Thermodynamics (NEET Physics)
Why This Chapter Matters
Thermodynamics has 4-8 marks in NEET Physics. First law, second law, Carnot engine efficiency, and gas processes are all regularly tested.
Core Concepts
Laws of Thermodynamics
Zeroth: If A and B are in thermal equilibrium, and B and C are, then A and C are.
First: ΔU = Q - W (heat added minus work done by system = change in internal energy)
Second: Heat cannot flow from cold to hot spontaneously. Entropy never decreases in isolated system.
Third: At absolute zero temperature, entropy is minimum (zero for perfect crystals).
Gas Processes
Isothermal (T=const): PV = const | ΔU = 0 | Q = W = nRT ln(V₂/V₁)
Adiabatic (Q=0): PV^γ = const | ΔU = -W | W = nCv(T₁-T₂)
Isochoric (V=const): W = 0 | Q = ΔU = nCvΔT
Isobaric (P=const): W = PΔV = nRΔT | Q = nCpΔT
Specific Heats
Monoatomic: Cv = 3R/2 | Cp = 5R/2 | γ = 5/3
Diatomic: Cv = 5R/2 | Cp = 7R/2 | γ = 7/5
Polyatomic: Cv = 3R | Cp = 4R | γ = 4/3
Mayer's relation: Cp - Cv = R
Carnot Engine
Efficiency: η = 1 - T₂/T₁ = W/Q₁
where T₁ = source temperature, T₂ = sink temperature (must be in Kelvin)
Maximum possible efficiency for given temperatures.
Heat Engines
Efficiency = Work output / Heat input = (Q₁ - Q₂)/Q₁
PYQs (NEET)
NEET 2023: Carnot engine efficiency = 50% when T_source = 500K. Find T_sink.
η = 1 - T₂/T₁ → 0.5 = 1 - T₂/500 → T₂ = 250K
NEET 2022: For a gas, Cp - Cv = R. This is called:
Mayer's relation. It holds for IDEAL gases only.
NEET 2021: In adiabatic compression, temperature:
Increases (no heat exchange, work done ON gas increases internal energy, hence temperature).