and energy calculations
Second Law	Entropy and direction of processes	Entropy of an isolated system never decreases	Refrigerators and heat engines
Third Law	Absolute zero	Entropy of a perfect crystal approaches zero at 0 K	Low-temperature physics

Important Heat and Temperature Facts for Competitive Exams

Question	Important fact
SI unit of heat	Joule
SI unit of temperature	Kelvin
CGS unit of heat	Calorie
1 calorie	Approximately 4.186 joules
Absolute zero	0 K or −273.15°C
Maximum density of water	At approximately 4°C
Specific heat capacity of water	Approximately 4,200 J kg⁻¹ K⁻¹
Latent heat of fusion of ice	Approximately 3.34 × 10⁵ J/kg
Latent heat of vaporisation of water	Approximately 2.26 × 10⁶ J/kg
Heat transfer through vacuum	Radiation
Principle of thermometer	Zeroth Law of Thermodynamics
Law of conservation of energy	First Law of Thermodynamics
Entropy and irreversibility	Second Law of Thermodynamics
Entropy of perfect crystal at 0 K	Zero
Thermal expansion of solids	Linear, superficial and cubical

Heat, Temperature and Thermodynamics – Formula Sheet

Quantity	Formula	SI unit
Celsius to Kelvin	K = C + 273.15	K
Celsius to Fahrenheit	F = (9/5)C + 32	°F
Heat supplied	Q = mcΔT	J
Latent heat	Q = mL	J
Linear expansion	ΔL = αL₀ΔT	m
Area expansion	ΔA = βA₀ΔT	m²
Volume expansion	ΔV = γV₀ΔT	m³
Expansion coefficients (isotropic solid)	β ≈ 2α; γ ≈ 3α	K⁻¹
Charles's law	V₁/T₁ = V₂/T₂	–
Thermal stress	σ = YαΔT	Pa
First Law	Q = ΔU + W	J
Entropy change (reversible)	dS = δQrev/T	J/K
Heat engine efficiency	η = W/QH	–
Refrigerator COP	COP = QC/W	–

West Bengal Connection – Heat and Thermal Expansion in Everyday Life

The concepts of heat and temperature can be observed in everyday life across West Bengal. During the summer, Kolkata and other parts of the state experience high temperatures, while coastal regions are influenced by the moderating effect of the Bay of Bengal. Water's high specific heat capacity helps explain why large water bodies heat up and cool down more slowly than nearby land.

The Howrah Bridge and Vidyasagar Setu are examples of large steel structures where temperature variations must be considered in engineering design. Expansion arrangements in bridges and other infrastructure help accommodate changes in dimensions caused by seasonal heating and cooling.

In the Sundarbans, the movement of heat and moisture between land, water and the atmosphere influences local weather conditions. Sea breeze and land breeze are practical examples of convection caused by unequal heating of land and water.

These examples connect basic Physics with daily life, geography, engineering and the environment – an approach useful for WBCS, West Bengal Police and other State PSC examinations.

Quick Revision Box – Easy Tricks to Memorise

Trick 1 – Heat vs Temperature
Heat = Energy transferred; Temperature = Degree of hotness or coldness.

Trick 2 – Temperature Conversion
Kelvin = Celsius + 273.15. Remember: Kelvin has no degree symbol.

Trick 3 – Specific Heat
Q = mcΔT. More mass or a greater temperature change requires more heat for the same substance.

Trick 4 – Latent Heat
Q = mL. During a change of state, heat is absorbed or released without a temperature change at constant pressure.

Trick 5 – Three Modes of Heat Transfer
Conduction = Contact; Convection = Movement of fluid; Radiation = Electromagnetic waves.

Trick 6 – Expansion Ratio
Linear : Superficial : Cubical = 1 : 2 : 3 (approximately, for isotropic solids).

Trick 7 – Anomalous Water
Water is densest at 4°C. Ice floats because it is less dense than liquid water.

Trick 8 – Four Laws of Thermodynamics
Zeroth = Temperature; First = Energy; Second = Entropy; Third = Absolute zero.

Trick 9 – First Law
Q = ΔU + W. Heat supplied = Increase in internal energy + Work done by the system.

Trick 10 – Second Law
Heat naturally flows from hot to cold. Entropy of an isolated system never decreases.

Trick 11 – Third Law
Perfect crystal at 0 K → Entropy approaches zero.

Trick 12 – Refrigerator
A refrigerator removes heat from a cold space by using external work.

20 MCQs on Heat, Temperature, Thermal Expansion and Thermodynamics

Q1. What is the SI unit of heat?
(a) Calorie
(b) Joule
(c) Kelvin
(d) Watt

Answer: (b) Joule. Heat is energy transferred because of a temperature difference, and its SI unit is the joule.

Q2. Which of the following is the SI unit of temperature?
(a) Celsius
(b) Fahrenheit
(c) Kelvin
(d) Calorie

Answer: (c) Kelvin. Kelvin is the SI base unit of thermodynamic temperature.

Q3. Absolute zero is equal to:
(a) 0°C
(b) −100°C
(c) −273.15°C
(d) 100°C

Answer: (c) −273.15°C. Absolute zero is 0 K, equivalent to −273.15°C.

Q4. The temperature at which water has maximum density is:
(a) 0°C
(b) 4°C
(c) 10°C
(d) 100°C

Answer: (b) 4°C. Water shows anomalous expansion and reaches maximum density at approximately 4°C.

Q5. Which substance has a relatively high specific heat capacity?
(a) Copper
(b) Lead
(c) Water
(d) Silver

Answer: (c) Water. Water has a specific heat capacity of approximately 4,200 J kg⁻¹ K⁻¹.

Q6. Which formula is used to calculate the heat required to change the temperature of a substance without changing its state?
(a) Q = mL
(b) Q = mcΔT
(c) F = ma
(d) P = W/t

Answer: (b) Q = mcΔT. It represents heat transfer based on mass, specific heat capacity and temperature change.

Q7. During the melting of ice at 0°C, its temperature:
(a) Increases continuously
(b) Decreases continuously
(c) Remains constant until melting is complete
(d) Immediately reaches 100°C

Answer: (c). The supplied energy is used as latent heat of fusion rather than increasing temperature.

Q8. Heat from the Sun reaches the Earth through:
(a) Conduction
(b) Convection
(c) Radiation
(d) Conduction and convection only

Answer: (c) Radiation. Radiation can travel through the vacuum of space without a material medium.

Q9. Which of the following is a good conductor of heat?
(a) Wood
(b) Rubber
(c) Copper
(d) Plastic

Answer: (c) Copper. Copper is a good conductor of heat and is widely used in cookware and heat exchangers.

Q10. The transfer of heat through the actual movement of a fluid is called:
(a) Radiation
(b) Conduction
(c) Convection
(d) Reflection

Answer: (c) Convection. Convection transfers heat through the bulk movement of liquids and gases.

Q11. The coefficient of linear expansion is represented by:
(a) α
(b) β
(c) γ
(d) μ

Answer: (a) α. Alpha represents the coefficient of linear thermal expansion.

Q12. For an isotropic solid, the approximate relationship between linear and cubical expansion coefficients is:
(a) γ = α
(b) γ = 2α
(c) γ = 3α
(d) γ = 4α

Answer: (c) γ = 3α. For small temperature changes, the cubical expansion coefficient is approximately three times the linear coefficient.

Q13. Railway tracks are provided with expansion arrangements to:
(a) Increase their weight
(b) Prevent thermal expansion
(c) Accommodate temperature-related expansion and contraction
(d) Increase friction

Answer: (c). Thermal expansion arrangements help prevent excessive stress and deformation.

Q14. A bimetallic strip works because two metals have different:
(a) Densities only
(b) Coefficients of thermal expansion
(c) Melting points only
(d) Electrical resistances only

Answer: (b). Different expansion rates cause the joined metals to bend when heated.

Q15. The Zeroth Law of Thermodynamics establishes the concept of:
(a) Work
(b) Entropy
(c) Thermal equilibrium
(d) Absolute zero

Answer: (c) Thermal equilibrium. It provides the foundation for comparing temperatures and using thermometers.

Q16. Which law of thermodynamics is based on the conservation of energy?
(a) Zeroth Law
(b) First Law
(c) Second Law
(d) Third Law

Answer: (b) First Law. It relates heat supplied, work done and the change in internal energy.

Q17. Which law states that the entropy of an isolated system never decreases?
(a) Zeroth Law
(b) First Law
(c) Second Law
(d) Third Law

Answer: (c) Second Law. The entropy of an isolated system remains constant for reversible processes and increases for irreversible processes.

Q18. According to the Third Law of Thermodynamics, the entropy of a perfect crystal approaches zero at:
(a) 100°C
(b) 0°C
(c) 273 K
(d) 0 K

Answer: (d) 0 K. A perfect crystal in its unique ground state has zero entropy at absolute zero.

Q19. Which thermodynamic process takes place at constant temperature?
(a) Adiabatic
(b) Isothermal
(c) Isobaric
(d) Isochoric

Answer: (b) Isothermal. An isothermal process occurs at constant temperature.

Q20. A refrigerator transfers heat from a colder region to a hotter region by:
(a) Producing heat without energy
(b) Using external work
(c) Eliminating entropy completely
(d) Stopping all molecular motion

Answer: (b) Using external work. A refrigerator requires external energy to transfer heat from its cold interior to the warmer surroundings.

Want more practice? Try our Daily GK Quiz and Mock Tests.

Conclusion

Heat, temperature, thermal expansion and the laws of thermodynamics are closely connected concepts that explain many natural and technological processes. Heat is energy transferred because of a temperature difference, while temperature describes the thermal state of a body. Specific heat capacity and latent heat explain how substances respond to heating and changes of state, while conduction, convection and radiation describe the three major methods of heat transfer.

Thermal expansion explains the behaviour of railway tracks, bridges, overhead wires, thermometers and bimetallic strips. The anomalous expansion of water, particularly its maximum density at 4°C, is an important scientific fact with significant environmental consequences.

For competitive examinations, remember the four laws of thermodynamics in order: Zeroth Law – thermal equilibrium; First Law – conservation of energy; Second Law – entropy and direction of processes; Third Law – entropy of a perfect crystal at absolute zero. Revise the formulas, numerical examples, comparison tables and MCQs regularly to strengthen your Physics preparation for SSC, Railway, WBCS, UPSC, Police, Defence and other government examinations.

Reference: NCERT Science Class VII (Heat); NCERT Science Class IX (Matter in Our Surroundings); NCERT Physics Class XI (Thermal Properties of Matter); NCERT Physics Class XI (Thermodynamics); standard introductory Physics and thermodynamics principles.