JEE Main Physics · Thermodynamics PYQ
JEE Main Thermodynamics Previous Year Questions (2002–2025)
Thermodynamics & Kinetic Theory is the single highest-weightage chapter in JEE Main Physics — ~7% across 10 years, with 2 questions per session routinely. The chapter is formula-driven: learn the six standard processes (isothermal, adiabatic, isobaric, isochoric, cyclic, polytropic) and the PYQs become template matching.
Thermodynamics PYQs from 2002 to 2025, split across Laws of Thermodynamics, Heat Engines, PV diagrams and Kinetic Theory. Every question is tagged with the process type and the law being tested, so you can drill down to exactly the pattern you're weak on.
Thermodynamics & Kinetic Theory at a Glance
Key Sub-Topics & What's Tested
First Law of Thermodynamics
dU = dQ - dW convention, work done in isothermal/adiabatic/isobaric/isochoric processes, internal energy of ideal gas.
Standard Thermodynamic Processes
Isothermal (PV = const), Adiabatic (PV^γ = const), Isobaric, Isochoric, Cyclic, Polytropic. Work, heat and ΔU for each.
Second Law & Heat Engines
Carnot cycle, efficiency η = 1 - T2/T1, refrigerator COP, Kelvin-Planck and Clausius statements.
Kinetic Theory of Gases
RMS / average / most probable speed, mean free path, degrees of freedom, equipartition, Maxwell-Boltzmann distribution.
Specific Heat Capacities
Cp, Cv, γ = Cp/Cv, Mayer's relation, variation with degrees of freedom for monoatomic/diatomic/triatomic gases.
PV Diagrams
Reading and drawing PV diagrams, work as area under curve, cyclic processes and net work calculation.
Entropy (basic)
Entropy change in reversible/irreversible processes, entropy of universe in isolated systems, qualitative second-law statements.
Heat Transfer
Conduction, convection, radiation (Stefan's law), thermal resistance, Newton's law of cooling.
Question Type Distribution
| Question Type | Share (approx) | Example Pattern |
|---|---|---|
| Process Identification | 30% | Given a PV diagram segment, identify the process type and compute work done. |
| First Law Application | 25% | Gas undergoes a cyclic process — find net heat absorbed per cycle. |
| Carnot / Engine Efficiency | 15% | Carnot engine between temperatures T1 and T2 — find efficiency or heat rejected. |
| Kinetic Theory (speeds, degrees of freedom) | 20% | Compare rms speeds of H2 and O2 at the same temperature. |
| Specific Heat / Mayer's Relation | 10% | Given Cv for a diatomic gas, find γ and Cp. |
How to Solve Thermodynamics & Kinetic Theory PYQs
- 1Memorise the four standard-process table. For each (isothermal, adiabatic, isobaric, isochoric), know W, Q, ΔU in terms of n, R, T. This single table unlocks ~60% of PYQs instantly.
- 2Use the PV diagram as your primary tool. Work = area under curve. Cyclic process net work = enclosed area. Draw the diagram before doing any integration.
- 3Sign convention discipline. Work done BY the gas is positive in physics (dU = dQ - dW). Use one convention consistently.
- 4For kinetic theory, use v_rms = √(3kT/m). Ratios of speeds for two gases at same T reduce to √(m2/m1). Most kinetic theory PYQs collapse to this ratio.
- 5Carnot questions are direct. η = 1 - T2/T1 (absolute temperatures). If η is given, back-solve for T. These are free marks — don't over-think them.
Common Mistakes That Cost Marks
- Using Celsius in efficiency formulas. Carnot efficiency requires absolute (Kelvin) temperatures. Celsius will wreck your answer.
- Confusing isothermal and adiabatic work formulas. Isothermal: W = nRT ln(V2/V1). Adiabatic: W = (P1V1 - P2V2) / (γ - 1). Using the wrong one is the most common Thermodynamics error.
- Forgetting that ΔU = 0 in cyclic processes. Internal energy is a state function — in any cycle, net ΔU = 0, so net Q = net W.
- Mixing up Cp and Cv usage. Constant pressure → Cp. Constant volume → Cv. Heat at constant pressure ≠ heat at constant volume.
- Missing the difference between reversible and irreversible processes. Efficiency formulas are for reversible (Carnot) engines. Real engines have lower efficiency.
Related JEE Main Practice
Frequently asked questions
How many Thermodynamics PYQs should I solve?
Target 80–100 Thermodynamics PYQs across 2010–2025 for P1 chapter mastery. Given its #1 weightage rank (~7%), this is one chapter where over-practising pays off.
Is Thermodynamics in Physics separate from Thermodynamics in Chemistry?
Yes, though the underlying laws are the same. JEE Physics Thermodynamics focuses on gas processes, heat engines and kinetic theory. JEE Chemistry Thermodynamics focuses on enthalpy, entropy, Gibbs free energy and spontaneity. Both chapters are P1 in their respective subjects.
Do I need to memorise Maxwell-Boltzmann distribution formulas?
Not the full distribution formula. Memorise v_rms, v_avg and v_mp expressions — these three appear in PYQs. The detailed probability distribution is a coaching-level overkill for JEE Main.
What's a common trap in Carnot PYQs?
NTA often gives efficiency as a percentage but expects you to use it as a decimal. Convert η = 40% to η = 0.4 before substituting into 1 - T2/T1. Also, the Carnot formula uses absolute temperatures (Kelvin) only.
Are Heat Transfer questions part of Thermodynamics PYQs?
In the NCERT/NTA syllabus split, Heat Transfer (conduction, convection, radiation) is usually grouped with Thermal Physics. It contributes ~10% of the Thermodynamics PYQ bank in practice.
Do I need calculus for these PYQs?
Only for deriving work done in adiabatic and isothermal processes. PYQ answers mostly use pre-derived formulas — you integrate once to derive, then reuse the formula across problems.
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