Carnot’s Theorem

Team Physics -
Created by: Team Physics -, Last Updated: May 17, 2024

Carnot’s Theorem

Carnot’s Theorem is a fundamental principle in thermodynamics, formulated by Sadi Carnot in 1824. It provides a crucial understanding of the efficiency of heat engines and lays the foundation for the second laws of thermodynamics.

What is Carnot’s Theorem?

Carnot’s Theorem is a fundamental principle in thermodynamics that describes the maximum possible efficiency of a heat engine. Sadi Carnot, a French physicist, formulated this theorem in 1824. It plays a crucial role in understanding the limitations and potentials of heat engines.

Statements of Carnot’s Theorem

  1. No heat engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between the same reservoirs.
  2. The efficiency of a Carnot engine depends only on the temperatures of the heat reservoirs.
Carnot's Theorem

History of Nicolas Léonard Sadi Carnot

Nicolas Léonard Sadi Carnot was a French physicist and military engineer, widely regarded as the father of thermodynamics. His work laid the foundation for the second law of thermodynamics and introduced the concept of the Carnot cycle, which remains fundamental in the field of thermodynamics.

  • Birth: Carnot was born on June 1, 1796, in Paris, France. He was the son of Lazare Carnot, a prominent military engineer and politician during the French Revolution.
  • Education: Carnot showed early aptitude for science and mathematics. He entered the prestigious École Polytechnique in 1812, where he received a rigorous education in engineering, physics, and mathematics. Later, he attended the École du Génie at Metz to further his studies in military engineering.

Carnot’s Efficiency Formula

The efficiency (𝜂) of a Carnot engine is given by the formula:



  • 𝜂 is the efficiency of the Carnot engine.
  • 𝑇꜀​ is the absolute temperature of the cold reservoir (in Kelvin).
  • 𝑇ₕ is the absolute temperature of the hot reservoir (in Kelvin).

Reversible Engine

The efficiency of all reversible engines remains the same when they operate between the same heat reservoirs.

Reversible engine formula


  • Δ: Change in entropy
  • T: Temperature
  • ∫ᵇₐ ​: Indicates that this is a path function

Irreversible Engine

There is no irreversible engine that is more efficient than the Carnot engine working between the same reservoirs.

Examples of Irreversible Engines

  • Plastic deformation
  • Friction
  • Spontaneous chemical reaction

Uses of Carnot’s Theorem

Uses of Carnot's Theorem
  • Evaluating Engine Performance: Carnot’s Theorem helps compare real engines to the ideal Carnot engine. It provides a benchmark for the maximum possible efficiency that any heat engine can achieve when operating between two temperature reservoirs.
  • Designing Efficient Engines: Engineers use Carnot’s Theorem to guide the design of more efficient heat engines and refrigeration systems. By understanding the limits set by Carnot efficiency, they can optimize the components and processes to approach this ideal efficiency as closely as possible.
  • Understanding Thermodynamic Efficiency: Carnot’s Theorem illustrates the second law of thermodynamics, emphasizing that no engine can be 100% efficient if it operates between two heat reservoirs. This fundamental insight is crucial for both theoretical studies and practical applications in thermodynamics.
  • Setting Efficiency Benchmarks: The theorem sets an upper limit on the efficiency of heat engines. This benchmark is used to evaluate the performance of existing engines and to develop new technologies that aim to reach or exceed these efficiency standards.
  • Analyzing Irreversibilities: By comparing real engine efficiencies to the Carnot efficiency, scientists and engineers can identify and analyze sources of irreversibility in the system. This helps in improving engine designs by minimizing energy losses due to friction, heat transfer, and other non-ideal processes.
  • Optimizing Energy Systems: Carnot’s Theorem is used to improve the performance of power plants and refrigeration systems. It provides a theoretical foundation for optimizing the conversion of heat into work and vice versa, leading to more efficient energy systems.

Examples for Carnot’s Theorem

  1. Ideal Heat Engine: Consider an ideal heat engine operating between a hot reservoir at 600 K and a cold reservoir at 300 K. Using Carnot’s Theorem, the maximum efficiency of this engine is 50%. This means that the engine can convert up to 50% of the heat energy into work, with the rest being expelled to the cold reservoir.
  2. Power Plants: A modern coal-fired power plant operates with steam temperatures around 600 K and a cooling tower temperature around 300 K. According to Carnot’s Theorem, the maximum theoretical efficiency is 50%. Although real-world efficiency is lower due to practical irreversibilities, this sets an upper limit for the plant’s efficiency.
  3. Refrigerators: A refrigerator operates with an internal temperature of 4°C (277 K) and an external temperature of 30°C (303 K). Using Carnot’s Theorem, the maximum theoretical coefficient of performance (COP) is about 10.65. This indicates the ideal performance of the refrigerator in terms of efficiency.
  4. Heat Pumps: A heat pump heats a house to 20°C (293 K) using outside air at 0°C (273 K). According to Carnot’s Theorem, the maximum theoretical coefficient of performance (COP) is approximately 14.65. This shows the ideal efficiency for heating the house, highlighting the potential savings in energy costs.
  5. Comparing Two Engines: Engine A operates between 500 K and 300 K, while Engine B operates between 700 K and 300 K. Calculating the efficiencies, Engine A has an efficiency of 40%, and Engine B has an efficiency of 57.1%. This comparison shows that Engine B is more efficient than Engine A due to a higher temperature difference between the heat reservoirs.
  6. Solar Thermal Power: A solar thermal power plant operates between the temperature of the sun’s heat collector (about 800 K) and the ambient temperature (about 300 K). Carnot’s Theorem indicates that the maximum efficiency of such a power plant is around 62.5%. This theoretical limit helps in designing efficient solar power systems.


Who formulated Carnot’s Theorem?

Sadi Carnot, a French physicist, formulated Carnot’s Theorem in 1824.

What does Carnot’s Theorem imply about real engines?

Carnot’s Theorem implies that all real engines are less efficient than the ideal Carnot engine.

Why is Carnot’s Theorem important?

Carnot’s Theorem sets the theoretical limit for the efficiency of heat engines, guiding the design of efficient thermal systems.

Can any engine achieve Carnot efficiency?

No real engine can achieve Carnot efficiency due to irreversibilities and non-ideal processes.

What is the efficiency of a Carnot engine operating between 500 K and 300 K?

The efficiency is 40%.

How does temperature affect Carnot efficiency?

Higher hot reservoir temperatures and lower cold reservoir temperatures increase Carnot efficiency.

What is a Carnot cycle?

A Carnot cycle is an idealized thermodynamic cycle that provides the maximum possible efficiency for a heat engine.

How is Carnot’s Theorem related to the second law of thermodynamics?

Carnot’s Theorem is a direct consequence of the second law, which states that entropy must increase in real processes.

Why can’t real engines be as efficient as Carnot engines?

Real engines experience irreversibilities such as friction and non-instantaneous heat transfer.

What is the significance of the cold reservoir in Carnot’s Theorem?

The cold reservoir absorbs waste heat, limiting the engine’s efficiency.

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