🔥 Thermodynamics – Stage 1

Page 3 : First Law of Thermodynamics & Internal Energy

Central Law of Thermodynamics:
Energy can neither be created nor destroyed, it can only be transformed from one form to another.

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1️⃣ Internal Energy (U)

Internal energy is the total microscopic energy of a system.

Includes:
• Translational KE of molecules
• Rotational KE of molecules
• Vibrational KE of molecules
• Intermolecular potential energy

❌ Internal energy does NOT include macroscopic KE or PE (motion or height of the system as a whole)

Important:
Internal energy is a state function

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2️⃣ First Law of Thermodynamics

ΔQ = ΔU + W

This equation connects heat, work and internal energy.

Meaning of terms:
ΔQ → Heat supplied to system
ΔU → Change in internal energy
W → Work done by system

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3️⃣ Physical Interpretation

Heat supplied to a system is used for:
• Increasing internal energy
• Doing external work

If no work is done → All heat changes internal energy If no heat is supplied → Work changes internal energy

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4️⃣ Special Thermodynamic Processes (First Law)

(a) Isochoric Process (V = constant)
• Work W = 0
• ΔQ = ΔU

(b) Isothermal Process (T = constant)
• ΔU = 0 (for ideal gas)
• ΔQ = W

(c) Adiabatic Process (Q = 0)
• ΔU = −W

(d) Cyclic Process
• Final state = Initial state
• ΔU = 0
• Net heat = Net work

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5️⃣ Internal Energy of Ideal Gas

For an ideal gas:
Internal energy depends only on temperature

Monoatomic Gas:
U = (3/2) nRT

Diatomic Gas:
U = (5/2) nRT

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6️⃣ JEE Favourite Conceptual Traps

• ΔU = 0 does NOT mean Q = 0
• Q = 0 does NOT mean W = 0
• State function ≠ Path function
• Same initial & final states → same ΔU

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7️⃣ Typical JEE Question Patterns

✔ Identify the process first
✔ Apply First Law accordingly
✔ Check sign convention carefully
✔ Eliminate options using ΔU logic

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👉 Next Page:
Page 4 – Specific Heat Capacities (C_p, C_v) & Mayer’s Relation

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🔥 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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• Work, Energy & Power
• WEP – Intermediate Notes
• Motion in a Plane

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