Carnot Engine

Carnot engine: theoretical ideal engine — most efficient possible between two reservoirs at temperatures T_hot and T_cold.

Cycle: isothermal expansion at T_hot → adiabatic expansion → isothermal compression at T_cold → adiabatic compression.

Carnot efficiency: η_Carnot = 1 − T_cold/T_hot (temperatures in KELVIN!).

All reversible engines between same two reservoirs have SAME efficiency = η_Carnot.

No real engine can exceed Carnot efficiency (2nd law of thermodynamics).

Engine output = work; engine sucks heat Q_h from hot, dumps Q_c to cold, produces W = Q_h − Q_c.

Carnot engine is REVERSIBLE — can run backward as a refrigerator with max COP.

Higher T_hot or lower T_cold ⇒ higher efficiency.