🌡️ Thermodynamics – Stage 2

Page 3 : First Law in Multi-Process & Mixed Systems

JEE Advanced loves problems where a system undergoes multiple processes. One wrong step → entire answer collapses.

1️⃣ First Law – Universal Backbone

ΔU = Q − W
(Always true, regardless of process)

👉 First Law is applied process-wise in multi-step questions

2️⃣ Multi-Step Process Strategy (Toppers’ Method)

For each step:

Identify the process
Find W (work)
Find ΔU
Use ΔU = Q − W

👉 Final values = algebraic sum of all steps

3️⃣ Common Multi-Process Combination (Very High Yield)

Isothermal → Isochoric → Isobaric

Step 1: ΔU = 0
Step 2: W = 0
Step 3: Both Q & W present

⚠ Many students incorrectly assume same formula for all steps

4️⃣ Total Internal Energy Change

ΔU depends only on:

Initial & Final Temperature

ΔU = nCᵥ (T₂ − T₁)

👉 Path is irrelevant for ΔU

5️⃣ Heat Calculation – Stepwise Only

Total heat:
Q_total = Q₁ + Q₂ + Q₃ + …

❌ Never use one formula for whole process

6️⃣ Cyclic + Non-Cyclic Hybrid Problems

If system returns to initial state:

ΔU = 0
Q = W

👉 Check if final temperature equals initial temperature

7️⃣ Sign Convention – Silent Killer

Heat absorbed → Q +ve
Heat rejected → Q −ve
Expansion → W +ve
Compression → W −ve

⚠ One sign error = wrong option

8️⃣ IIT Advanced Thinking Pattern

Ask yourself:

Which step contributes maximum work?
Where is temperature changing?
Is any step redundant?

✅ Stage 2 – Page 3 Complete
👉 Next: Stage 2 – Page 4 : Heat Capacities, Cp vs Cv & Graphical Traps

📚 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

MIND GROW Magazine