Watch Video
What are the 3 Laws of Thermodynamics?
Thermodynamics explores how energy changes form, controls temperature, and drives every physical and chemical process you encounter—making it a top scoring topic in JEE Physics. From boiling water to running an engine, thermodynamics connects theory to practical reality. Understanding core laws, formulas, and key definitions will help you solve numericals and interpret the logic behind energy transfers. Aspiring engineers preparing for JEE must focus on core principles, critical differences, and mistake-proofing their revision to excel in exams. Vedantu Physics teachers emphasise this chapter for both its conceptual depth and its straightforward scoring potential.
The word thermodynamics combines “thermo” (heat) and “dynamics” (movement/change) to describe the study of heat, work, and energy transformations. Central laws state how energy moves and explain why certain processes—like mixing hot and cold water—proceed only one way. In competitive exams, questions often test how well you can distinguish between process types or apply formulas like Q = mcΔT or ΔU = Q – W. Real-world applications range from air conditioners to chemical reactions, making this a vital area of physics for all JEE aspirants.
Fundamental Concepts of Thermodynamics
At its core, thermodynamics classifies everything into systems and surroundings to analyse energy exchange. For JEE, you must know critical terms, process types, and the importance of distinguishing between state function and path function—common areas of confusion in MCQs and theory questions.
- An isolated system exchanges neither energy nor matter with its surroundings.
- A closed system allows energy but not matter to pass through its boundary.
- An open system exchanges both energy and matter.
- Thermal equilibrium means no net heat flow between objects.
- State functions—like internal energy and entropy—depend only on initial and final states.
| Term | Definition |
|---|---|
| System | Region under study, e.g., a gas in a cylinder |
| Surroundings | Everything outside the system |
| State | Set of properties (P, V, T) describing the system |
| Process | How a system changes state (e.g., isothermal, adiabatic) |
| Equilibrium | No net change in macroscopic properties |
Laws of Thermodynamics Explained
The laws of thermodynamics form the backbone of JEE Physics, each law setting essential boundaries for all physical phenomena. Remembering these laws and their formulas is crucial in exams and practical problem solving.
- Zeroth Law of Thermodynamics: If object A is in thermal equilibrium with B, and B with C, then A is also in thermal equilibrium with C. This law defines temperature.
- First Law of Thermodynamics: ΔU = Q – W. Energy cannot be created or destroyed—only transformed. Here, ΔU is change in internal energy, Q is heat supplied, W is work done by the system.
- Second Law of Thermodynamics: No engine can convert all heat into work; entropy (disorder) of an isolated system always increases.
- Third Law: Entropy of a perfect crystal at absolute zero is zero.
Key Thermodynamics Formulas and Equations
Critical for JEE Main, these equations connect core laws and allow rapid calculations in diverse numericals. Always use SI units and double-check for proper sign convention. Direct application during mock tests enables fast MCQ-solving.
- First law: ΔU = Q – W
- Work done by gas in an isothermal process: W = nRT ln(V2/V1)
- For adiabatic process: PVγ = constant; γ = Cp/Cv
- Heat added at constant pressure: Q = nCpΔT
- Relation for efficiency of a Carnot engine: η = 1 – (Tc/Th)
Work and heat are path functions; internal energy and entropy are state functions. For a system returning to its initial state, ΔU = 0 but Q and W may not be zero. Practice distinguishing such scenarios in JEE numericals.
Thermodynamics in JEE: Processes, Pitfalls, and Strategy
Much of JEE’s thermodynamics portion focuses on differentiating process types, interpreting PV diagrams, and avoiding classic mistakes. Directly compare isothermal and adiabatic processes using equations and PV graphs to clarify concepts.
- In isothermal processes, temperature (T) stays constant.
- For adiabatic expansion, no heat is exchanged (Q = 0).
- Entropy increases in all real irreversible processes; remains constant in reversible adiabatic ones.
- Sign convention: Work done by system is positive (JEE standard).
- Don’t confuse state and path functions while selecting answers during revision or mock tests.
Graphical questions often require drawing PV and TS diagrams. For an isothermal curve, the PV graph is a hyperbola; for adiabatic, it’s steeper. Use work done during adiabatic expansion and Carnot engine examples for practice.
- Most common JEE errors:
- Mixing up isothermal and adiabatic conditions
- Sign mistakes in work (ΔU vs. Q vs. W)
- Assuming entropy can decrease in an isolated system
- Using incorrect constants or units
Sample Example: Calculate work done when 1 mol of ideal gas expands isothermally at 300 K from 2 L to 4 L.
W = nRT ln(V2/V1);
W = 1 × 8.314 × 300 × ln(4/2) = 1 × 8.314 × 300 × 0.693 ≈ 1726 J.
Linking theory to practice, major applications of thermodynamics include car engines, refrigerators, and even predicting chemical reaction spontaneity. For more advanced connections, explore kinetic theory of gases, specific heats, and energy transformations that directly relate to thermal concepts.
- Test your readiness with thermodynamics MCQs
- Review thermodynamics revision notes
- Compare isothermal vs adiabatic process differences
- Read more about entropy and the second law
- Practice with topic-specific mock tests
Mastering thermodynamics makes tackling many high-weightage JEE problems simpler. For targeted revision, consult formula guides and process comparisons in Vedantu’s concise notes. Always double-check system boundaries, formula units, and the direction of heat/work transfer. Deep understanding of entropy, process types, and the first law will help you answer conceptual and numerical questions more confidently on exam day.
Thermodynamics: Concepts, Laws, and Applications
ShareFAQs on Thermodynamics: Concepts, Laws, and Applications
1. What are the 3 laws of thermodynamics?
The three laws of thermodynamics summarize the basic principles governing energy and heat transfer in physical and chemical systems.
Their key points are:
- First Law: Energy cannot be created or destroyed; only transformed (also called the law of conservation of energy).
- Second Law: The entropy (disorder) of an isolated system always increases with time, and processes occur in the direction of greater entropy.
- Third Law: As temperature approaches absolute zero, the entropy of a perfect crystal approaches zero.
2. What is thermodynamics in simple terms?
Thermodynamics is the study of heat, energy, and how they interact with matter.
In simple terms:
- It explains how energy is transferred or changed from one form to another.
- It covers systems in chemistry and engineering, such as engines, refrigerators, and chemical reactions.
- The subject uses laws, equations, and principles to predict how systems behave in different conditions.
3. Is thermodynamics a law or theory?
Thermodynamics is a branch of science based on natural laws, not just a theory.
The subject is governed by fundamental laws (First, Second, and Third Laws of Thermodynamics) which have been repeatedly tested and confirmed by experiment. They describe how energy operates in the universe – in physical, chemical, and biological systems.
4. Is thermodynamics a hard subject?
Thermodynamics can be challenging for many students because it combines abstract concepts, mathematics, and real-world applications.
To succeed with this topic:
- Focus on understanding the basic laws and definitions first (system, surroundings, state variables).
- Practice applying key formulas and equations (energy conservation, entropy changes).
- Refer to thermodynamics books, PDFs, notes, and solved examples for step-by-step guidance.
5. What is the meaning of thermodynamics in chemistry?
In chemistry, thermodynamics explains how energy is exchanged in chemical reactions and predicts whether a reaction will occur spontaneously.
Important points include:
- It analyses heat changes, work done, and energy flow during reactions.
- It uses terms like enthalpy, entropy, and Gibbs free energy to forecast reaction feasibility.
- Understanding thermodynamic systems and surroundings is key for chemical equilibrium and phase changes.
6. What are some common examples of thermodynamics in everyday life?
Thermodynamic principles are part of many everyday situations.
Some examples include:
- Melting ice: Heat from surroundings flows into ice, causing it to melt (energy transfer).
- Boiling water: Turning liquid into vapor involves entropy and heat changes.
- Refrigerators: Use work to remove heat and keep food cool (second law in action).
- Car engines: Convert chemical energy from fuel to mechanical work and heat.
7. What is a thermodynamic system?
A thermodynamic system is the specific part of the universe being studied for energy and heat changes.
A system can be:
- Open: Exchanges both energy and matter with surroundings (like a pot of boiling water).
- Closed: Exchanges energy, not matter (like a sealed can of soda warming up).
- Isolated: No exchange of energy or matter (like an insulated flask).
8. What are some important thermodynamics formulas?
Key thermodynamic formulas help solve heat, work, and energy questions.
Examples include:
- First law: ΔU = Q – W (change in internal energy = heat added – work done)
- Enthalpy: ΔH = ΔU + PΔV
- Entropy: ΔS = Qrev/T (at constant temperature, reversible process)
- Gibbs free energy: ΔG = ΔH – TΔS
9. What are the laws of thermodynamics in order?
The laws of thermodynamics are usually stated in the following order:
- First Law: Conservation of energy (ΔU = Q – W).
- Second Law: Entropy always increases in a spontaneous process.
- Third Law: Entropy of a perfect crystal is zero at absolute zero temperature.
10. What are some recommended books for studying thermodynamics?
Choosing the right books is important for mastering thermodynamics.
Popular recommendations include:
- “Thermodynamics: An Engineering Approach” by Yunus Çengel and Michael Boles
- “Thermodynamics, Statistical Thermodynamics, and Kinetics” by Thomas Engel and Philip Reid
- “Physical Chemistry” by Peter Atkins (for chemistry students)
- Class PDF notes or NCERT textbooks for exam preparation
