Ch 12 covers the laws of thermodynamics — zeroth (temperature), first (energy conservation), and second (entropy/direction of processes), along with thermodynamic processes and heat engines.
Zeroth law: if A is in thermal equilibrium with B, and B with C, then A with C (defines temperature). First law: ΔU = Q − W (energy conservation; internal energy change = heat added − work done by system). Second law: (Kelvin-Planck) no engine can convert heat entirely to work; (Clausius) heat cannot spontaneously flow from cold to hot.
Isothermal (T constant): PV = nRT; W = nRT ln(V₂/V₁). Adiabatic (Q = 0): PVᵞ = const; W = (P₁V₁ − P₂V₂)/(γ−1). Isobaric (P constant): W = PΔV. Isochoric (V constant): W = 0. Cyclic process: ΔU = 0 so Q = W.
Heat engine: takes heat Q₁ from hot reservoir, does work W, rejects Q₂ to cold reservoir. Efficiency η = W/Q₁ = 1 − Q₂/Q₁. Carnot cycle (most efficient): two isothermals + two adiabatics. Carnot efficiency = 1 − T₂/T₁. No real engine can exceed Carnot efficiency. Refrigerator: COP = Q₂/W = T₂/(T₁−T₂).
Download: https://ncert.nic.in/textbook/pdf/keph204.pdf | Part II: https://ncert.nic.in/textbook/pdf/keph2ps.zip
The second law of thermodynamics states that some heat must always be rejected to a cold reservoir. Carnot efficiency η = 1 − T₂/T₁ equals 1 only if T₂ = 0 K (absolute zero), which is physically unattainable. In practice, friction, irreversibilities, and the need for a finite cold reservoir ensure that real engines have even lower efficiency.
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