Steam turbines (power plants) operate on the Rankine cycle — an approximation of the Carnot ideal. Typical efficiency 35–40% due to friction, heat loss, and non-ideal processes.

Internal combustion engines (cars) operate on the Otto cycle. Practical efficiency ≈ 25–30%. Most heat is wasted in exhaust gases (rejected to the environment as $Q_{2}$).

Air conditioners and refrigerators are Carnot refrigerators in reverse. For a room at 295 K and outside at 310 K: COP = 295/15 ≈ 19.7, meaning 19.7 J of heat is removed per joule of electrical work — far more efficient than simple electric heating.

Adiabatic cooling in the atmosphere: Rising air masses expand adiabatically (no heat exchange with surroundings due to low thermal conductivity of air). Temperature drops by about 10°C per km. This is why mountain peaks are cold and why clouds form at a certain altitude.

Diesel engine vs petrol engine: Diesel uses higher compression ratio → higher $T_{1}$ → higher Carnot efficiency. This is why diesel engines are more fuel-efficient.

Spray deodorant feels cold: Compressed gas exits through a nozzle, expanding rapidly (approximately adiabatic, Q ≈ 0). Work is done against external pressure; $\Delta U$ = −W < 0; temperature drops.

Speed of sound uses γ: v_sound = √(γP/ρ) = √(γRT/M). Sound propagation is adiabatic (fast oscillations, no time for heat transfer). The adiabatic bulk modulus γP is used, not the isothermal P.

Molecular speeds in gas mixtures: At the same temperature, lighter gases (smaller M) have higher v_rms. Hydrogen (M = 2) is 4× faster than oxygen (M = 32): v_rms($H_{2}$)/v_rms($O_{2}$) = √(32/2) = 4.

Human body as heat engine: Core temperature ≈ 310 K, skin temperature ≈ 303 K. Theoretical Carnot efficiency = 1 − 303/310 ≈ 2.3%. Actual metabolic efficiency ≈ 25% for muscle work — much higher due to direct chemical energy conversion, not a simple heat-engine cycle.

Entropy and life: Living organisms locally decrease entropy by metabolising food (highly ordered molecules). But they increase entropy of surroundings (heat + waste) by a larger amount. Total entropy of universe always increases.