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Related Concept Videos

The Carnot Cycle and the Second Law of Thermodynamics01:20

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The Carnot engine works between two heat reservoirs of fixed temperatures. The Carnot cycle begs the following question: Is it possible to devise a heat engine that is more efficient than a Carnot engine between two fixed temperatures? The answer lies in designing a Carnot refrigerator.
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Converting work to heat is an irreversible process, and the purpose of a heat engine is to reverse the effect partially. Heat engines aim to increase the efficiency of the reversal, that is, maximize the work retrieved from heat. If the efficiency of a heat engine were 100%, it would imply reversing the process completely without introducing any other effect. Thus, it would violate the second law of thermodynamics.
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The hypothetical Carnot cycle consists of an ideal gas subjected to two isothermal and two adiabatic processes. Since the internal energy of an ideal gas depends only on its temperature, which is the same before and after the completion of the Carnot cycle, there is no change in its internal energy. Hence, using the first law of thermodynamics, the total heat exchanged by the ideal gas equals the total work done. Thus, we can quantify the efficiency of the Carnot cycle via the heat exchanged...
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In the Carnot engine, which achieves the maximum efficiency between two reservoirs of fixed temperatures, the total change in entropy is zero. The observation can be generalized by considering any reversible cyclic process consisting of many Carnot cycles. Thus, it can be stated that the total entropy change of any ideal reversible cycle is zero.
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The Joule-Thomson effect, also known as the Joule-Kelvin effect, describes the temperature change of a fluid when it is forced through a valve or porous plug while keeping it in a thermally insulated environment. This experiment is called a throttling process. This is an important effect widely used in refrigeration and the liquefaction of gases.
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Updated: Jun 3, 2025

Ice Generation and the Heat and Mass Transfer Phenomena of Introducing Water to a Cold Bath of Brine
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A Contemporary View on Carnot's Réflexions.

Jan-Peter Meyn1

  • 1Department Physik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Staudtstraße 7, 91058 Erlangen, Germany.

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|January 8, 2025
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Summary

Carnot's foundational work on heat engines can be reinterpreted using modern concepts of entropy and temperature, offering a clearer understanding of thermal processes and engine efficiency.

Keywords:
Carnot cyclecombustion enginediffusionefficiencygas turbineheat enginewaste heatwaterfall analogy

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Area of Science:

  • Thermodynamics
  • History of Physics

Background:

  • Carnot's 1824 treatise "Réflexions sur la puissance motrice du feu" predates key thermodynamic concepts.
  • Scholarly debate continues regarding Carnot's original intentions and potential misconceptions.

Purpose of the Study:

  • To re-evaluate Carnot's work on heat engines through the lens of modern thermodynamics.
  • To demonstrate how Carnot's concepts align with entropy and temperature as fundamental quantities.

Main Methods:

  • Reinterpreting Carnot's term "calorique" as entropy.
  • Adopting Carnot's analogies (e.g., waterfall) and classifications (reversible/irreversible processes).
  • Explaining heat engine physics using only temperature and entropy initially.

Main Results:

  • Carnot's work provides a valid, albeit early, framework for heat engine physics when "calorique" is replaced by entropy.
  • Modern heat engines can be detailed using only temperature and entropy.
  • Energy is introduced later to enable efficiency comparisons with other engine types.

Conclusions:

  • Carnot's core ideas on heat engines remain relevant and can be accurately described using modern thermodynamic quantities.
  • A conceptual framework based on temperature and entropy offers a robust method for analyzing heat engines before introducing energy conservation principles.