• id: JTHERM-01-N11
• title: Heat Engines and Carnot Efficiency
• tags: carnot, efficiency, heat-engine

A heat engine operates cyclically: absorbs $Q_H$ from hot reservoir ($T_H$), produces work $W = Q_H - Q_C$, rejects $Q_C$ to cold reservoir ($T_C$). Efficiency: $\eta = W/Q_H = 1 - Q_C/Q_H$. The Carnot engine (reversible, using ideal gas) achieves maximum efficiency: $\eta_C = 1 - T_C/T_H$. No real engine exceeds Carnot efficiency between the same temperatures. The Carnot cycle consists of two isothermals and two adiabatics. Efficiency depends only on temperature ratio, not on the working substance. For $T_C = 0$ K, $\eta = 100\%$ — but absolute zero is unattainable (Third Law).