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Laws of Thermodynamics is one of the most important and high-weightage chapters in the Thermal Physics unit of JEE Physics. It governs how heat, work, and internal energy interact, and it underpins engines, refrigerators, and the behaviour of gases under various processes. Because it blends conceptual understanding with reliable numerical application, JEE Laws of Thermodynamics questions appear consistently in both JEE Main and JEE Advanced. This chapter covers the first law of thermodynamics, thermodynamic processes (isothermal, adiabatic, isobaric, isochoric), work done by gases, heat capacities, and the second law with engine and refrigerator efficiency. JEE Main tests process-based numericals, while JEE Advanced often combines thermodynamics with kinetic theory and PV-diagram reasoning. Practising topic-wise JEE Questions helps you interpret PV diagrams quickly and apply the first law without confusion over sign conventions. A strong command of thermodynamics ties together kinetic theory and heat transfer, completing your mastery of the thermal physics unit.
A. Pressure varies inversely with volume of an ideal gas.
For an isothermal process,
$$PV=\text{constant}$$
So
$$P\propto\frac{1}{V}$$
A → III
B. Heat absorbed goes partly to increase internal energy and partly to do work.
That is true for isobaric process:
$$Q=\Delta U+W$$
Both $$ΔU$$ and W are generally nonzero.
B → IV
C. Heat is neither absorbed nor released.
That is adiabatic process:
Q=0
C → I
D. No work is done on or by gas.
In isochoric process volume is constant:
$$W=P\Delta V=0$$
D → II
Parameter | Details |
|---|---|
Topic Name | Laws of Thermodynamics |
Subject | Physics |
JEE Main Weightage | ~4–6% (2 questions on average) |
JEE Advanced Weightage | ~5–7% (often combined) |
Difficulty Level | Moderate to High |
Important Concepts | First Law, Thermodynamic Processes, Work by Gas, Heat Capacities, Second Law & Efficiency |
Recommended Practice Level | High – attempt 70+ mixed problems |
Concept | Importance | Difficulty Level | Frequently Asked In |
|---|---|---|---|
First Law of Thermodynamics | Very High | Moderate | JEE Main & Advanced |
Thermodynamic Processes & PV Diagrams | Very High | Moderate–High | JEE Main & Advanced |
Work Done by a Gas | Very High | Moderate | JEE Main & Advanced |
Heat Capacities (Cp, Cv) | High | Moderate | JEE Main |
Adiabatic & Isothermal Relations | High | Moderate–High | JEE Advanced |
Second Law, Carnot Engine & Efficiency | Very High | Moderate–High | JEE Main & Advanced |
Concept learning: Master the first law as energy conservation linking heat, work, and internal energy, and fix a consistent sign convention. Understand how each process appears on a PV diagram and what stays constant in each.
Formula revision: Keep work-done expressions for each process, adiabatic and isothermal relations, heat-capacity links, and efficiency formulas handy. Organised JEE Study Material helps you keep these process relations in one place for efficient revision.
Problem-solving techniques: Identify the process first, then choose the matching work and heat relations. Use the area under the PV curve for work done, and apply the first law to find the missing quantity.
Common mistakes: Sign errors in heat added versus work done, confusing isothermal and adiabatic relations, forgetting that internal energy depends only on temperature for an ideal gas, and misreading PV diagrams.
Exam strategy: Attempt single-process numericals first, then tackle cyclic-process and efficiency questions that require multiple steps.
Exam | Average Questions | Expected Marks |
|---|---|---|
JEE Main | 2 | 8 |
JEE Advanced | 2–3 (often combined) | 8–14 |
Laws of Thermodynamics is a consistently heavy chapter in JEE Main and a frequent source of combined problems in JEE Advanced, especially those linking processes with kinetic theory.
Reinforcing these methods with a timed JEE Mock Test helps you interpret PV diagrams and apply the first law quickly under pressure.
JEE Thermodynamics questions focus on the laws of thermodynamics, heat transfer, thermodynamic processes, work done by gases, entropy, and heat engine efficiency. These concepts are frequently tested in both JEE Main and JEE Advanced.
Yes. Thermodynamics is one of the most important Physics chapters for JEE Main 2026 and typically contributes 1–2 questions every year.
The first law of thermodynamics, isothermal and adiabatic processes, PV diagrams, heat engines, refrigerators, and Carnot engine efficiency are the most frequently tested topics.
Thermodynamics is considered moderately difficult because it combines conceptual understanding with numerical problem-solving and requires careful application of formulas and sign conventions.
JEE Advanced generally includes 2–3 questions from Thermodynamics, often combined with topics such as Kinetic Theory of Gases and Calorimetry.
To prepare Thermodynamics effectively, study the theory thoroughly, master thermodynamic processes, solve previous year JEE questions, and practise PV-diagram-based numerical problems regularly.
Students often make mistakes with sign conventions, confuse isothermal and adiabatic processes, use incorrect formulas, or misinterpret PV diagrams during calculations.
Key Thermodynamics formulas for JEE include the first law of thermodynamics, work done in different processes, efficiency of heat engines, Carnot efficiency, and ideal gas relations used in thermodynamic systems.
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