Thermodynamics Revision Notes for NEET Physics PDF Download
🔥 THERMODYNAMICS · Laws & Engines
Zeroth Law • First Law • Second Law • Carnot Engine • Entropy • NEET problems
📜 LAWS OF THERMODYNAMICS
Zeroth Law
If two systems are in thermal equilibrium with a third, they are in equilibrium with each other. Basis of temperature measurement.
First Law (Energy Conservation)
ΔU = Q – W
Change in internal energy = heat added – work done by system.
For infinitesimal: dU = dQ – dW
Second Law (Entropy)
Kelvin‑Planck: No engine can convert all heat into work.
Clausius: Heat cannot flow spontaneously from cold to hot.
Entropy of isolated system never decreases: ΔS ≥ 0.
Third Law
As T → 0, entropy → 0 (perfect crystal).
⚙️ THERMODYNAMIC PROCESSES
Isothermal (T constant): ΔU = 0 ⇒ Q = W. W = nRT ln(V₂/V₁)
Adiabatic (Q = 0): ΔU = –W. PVγ = constant, TVγ–1 = constant. W = (P₁V₁ – P₂V₂)/(γ–1)
Isobaric (P constant): W = PΔV, Q = nCpΔT
Isochoric (V constant): W = 0, Q = ΔU = nCvΔT
📈 PV diagrams for processes
🏭 CARNOT ENGINE (Ideal Heat Engine)
Most efficient engine operating between two temperatures T₁ (hot) and T₂ (cold). Cycle: isothermal expansion → adiabatic expansion → isothermal compression → adiabatic compression.
Efficiency: η = 1 – T₂/T₁ = 1 – Q₂/Q₁ (maximum possible).
Work done: W = Q₁ – Q₂
🔄 Carnot cycle on PV diagram
❄️ HEAT ENGINES & REFRIGERATORS
Heat engine: η = W/Q₁ = 1 – Q₂/Q₁
Refrigerator: COP = Q₂/W = Q₂/(Q₁–Q₂)
Heat pump: COP = Q₁/W = Q₁/(Q₁–Q₂)
For Carnot refrigerator: COPmax = T₂/(T₁–T₂)
📈 ENTROPY & SECOND LAW
Entropy (S): Measure of disorder. ΔS = ∫ dQrev/T (reversible path).
For reversible process, ΔStotal = 0; for irreversible, ΔStotal > 0.
Entropy change in:
• Isothermal expansion: ΔS = nR ln(V₂/V₁)
• Phase change: ΔS = mL/T
• Heating: ΔS = nC ln(T₂/T₁)
💡 NEET TIPS & SHORTCUTS
- For isothermal process, ΔU = 0, Q = W.
- For adiabatic process, Q = 0, ΔU = –W.
- In cyclic process, net work = area enclosed in PV diagram.
- Cp – Cv = R for ideal gases. γ = Cp/Cv = 1 + 2/f (f = degrees of freedom).
- For monatomic gas: f = 3, γ = 5/3; diatomic: f = 5, γ = 7/5; polyatomic: f = 6, γ = 4/3.
⚠️ COMMON MISTAKES
- Using Cv for isobaric process instead of Cp.
- Forgetting sign convention in First Law: ΔU = Q – W (work done BY system).
- Assuming adiabatic process means constant temperature – it's constant heat.
- Applying Carnot efficiency formula without converting temperatures to Kelvin.
📌 QUICK REVISION CARD
First law: ΔU = Q – W
Isothermal work: W = nRT ln(V₂/V₁)
Adiabatic work: W = (P₁V₁ – P₂V₂)/(γ–1)
Efficiency (Carnot): η = 1 – T₂/T₁
Cp – Cv = R
γ = Cp/Cv