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Thermal Physics · Physics

The Carnot cycle sets the ceiling.

The most efficient possible engine between two temperatures is reversible. Carnot’s result tells you the best anyone can do.

This topic

Carnot Cycle and Efficiency Limits

Understand why reversible cycles are special, how temperature sets limits, and what “maximum efficiency” really means.

Cycle
Carnot engine
The Carnot engine is an idealized reversible engine operating between a hot and cold reservoir.
  • Two reservoirs define the temperature limits
  • Reversible steps (conceptual overview)
  • Why it’s a benchmark, not a blueprint
Start nano-lesson Idealized, but powerful.
Reversibility
Reversible cycles
Reversible cycles produce no net entropy. They represent the best-case limit for converting heat into work.
  • No entropy production in the cycle
  • Why reversibility implies “no wasted opportunity”
  • How irreversibility lowers performance
Start nano-lesson Reversible = limit case.
Limit
Maximum possible efficiency
Between the same two reservoirs, no engine can be more efficient than a reversible (Carnot) engine.
  • Efficiency depends only on reservoir temperatures
  • Why “100% efficiency” is impossible in a cycle
  • Comparing real engines to the Carnot bound
Start nano-lesson Two temperatures set the ceiling.
Scale
Absolute temperature scale
Carnot limits use absolute temperature (Kelvin). Using Celsius directly will give incorrect results.
  • Why zero on the Kelvin scale matters
  • Reservoir temperatures must be in Kelvin
  • Quick conversion and common mistakes
Start nano-lesson Always convert to Kelvin.
Practice
Practice & Exercises
Practice applying the Carnot bound, comparing engines, and reasoning about how changing temperatures affects efficiency.
  • Compute Carnot efficiency from Th and Tc
  • Compare real engine efficiency to the limit
  • Concept checks: what changes raise/lower the bound?
  • Kelvin conversion drills
  • Exam-style Carnot and limits sets
Open practice Keep Th and Tc in Kelvin.