Thermodynamics is Chapter 11 of NCERT Class 11 Physics and Unit 8 in both the NEET UG and JEE Main syllabi. These notes cover thermal equilibrium and the zeroth law, heat, internal energy and work, the first law, specific heat capacity, thermodynamic processes, the second law, reversible processes and the Carnot engine, with a free PDF.
Thermodynamics Notes for NEET and JEE Main
Section-wise key points from NCERT Chapter 11, process tables, high-yield facts and common mistakes for 2027 revision.
Thermodynamics at a Glance
| Item | Details |
|---|---|
| NEET UG syllabus unit | Unit 8: Thermodynamics |
| JEE Main syllabus unit | Unit 8: Thermodynamics |
| Exams that cover it | Both NEET UG and JEE Main (Paper 1, B.E./B.Tech.) |
| NCERT textbook | Physics Part II, Class 11, Chapter 11 (rationalised book) |
| Syllabus topics | Thermal equilibrium, zeroth law of thermodynamics and the concept of temperature; heat, work and internal energy; first law of thermodynamics; isothermal and adiabatic processes; second law of thermodynamics; reversible and irreversible processes |
| Official chapter | NCERT Chapter 11 |
| Notes PDF | View PDF |
Topics follow the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025 and the JEE Main 2026 syllabus published by NTA. Both stay the reference until the 2027 syllabi are out. Full lists: NEET syllabus and JEE Main syllabus.
Heat as Energy and What Thermodynamics Studies
Thermodynamics deals with heat, temperature and the inter-conversion of heat and other forms of energy. It is a macroscopic science that uses bulk variables such as pressure, volume, temperature, mass and composition.
- Heat was once thought to be an invisible fluid called caloric.
- Benjamin Thomson (Count Rumford, 1798) found that boring a brass cannon produced heat that depended on the work done, not on the sharpness of the drill, showing heat is a form of energy.
- Mechanics deals with the motion of a body as a whole; thermodynamics deals with its internal macroscopic state.
- A bullet fired from a gun is not hotter because of its speed; temperature relates to internal disordered motion, not to motion of the centre of mass.
Thermal Equilibrium and the Zeroth Law
A system is in thermodynamic equilibrium when the macroscopic variables that describe it do not change with time.
- An adiabatic wall is insulating and does not allow heat to flow; a diathermic wall is conducting and allows heat flow.
- Two systems in contact through a diathermic wall change until they reach thermal equilibrium, when their temperatures are equal.
- Zeroth law: two systems in thermal equilibrium with a third system separately are in thermal equilibrium with each other.
- R.H. Fowler formulated the zeroth law in 1931, after the first and second laws had been named.
- The zeroth law leads to the concept of temperature: the quantity that is equal for systems in thermal equilibrium.
Heat, Internal Energy and Work
Internal energy U is the sum of the kinetic and potential energies of the molecules, measured in the frame where the centre of mass of the system is at rest.
| Quantity | Nature | Depends on path? |
|---|---|---|
| Internal energy U | State variable of the system | No; depends only on the state |
| Heat Q | Energy in transit due to a temperature difference | Yes |
| Work W | Energy transfer by other means, such as moving a piston | Yes |
- The kinetic energy of the system moving as a whole is not part of U.
- If intermolecular forces in a gas are neglected, U is the sum of the kinetic energies of its random translational, rotational and vibrational motions.
- ‘A gas has a certain amount of heat’ is meaningless; ‘a gas has a certain internal energy’ is meaningful.
First Law of Thermodynamics
First law: ΔQ = ΔU + ΔW, where ΔQ is heat supplied to the system, ΔU the change in internal energy and ΔW the work done by the system. It is the law of conservation of energy applied to a system.
| Sign | Meaning |
|---|---|
| Q > 0 | Heat is added to the system |
| Q < 0 | Heat is removed from the system |
| W > 0 | Work is done by the system |
| W < 0 | Work is done on the system |
- ΔQ and ΔW depend on the path, but ΔQ − ΔW = ΔU does not.
- Work against constant pressure: ΔW = PΔV, so ΔQ = ΔU + PΔV.
- Boiling 1 g of water at atmospheric pressure: ΔQ = 2256 J (latent heat), ΔW = P(V_g − V_l) ≈ 169.2 J, so ΔU ≈ 2086.8 J. Most of the heat raises the internal energy.
- If ΔU = 0 (for example, isothermal expansion of an ideal gas), ΔQ = ΔW.
Specific Heat Capacity and Cp − Cv = R
Specific heat capacity s = (1/m)(ΔQ/ΔT), unit J kg⁻¹ K⁻¹; molar specific heat capacity C = (1/μ)(ΔQ/ΔT), unit J mol⁻¹ K⁻¹.
- Heat capacity depends on the process, so gases have two molar specific heats: Cp at constant pressure and Cv at constant volume.
- For an ideal gas, Cp − Cv = R (from the first law and PV = RT for one mole).
- For solids, the law of equipartition of energy gives C = 3R, which agrees with experiment at ordinary temperatures (carbon is an exception) but breaks down at low temperatures.
- One calorie is the heat needed to raise 1 g of water from 14.5 °C to 15.5 °C; 1 cal = 4.186 J.
- Specific heat of water: 4186 J kg⁻¹ K⁻¹; it varies slightly with temperature between 0 and 100 °C.
- Since heat is energy measured in joules, the term ‘mechanical equivalent of heat’ is now superfluous.
State Variables, Equation of State and Quasi-static Processes
Equilibrium states are described by state variables such as pressure, volume, temperature and mass. The relation between them is the equation of state, such as PV = μRT for an ideal gas.
| Extensive variables | Intensive variables |
|---|---|
| Depend on the size of the system; halve when the system is split into two equal parts | Do not change when the system is split |
| Internal energy U, volume V, total mass M | Pressure P, temperature T, density ρ |
- Free expansion of a gas into vacuum and an explosive chemical reaction pass through non-equilibrium states that state variables cannot describe.
- A quasi-static process is infinitely slow, so the system stays in thermal and mechanical equilibrium with its surroundings at every stage.
- In a quasi-static process the pressure and temperature of the surroundings differ from the system’s only infinitesimally.
Thermodynamic Processes of an Ideal Gas
NCERT treats these processes for an ideal gas taken quasi-statically.
| Process | Condition | Work done by the gas | First-law result |
|---|---|---|---|
| Isothermal | T constant; PV = constant (Boyle’s law) | W = μRT ln(V₂/V₁) | ΔU = 0, so Q = W |
| Adiabatic | No heat exchange; PV^γ = constant | W = μR(T₁ − T₂)/(γ − 1) | Q = 0, so W = −ΔU |
| Isochoric | V constant | W = 0 | Q = ΔU, set by Cv |
| Isobaric | P constant | W = P(V₂ − V₁) = μR(T₂ − T₁) | Q goes partly to ΔU and partly to W, set by Cp |
| Cyclic | System returns to its initial state | W = net heat absorbed | ΔU = 0, so Q = W |
- In isothermal expansion the gas absorbs heat and does work; in isothermal compression work is done on the gas and heat is released.
- γ is the ratio of specific heats at constant pressure and constant volume, Cp/Cv.
- In adiabatic expansion the gas does work and cools (T₂ < T₁); in adiabatic compression its temperature rises.
- Heat can flow in an isothermal process because of an infinitesimal temperature difference between the system and the reservoir.
Second Law, Reversible and Irreversible Processes
The second law rules out many processes that the first law would allow. It limits the efficiency of a heat engine and the coefficient of performance of a refrigerator.
| Statement | What it says |
|---|---|
| Kelvin-Planck | No process is possible whose sole result is the absorption of heat from a reservoir and the complete conversion of the heat into work |
| Clausius | No process is possible whose sole result is the transfer of heat from a colder object to a hotter object |
- The two statements are completely equivalent.
- In simple terms, no heat engine can have efficiency 1 and no refrigerator can have an infinite coefficient of performance.
- A process is reversible if both the system and the surroundings can be returned to their original states with no other change anywhere.
- A process is reversible only if it is quasi-static and has no dissipative effects such as friction or viscosity.
- Spontaneous processes in nature are irreversible: heat spreading from the hot base of a vessel, free expansion of a gas, combustion, diffusion of gas leaking from a cylinder.
- Irreversibility comes mainly from non-equilibrium states and from dissipative effects.
Carnot Engine and Its Efficiency
A Carnot engine is a reversible engine working between two temperatures, a source at T₁ and a sink at T₂. Sadi Carnot, a French engineer, studied it in 1824.
| Step of the Carnot cycle | Process | Heat |
|---|---|---|
| 1 → 2 | Isothermal expansion at T₁ | Absorbs Q₁ from the hot reservoir |
| 2 → 3 | Adiabatic expansion, T₁ to T₂ | None |
| 3 → 4 | Isothermal compression at T₂ | Releases Q₂ to the cold reservoir |
| 4 → 1 | Adiabatic compression, T₂ to T₁ | None |
- Efficiency η = 1 − Q₂/Q₁ = 1 − T₂/T₁.
- Q₁/Q₂ = T₁/T₂ for a Carnot engine, a universal relation used to define the thermodynamic temperature scale.
- Carnot’s theorem: no engine working between the same two temperatures can be more efficient than a Carnot engine, and its efficiency does not depend on the working substance.
- Running the Carnot cycle in reverse gives a reversible refrigerator.
Most-asked Facts for NEET and JEE Main: Thermodynamics
Revise these one-line facts from the NCERT chapter just before the exam.
- Zeroth law gives the concept of temperature; R.H. Fowler stated it in 1931.
- Internal energy is a state variable; heat and work are not.
- First law: ΔQ = ΔU + ΔW (W done by the system).
- For an ideal gas, Cp − Cv = R.
- For solids, molar heat capacity ≈ 3R at ordinary temperatures.
- 1 cal = 4.186 J; specific heat of water = 4186 J kg⁻¹ K⁻¹.
- Isothermal: ΔU = 0 and Q = W = μRT ln(V₂/V₁).
- Adiabatic: Q = 0, PV^γ = constant, W = μR(T₁ − T₂)/(γ − 1).
- Isochoric: W = 0; isobaric: W = P(V₂ − V₁).
- Cyclic process: ΔU = 0.
- U, V and M are extensive; P, T and ρ are intensive.
- Kelvin-Planck and Clausius statements of the second law are equivalent.
- A reversible process must be quasi-static and free of dissipative effects.
- Carnot efficiency η = 1 − T₂/T₁, independent of the working substance.
Common Mistakes to Avoid
Watch for these mix-ups when you answer questions on this chapter.
- Mixing sign conventions for work. In NCERT’s ΔQ = ΔU + ΔW, ΔW is work done by the system; work done on it is negative.
- Saying no heat flows in an isothermal process. Heat flows; it is the temperature, and so the internal energy of an ideal gas, that stays constant.
- Confusing adiabatic with isothermal. Adiabatic means no heat exchange; the temperature changes.
- Calling heat or work a property of a state. Only internal energy and other state variables are; heat and work depend on the path.
- Using Celsius temperatures in η = 1 − T₂/T₁. Use kelvin.
- Thinking a perfect engine is ruled out by the first law. It is the second law (Kelvin-Planck statement) that forbids it.
- Assuming any slow process is reversible. It must also be free of friction, viscosity and other dissipative effects.
- Linking temperature to the speed of the whole body. A fast bullet is not hotter because of its speed.
Thermodynamics Notes PDF and Related Pages
Download the AglaSem notes for Thermodynamics (key notes on thermometry, calorimetry, the laws of thermodynamics, thermodynamic processes, the Carnot engine and heat transfer, with important questions and answers) free: View PDF
- All chapters: NEET and JEE Main Physics notes
- Full topic lists: NEET syllabus and JEE Main syllabus
- Practice: NEET question papers and JEE Main question papers
- Source text: NCERT Chapter 11 (NCERT, Class 11 Physics Part II)
Physics Notes for NEET and JEE Main: All Chapters
- Units and Measurement
- Motion in a Straight Line
- Motion in a Plane
- Laws of Motion
- Work, Energy and Power
- Systems of Particles and Rotational Motion
- Gravitation
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- Mechanical Properties of Fluids
- Thermodynamics
- Kinetic Theory of Gases
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- Dual Nature of Radiation and Matter
- Atoms
- Nuclei
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All chapters with key points and notes PDFs are on the Physics notes for NEET and JEE Main page.
Thermodynamics Notes for NEET and JEE Main FAQs
Which NCERT chapter is Thermodynamics?
It is Chapter 11 of the rationalised NCERT Class 11 Physics Part II textbook. It is Unit 8 in both the NEET UG 2026 and JEE Main 2026 syllabi.
Is Thermodynamics in the NEET and JEE Main 2027 syllabus?
Yes. Thermal equilibrium, the zeroth law, heat, work and internal energy, the first law, isothermal and adiabatic processes and the second law with reversible and irreversible processes are in both syllabi. They remain the reference until the 2027 syllabi are released.
What is the difference between isothermal and adiabatic processes?
In an isothermal process the temperature stays constant and heat flows in or out; for an ideal gas ΔU = 0 and Q = W. In an adiabatic process no heat is exchanged, so the work done comes from internal energy and the temperature changes.
Why can no engine be more efficient than a Carnot engine?
A Carnot engine is reversible. If an irreversible engine working between the same two temperatures were more efficient, coupling it to a reversed Carnot engine would convert heat fully into work, which the Kelvin-Planck statement of the second law forbids.
Are these notes enough for Thermodynamics questions?
No. Read NCERT Chapter 11 first, then use these notes and the PDF for revision, and practise numerical questions on work done in processes and Carnot efficiency from previous year papers.