The Carnot cycle is the most efficient thermodynamic cycle possible. It consists of four reversible processes that convert heat into work. Named after French physicist Sadi Carnot, it represents the theoretical maximum efficiency for any heat engine operating between two temperature reservoirs.
The first step is isothermal expansion. The gas starts at point A and expands to point B while maintaining constant temperature T H. During this process, the gas absorbs heat Q H from the hot reservoir and does work on its surroundings. The pressure decreases as the volume increases, following the hyperbolic curve of Boyle's law.
The second step is adiabatic expansion. The gas continues to expand from point B to point C, but now there is no heat exchange with the surroundings. The gas does work using its internal energy, causing the temperature to drop from T H to T C. This process follows a steeper curve than the isothermal process.
Steps three and four complete the cycle. In step three, isothermal compression occurs at constant low temperature T C, where the gas rejects heat Q C to the cold reservoir. In step four, adiabatic compression with no heat exchange brings the gas back to its initial state, with temperature rising from T C back to T H. The cycle is now complete.
To summarize the Carnot cycle: It represents the most efficient thermodynamic cycle possible, consisting of four reversible processes. Its efficiency depends only on the temperature ratio between hot and cold reservoirs. The Carnot cycle sets the theoretical limit for all real heat engines and serves as the foundation for understanding thermodynamic efficiency in engineering applications.