🔥 Thermodynamics – Stage 3

Page 3 : Extreme PYQ-Style Multi-Concept Problems

This page is designed to separate Top-500 rankers from others.

Problem 1️⃣ : Isothermal + Adiabatic Combination

An ideal gas undergoes:
① Isothermal expansion
② Followed by adiabatic compression back to initial volume

Thinking Method:

Split process into two independent steps
Calculate work separately
Use PV^γ only in adiabatic part

❌ Never mix isothermal & adiabatic equations

Problem 2️⃣ : Negative Work but Temperature Rise

A gas is compressed while heat is supplied externally.

Question: Can temperature increase even if work is negative?

First Law:
ΔU = Q − W

If Q > |W| → ΔU positive → Temperature rises

❌ Many students think compression always cools gas

Problem 3️⃣ : Variable Heat Capacity Trap

Heat capacity of gas varies with temperature.

Correct Method:
Q = ∫ C(T) dT

❌ Using Q = CΔT directly is wrong

Problem 4️⃣ : Entropy + Carnot Engine

Engine operates between T₁ and T₂.

Efficiency:
η = 1 − T₂/T₁

Total entropy change = 0 (reversible)

❌ Efficiency cannot exceed Carnot limit

Problem 5️⃣ : Partial Heat Rejection

Gas absorbs Q₁ at T₁ and rejects Q₂ at T₂.

For cyclic process:
Q₁/T₁ = Q₂/T₂

❌ Many assume Q₁ = Q₂ (wrong)

Problem 6️⃣ : Sudden Mixing of Two Gases

Two identical ideal gases mix freely.

ΔS > 0 even if gases identical

❌ Zero entropy change only if no increase in disorder

Problem 7️⃣ : Graph Interpretation (JEE Favorite)

Given P-V graph with curved path.

Work = Area under curve

❌ Area of rectangle gives wrong result

8️⃣ Ultra-Important Thinking Rule

In Thermodynamics:
✔ State variables → path independent
✔ Process variables → path dependent

9️⃣ Examiner Psychology

Examiner wants to see:

Concept selection
Not heavy calculation
Correct sign convention

✅ Stage 3 – Page 3 Completed
➡️ Next: Stage 3 – Page 4 (Top-Level Numerical PYQs with Full Solutions)

📚 IIT–JEE PHYSICS MASTER LIBRARY (ADVANCED)

One-stop structured learning hub for JEE Advanced Physics
Concepts • Objective Questions • Final Revision • Complete Libraries

🔥 Thermodynamics – Objective Final Revision

Thermodynamics – Set 1
Set 2
Set 3
Set 4
Set 5
Set 6
Set 7
Set 8
Set 9
Set 10
Set 11
Set 12
Set 13
Set 14
Set 15
Set 16
Set 17
Set 18
Set 19
Set 20
Set 21
Set 22
Set 23
Set 24
Set 25
Set 26
Set 27
Set 28
Set 29
Set 30
Set 31
Set 32
Set 33
Set 34
Set 35
Set 36
Set 37
Set 38
Set 39
Set 40
Set 41
Set 42
Set 43
Set 44
Set 45
Set 46
Set 47
Set 48
Set 49
Set 50
Set 51
Set 52
Set 53
Set 54
Set 55
Set 56
Set 57
Set 59

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