JEE · Physics

Thermodynamics

Understand the official JEE scope of Physics Thermodynamics, track energy across a system boundary, choose the correct process relation and diagnose why questions go wrong.

Subject: Physics
Syllabus unit: Thermodynamics
Updated: 7 September 2026

Mapped to JEE Main 2026 and JEE Advanced 2026
Every formula declares its sign convention and conditions
No invented weightage, question counts or trend percentages

Content status: draft. Verified academic content for this page has not been loaded yet, so the page is excluded from search indexing and the sitemap.

In short

Thermodynamics studies how a macroscopic system changes state when energy crosses its boundary as heat or work. A solution should declare the system, initial and final states, process path, gas model, and sign convention before using the first law.

This is Physics Thermodynamics. Chemical enthalpy, Gibbs energy, reaction spontaneity, and chemical equilibrium belong to Chemistry Thermodynamics, not this page.

Use this page to answer three questions:

What does the official syllabus actually require?
Which process relation and sign convention should I use for a problem?
If I get it wrong, what kind of gap should I repair?

Syllabus mapping

Syllabus mapping
Unit	Topics
Thermodynamics	Thermal equilibrium and definition of temperature (zeroth law of Thermodynamics), Heat, work and internal energy, First law of thermodynamics, Isothermal and adiabatic processes, Second law of thermodynamics, Reversible and irreversible processes, Broader Thermal Physics scope named by JEE Advanced: expansion, calorimetry, heat transfer and cooling, ideal-gas laws and specific heats, first law for ideal gases, Carnot engine and efficiency, thermal-radiation laws

Unit
Thermodynamics
Topics
Thermal equilibrium and definition of temperature (zeroth law of Thermodynamics), Heat, work and internal energy, First law of thermodynamics, Isothermal and adiabatic processes, Second law of thermodynamics, Reversible and irreversible processes, Broader Thermal Physics scope named by JEE Advanced: expansion, calorimetry, heat transfer and cooling, ideal-gas laws and specific heats, first law for ideal gases, Carnot engine and efficiency, thermal-radiation laws

What this chapter contains and why it matters

What this chapter contains and why it matters
Question	Direct answer
What is the chapter about?	How a macroscopic system changes state when energy crosses its boundary as heat or work.
What is the central method choice?	Declare the system, endpoints, process path, gas model and sign convention before applying the first law.
Where do most mistakes begin?	Sign convention, confusing state functions with path-dependent transfers, and applying a reversible ideal-gas relation without checking its conditions.
What should come before Thermodynamics?	Mechanical work and area under a graph, temperature, calorimetry and heat transfer, and the ideal-gas equation.
What comes after it?	Kinetic Theory of Gases builds the microscopic picture behind these macroscopic laws.

Question
What is the chapter about?
Direct answer
How a macroscopic system changes state when energy crosses its boundary as heat or work.
Question
What is the central method choice?
Direct answer
Declare the system, endpoints, process path, gas model and sign convention before applying the first law.
Question
Where do most mistakes begin?
Direct answer
Sign convention, confusing state functions with path-dependent transfers, and applying a reversible ideal-gas relation without checking its conditions.
Question
What should come before Thermodynamics?
Direct answer
Mechanical work and area under a graph, temperature, calorimetry and heat transfer, and the ideal-gas equation.
Question
What comes after it?
Direct answer
Kinetic Theory of Gases builds the microscopic picture behind these macroscopic laws.

The official JEE documents define content scope. They do not publish chapter weightage, so none is asserted here.

Official JEE syllabus mapping for Thermodynamics

Verified against the current JEE Main 2026 syllabus and JEE Advanced 2026 syllabus on 7 September 2026. This is a wording and scope mapping, not a claim about question difficulty or frequency.

Official JEE syllabus mapping for Thermodynamics
Concept group	JEE Main 2026	JEE Advanced 2026	Preparation note
Equilibrium and temperature	Thermal equilibrium and the zeroth law are explicitly listed.	Covered within the broader Thermal Physics scope.	Treat temperature as the property equalised at thermal equilibrium.
Heat, work and the first law	Heat, work, internal energy and the first law are explicitly listed.	The first law for ideal gases is explicitly listed within Thermal Physics.	Declare a sign convention before combining Q, W and Delta U.
Processes	Isothermal and adiabatic processes are explicitly listed.	Isothermal and adiabatic processes for ideal gases are explicitly listed, alongside expansion, calorimetry and heat transfer.	Main and Advanced scope should not be assumed identical from a combined coaching outline. Keep both official documents available.
Second law and reversibility	The second law and reversible and irreversible processes are explicitly listed.	Reversibility, the Carnot engine and efficiency are explicitly listed.	Efficiency of any real engine is bounded by the reversible Carnot value.
Related topics named only in Advanced Thermal Physics	Calorimetry and heat transfer sit under Properties of Solids and Liquids; ideal-gas kinetic ideas sit under Kinetic Theory of Gases.	Ideal-gas specific heats and thermal-radiation laws are explicitly listed.	Follow the links to Thermal Properties and Kinetic Theory of Gases rather than assuming this page silently absorbs them.

Concept group
Equilibrium and temperature
JEE Main 2026
Thermal equilibrium and the zeroth law are explicitly listed.
JEE Advanced 2026
Covered within the broader Thermal Physics scope.
Preparation note
Treat temperature as the property equalised at thermal equilibrium.
Concept group
Heat, work and the first law
JEE Main 2026
Heat, work, internal energy and the first law are explicitly listed.
JEE Advanced 2026
The first law for ideal gases is explicitly listed within Thermal Physics.
Preparation note
Declare a sign convention before combining Q, W and Delta U.
Concept group
Processes
JEE Main 2026
Isothermal and adiabatic processes are explicitly listed.
JEE Advanced 2026
Isothermal and adiabatic processes for ideal gases are explicitly listed, alongside expansion, calorimetry and heat transfer.
Preparation note
Main and Advanced scope should not be assumed identical from a combined coaching outline. Keep both official documents available.
Concept group
Second law and reversibility
JEE Main 2026
The second law and reversible and irreversible processes are explicitly listed.
JEE Advanced 2026
Reversibility, the Carnot engine and efficiency are explicitly listed.
Preparation note
Efficiency of any real engine is bounded by the reversible Carnot value.
Concept group
Related topics named only in Advanced Thermal Physics
JEE Main 2026
Calorimetry and heat transfer sit under Properties of Solids and Liquids; ideal-gas kinetic ideas sit under Kinetic Theory of Gases.
JEE Advanced 2026
Ideal-gas specific heats and thermal-radiation laws are explicitly listed.
Preparation note
Follow the links to Thermal Properties and Kinetic Theory of Gases rather than assuming this page silently absorbs them.

Sources: JEE Main 2026 syllabus and JEE (Advanced) 2026 syllabus, both linked in the sources section below.

Before this chapter

Prerequisites: what you should know before Thermodynamics

Prerequisites: what you should know before Thermodynamics
Prerequisite	You are ready if you can…	If not, repair this first
State variables	Distinguish pressure, volume and temperature as state variables.	Revise the ideal-gas equation and units of pressure, volume and temperature.
Mechanical work and graphs	Calculate mechanical work and read area under a graph.	Revise work-energy reasoning from Work, Energy and Power.
Moles and the gas equation	Use moles and the ideal-gas equation PV = nRT.	Revise Kinetic Theory of Gases fundamentals.
State versus path	Distinguish a state function from a path-dependent transfer.	Revise the difference between a property of a state and a process quantity.
Reading a P-V graph	Read the direction of a cyclic process on a P-V graph and its signed enclosed area.	Practise sketching and reading simple isothermal, isobaric and isochoric lines.

Prerequisite
State variables
You are ready if you can…
Distinguish pressure, volume and temperature as state variables.
If not, repair this first
Revise the ideal-gas equation and units of pressure, volume and temperature.
Prerequisite
Mechanical work and graphs
You are ready if you can…
Calculate mechanical work and read area under a graph.
If not, repair this first
Revise work-energy reasoning from Work, Energy and Power.
Prerequisite
Moles and the gas equation
You are ready if you can…
Use moles and the ideal-gas equation PV = nRT.
If not, repair this first
Revise Kinetic Theory of Gases fundamentals.
Prerequisite
State versus path
You are ready if you can…
Distinguish a state function from a path-dependent transfer.
If not, repair this first
Revise the difference between a property of a state and a process quantity.
Prerequisite
Reading a P-V graph
You are ready if you can…
Read the direction of a cyclic process on a P-V graph and its signed enclosed area.
If not, repair this first
Practise sketching and reading simple isothermal, isobaric and isochoric lines.

This is a readiness check, not a weightage or scoring-priority list.

Concepts in this chapter

1. Draw the system boundary

Heat and work are energy transfers across a boundary you choose; internal energy belongs to the system's state.

Choose the gas, gas plus piston, or another stated system. Heat and work are energy transfers across that boundary. Internal energy belongs to the state of the system.

2. Separate state from path

Internal-energy change depends only on the endpoints; heat and work depend on the process connecting them.

Internal-energy change depends only on the endpoints. Heat and work depend on the process connecting them. Two paths can reach the same final state with different Q and W but the same Delta U.

3. Declare the sign convention

State the convention before combining results from another source.

This page uses Q = Delta U + W, where Q is heat supplied to the system and W is work done by the system. If another source defines work done on the system as positive, translate the equation before combining results.

4. Identify the process constraint

Isothermal fixes temperature.
Adiabatic fixes heat transfer at zero.
Isochoric fixes volume, so boundary work is zero.
Isobaric fixes pressure.

5. Apply the material model

For an ideal gas, internal energy depends only on temperature. Relations such as PV^gamma = constant require additional reversible adiabatic and ideal-gas conditions.

6. Interpret cycles and engines

A complete cycle returns the system to its initial state, so Delta U = 0 over the cycle.

Over a complete cycle, the system returns to its initial state, so Delta U = 0. Net work is the signed area enclosed on the P-V diagram. Efficiency compares useful work output with heat absorbed, and can never exceed the reversible Carnot value between the same reservoir temperatures.

Choose the method before calculating

Six decisions cover most Thermodynamics questions. Select the representation before any algebra.

Choose the method before calculating
Given information	First move	Governing idea
Initial and final states only	Ask whether a state-function change is enough	Delta U is endpoint dependent
A path on a P-V graph	Determine signed area	W = integral(P dV) for quasistatic boundary work
Constant temperature, ideal gas	Test Delta U = 0	Isothermal ideal-gas process
Insulated or explicitly adiabatic	Set Q = 0	First law plus process relation
Closed cycle	Set total Delta U = 0	Net heat equals net work by the system
Heat engine between two reservoirs	Identify Q_H, Q_C and W	Energy balance and efficiency

Given information
Initial and final states only
First move
Ask whether a state-function change is enough
Governing idea
Delta U is endpoint dependent
Given information
A path on a P-V graph
First move
Determine signed area
Governing idea
W = integral(P dV) for quasistatic boundary work
Given information
Constant temperature, ideal gas
First move
Test Delta U = 0
Governing idea
Isothermal ideal-gas process
Given information
Insulated or explicitly adiabatic
First move
Set Q = 0
Governing idea
First law plus process relation
Given information
Closed cycle
First move
Set total Delta U = 0
Governing idea
Net heat equals net work by the system
Given information
Heat engine between two reservoirs
First move
Identify Q_H, Q_C and W
Governing idea
Energy balance and efficiency

Formula sheet

Pressure times volume equals n R T, using absolute temperature.
Ideal-gas equation of state relating pressure, volume, amount and temperature.
P
pressure (Pa)
V
volume (m^3)
n
amount of gas (mol)
T
absolute temperature (K)
Use when — Describing equilibrium states of an ideal gas.
Common trap — Using Celsius in place of kelvin.
Heat supplied equals the change in internal energy plus work done by the system.
First law with heat supplied to the system positive and work done by the system positive.
Q
heat supplied to the system (J)
Delta U
change in internal energy (J)
W
work done by the system (J)
Use when — Closed-system energy accounting under this convention.
Common trap — Combining it with a source using work-on-system positive without translating signs.
Work equals the integral of pressure with respect to volume from the initial to the final volume.
Quasistatic boundary work done by the gas along a known pressure-volume path.
P
pressure (Pa)
V
volume (m^3)
W
work (J)
Use when — The pressure as a function of volume, P(V), is known along the path.
Common trap — Using endpoint pressure without a constant-pressure condition.
Change in internal energy equals n times C V times the change in temperature.
Internal-energy change of a fixed amount of ideal gas.
n
amount of gas (mol)
C_V
molar heat capacity at constant volume (J mol^-1 K^-1)
Delta T
temperature change (K)
Use when — Fixed ideal-gas amount with an applicable C_V.
Common trap — Applying it to a phase change or an unspecified non-ideal system.
Work equals pressure times the change in volume.
Boundary work done by the gas at constant pressure.
P
constant pressure (Pa)
V_1, V_2
initial and final volume (m^3)
Use when — Pressure is constant throughout the process.
Common trap — Using final pressure for a varying-pressure path.
Work equals n R T times the natural log of the ratio of final volume to initial volume.
Reversible isothermal work done by an ideal gas.
n
amount of gas (mol)
T
constant absolute temperature (K)
V_1, V_2
initial and final volume (m^3)
Use when — Constant temperature, ideal gas, quasistatic path.
Common trap — Using it for a general isothermal non-ideal path.
Pressure times volume to the power gamma remains constant.
Reversible adiabatic ideal-gas relation.
gamma
ratio C_P over C_V
Use when — Quasistatic adiabatic ideal gas with applicable constant heat capacities.
Common trap — Treating every insulated process as reversible.
Efficiency equals net work output over heat absorbed, which equals one minus the ratio of rejected heat to absorbed heat.
Heat-engine efficiency over a complete cycle.
W_out
net work output (J)
Q_H
heat absorbed from the hot reservoir (J)
Q_C
heat rejected to the cold reservoir (J)
Use when — Analysing a cyclic engine.
Common trap — Using total heat magnitude in the denominator instead of heat absorbed.
Carnot efficiency equals one minus the ratio of cold-reservoir temperature to hot-reservoir temperature, both absolute.
Maximum possible efficiency of a reversible engine between two reservoirs.
T_C
cold-reservoir absolute temperature (K)
T_H
hot-reservoir absolute temperature (K)
Use when — Comparing a real engine's efficiency against the reversible limit.
Common trap — Using Celsius or claiming a real engine must attain the Carnot value.

Unit bridge

Use the equivalent form to check any result before accepting it.

Unit bridge
Quantity	SI unit	Equivalent form that helps checking
Pressure	pascal, Pa	N/m^2
Volume	m^3	—
Temperature	kelvin, K	Never substitute Celsius directly into PV = nRT
Heat and work	joule, J	Both are energy in transfer, not stored quantities
Internal energy	joule, J	State function, depends only on endpoints
Efficiency	dimensionless	Ratio of work output to heat input

Quantity
Pressure
SI unit
pascal, Pa
Equivalent form that helps checking
N/m^2
Quantity
Volume
SI unit
m^3
Equivalent form that helps checking
—
Quantity
Temperature
SI unit
kelvin, K
Equivalent form that helps checking
Never substitute Celsius directly into PV = nRT
Quantity
Heat and work
SI unit
joule, J
Equivalent form that helps checking
Both are energy in transfer, not stored quantities
Quantity
Internal energy
SI unit
joule, J
Equivalent form that helps checking
State function, depends only on endpoints
Quantity
Efficiency
SI unit
dimensionless
Equivalent form that helps checking
Ratio of work output to heat input

Worked examples

Worked reasoning: an ideal gas expands reversibly at constant temperature. What happens to Q, W and Delta U?

Answer: Delta U = 0, so Q = W, and positive heat must enter the gas to replace the energy transferred out as work.

For an ideal gas, internal energy depends only on temperature.
Constant temperature gives Delta U = 0.
With the declared convention, Q = Delta U + W, so Q = W.
During expansion, W = n R T ln(V_2 / V_1) > 0 because V_2 > V_1.
Therefore positive heat must enter the gas to replace the energy transferred out as work.

Common mistakes and what they actually indicate

Treating heat as stored inside a system
Knowledge gap
Why it happens
Heat is energy in transfer; internal energy is a state function.
How it is corrected
Ask whether the quantity belongs to a state or to a process before naming it.
Calling any constant-temperature process adiabatic
Knowledge gap
Why it happens
Isothermal and adiabatic are different constraints.
How it is corrected
State which quantity is fixed: temperature for isothermal, heat transfer for adiabatic.
Using PV^gamma = constant for an irreversible adiabatic change
Decision / selection error
Why it happens
The familiar relation needs the reversible ideal-gas model.
How it is corrected
Check reversibility and the ideal-gas assumption before applying it.
Forgetting the sign convention
Recall gap
Why it happens
Mixing a work-by-system convention with a work-on-system source silently flips a sign.
How it is corrected
Write the verbal definition of Q and W first.
Setting Delta U = 0 at one intermediate step of a cyclic process
Execution error
Why it happens
Delta U = 0 holds for the complete cycle, not for every intermediate leg.
How it is corrected
Apply the zero-Delta-U condition only across the full closed loop.
Mixing Physics and Chemistry Thermodynamics intent
Decision / selection error
Why it happens
Gas-process energy accounting and reaction thermodynamics are different entities.
How it is corrected
Keep this page's system-boundary model separate from Chemistry Thermodynamics.

Diagnose your Thermodynamics weakness with evidence

One wrong answer does not always reveal the cause. Use the smallest Preparation Intelligence v1.1 label supported by what the student actually did.

Diagnose your Thermodynamics weakness with evidence
PI v1.1 label	Evidence in Thermodynamics	Repair action	Retest
Knowledge Gap	System, state, path, heat, work, internal energy, or reversibility is not understood.	Rebuild the concept with a definition, relationship and one contrasting example.	Explain the idea aloud, then solve one direct and one contrast question.
Recall Gap	A process relation or efficiency definition was not retrieved.	Use closed-book retrieval with meaning and conditions, not formula copying.	Recall again after a delay, then apply in a fresh problem.
Execution Error	Graph area, logarithm, sign, unit, or algebra failed after correct setup.	Mark the exact failed step and add a direction, dimension or limit check.	Repeat the same method on a numerically different question.
Decision / Selection Error	The wrong system, sign convention, gas model, or process relation was selected.	Practise the method selector before calculation. Write the rejected alternative and why it was rejected.	Solve a mixed set where the process is not named in the prompt.
Needs Review	The response is blank, partially correct, ambiguous, guessed or inconsistent, so the evidence does not support a stable label.	Review the work or collect another response before assigning a gap.	Ask a short discriminating question that separates understanding, recall, execution and selection.

PI v1.1 label
Knowledge Gap
Evidence in Thermodynamics
System, state, path, heat, work, internal energy, or reversibility is not understood.
Repair action
Rebuild the concept with a definition, relationship and one contrasting example.
Retest
Explain the idea aloud, then solve one direct and one contrast question.
PI v1.1 label
Recall Gap
Evidence in Thermodynamics
A process relation or efficiency definition was not retrieved.
Repair action
Use closed-book retrieval with meaning and conditions, not formula copying.
Retest
Recall again after a delay, then apply in a fresh problem.
PI v1.1 label
Execution Error
Evidence in Thermodynamics
Graph area, logarithm, sign, unit, or algebra failed after correct setup.
Repair action
Mark the exact failed step and add a direction, dimension or limit check.
Retest
Repeat the same method on a numerically different question.
PI v1.1 label
Decision / Selection Error
Evidence in Thermodynamics
The wrong system, sign convention, gas model, or process relation was selected.
Repair action
Practise the method selector before calculation. Write the rejected alternative and why it was rejected.
Retest
Solve a mixed set where the process is not named in the prompt.
PI v1.1 label
Needs Review
Evidence in Thermodynamics
The response is blank, partially correct, ambiguous, guessed or inconsistent, so the evidence does not support a stable label.
Repair action
Review the work or collect another response before assigning a gap.
Retest
Ask a short discriminating question that separates understanding, recall, execution and selection.

Contributing factors such as rushed reading, weak diagram use, notation confusion or time pressure may be recorded separately. They do not replace the primary label.

How to review a Thermodynamics previous-year question

For every verified question, record five things.

How to review a Thermodynamics previous-year question
Review field	What to record
Required concept	Zeroth law, first law, process type, second law, reversibility, cycles or engines.
Representation	P-V graph, algebraic process relation, energy balance or efficiency.
Decisive choice	The system, sign convention and process relation that made the solution correct.
Error evidence	Knowledge Gap, Recall Gap, Execution Error, Decision / Selection Error or Needs Review.
Retest rule	The type of fresh question to attempt and when to attempt it.

Review field
Required concept
What to record
Zeroth law, first law, process type, second law, reversibility, cycles or engines.
Review field
Representation
What to record
P-V graph, algebraic process relation, energy balance or efficiency.
Review field
Decisive choice
What to record
The system, sign convention and process relation that made the solution correct.
Review field
Error evidence
What to record
Knowledge Gap, Recall Gap, Execution Error, Decision / Selection Error or Needs Review.
Review field
Retest rule
What to record
The type of fresh question to attempt and when to attempt it.

Use official previous papers without inventing chapter trends

Official repositories

Use official papers and record the system, endpoints, path, process constraint, gas model, sign convention, and first failed decision.

FAQ

Thermodynamics — questions

Straight answers about how Rank Sarthi fits into serious exam preparation.

With heat supplied to the system positive and work done by the system positive, it is Q = Delta U + W.

Sources and provenance

Evidence boundary: the syllabus mapping is tied to the official 2026 JEE Main and JEE Advanced documents. No chapter weightage, question frequency or forecast is asserted.

Syllabus for JEE (Main) 2026 — National Testing Agency (NTA) · official · checked 27 August 2026
JEE (Advanced) 2026 Syllabus (PDF) — JEE (Advanced) 2026, organising institute IIT Roorkee · official-pdf · checked 27 August 2026
JEE (Advanced) official past question-paper archive — JEE (Advanced), organising institute · official · checked 27 August 2026
JEE (Main) official site, Question Papers section — National Testing Agency (NTA) · official · checked 27 August 2026

Last updated: 7 September 2026

Contributor requirements for this page

Author: a JEE Physics educator or academic content specialist experienced in Thermal Physics and thermodynamic problem solving.
Academic reviewer: postgraduate qualification in Physics, Mechanical Engineering, or a closely relevant discipline, with documented JEE Thermal Physics teaching and assessment experience, covering system boundaries, sign conventions, ideal-gas processes, first and second laws, cycles, Carnot limits, and Main versus Advanced scope.
Independent checker: a physics educator or subject editor who verifies equations, sign conventions, process assumptions, SI units, worked reasoning and mobile rendering separately from the author.
No contributor is named on this page until their identity and qualification are verified, so no author, reviewer or rating is displayed yet.