Thermodynamics and Heat

Nicolas Léonard Sadi Carnot: The Father of Thermodynamics written by Richard Sottie

The Problem with Steam Engines

At the heart of the 19th-century Industrial Revolution were steam engines. These powerful machines drove factories, trains, and boats, allowing nations to grow and connect like never before. The problem was, however, that they were inefficient. No matter how much coal was burned, the energy somehow seemed to disappear! The metal components of the engines banged and clanged as steam blew out, but much of the energy of the fuel was lost as heat and noise into all the air. This kind of wastage made it difficult to achieve the maximum potential of these machines.

A Young Genius in a Revolutionary Time

Due to this issue, scientists and engineers were looking for better ways of understanding how engines worked and how to improve them. The solution to this issue began with the groundbreaking work of a then-unknown, young French scientist by the name of Sadi Carnot. Sadi was born on June 1, 1796, in Paris to Lazare Carnot, who was a key figure in the French Revolution. His father retired from the army in 1807 and devoted his life to the education of Sadi and his younger brother, instructing them in mathematics, science, languages, and music. Carnot entered one of France's most esteemed schools at the tender age of 16, where he was educated by some of the greatest minds of his day. Having graduated in 1814, he completed a two-year study of military engineering.

But after Napoleon’s defeat, his father was exiled to Germany—and Sadi’s life began to take a different path. He had a difficult time in the army. The roles assigned to him didn’t match his training, and he became unhappy as his complaint about promotion was consistently overlooked. He passed an examination in 1819 to enter an elite army corps in Paris, alongside focusing on scientific studies. Over time, he visited many factories and workshops, which made him interested in gas theory and engineering problem-solving in industries.

The Birth of the Carnot Cycle

In 1821, Sadi Carnot traveled to Germany to see his exiled relatives, where he heard discussions of steam engines that interested him in making them more efficient. Around that time, steam engines operated at only about 3% efficiency, i.e., out of every 100 units of energy they used, only 3 were used to do work. The rest was wasted as heat or otherwise lost, and scientists did not yet entirely understand how to improve their performance.

Carnot left Germany with the ambition of developing a better theory, believing that France's failure to improve the steam engines was a cause of Napoleon's defeat. Carnot had two big questions about steam engines:

1. Is there a limit to how much power we can get from heat?
2. Could there be a better fuel than steam to make engines work more efficiently?

In 1824, a year after his father's death, Carnot wrote a now-famous book, Reflections on the Motive Power of Fire. In it, he described a perfect kind of engine that uses heat in the most efficient way possible. Today, we call this the Carnot cycle — a design for the most efficient engine imaginable.

Carnot explained how this ideal engine works using four repeating steps. In two of the steps, the engine stays at the same temperature — scientists call this isothermal. During one isothermal step, the engine absorbs heat from a hot source, and during the other, it releases heat to a cold area. In the other two steps, the engine warms up or cools down without giving off heat to the surrounding— this is called adiabatic. In the adiabatic steps, the gas gets hotter or cooler just because it's being squeezed or stretched — not because heat is entering or leaving. These four steps show the best way to convert heat into work and set efficiency limits for any heat engine. Carnot also found that the type of fuel doesn’t matter for an ideal engine. What really matters is the temperature difference between the hot and cold parts — the bigger the difference, the better the engine performs.

Sadi Carnot also had a smart idea called reversibility. This means an engine could work backward — instead of turning heat into movement, it could use energy to create hot and cold areas. A good example is a refrigerator. It uses energy (like electricity) to cool the interior and push the heat out. That's the opposite of a steam engine, in which heat is converted into motion. Instead, the refrigerator uses energy to move heat from one place to another.

Legacy and Recognition

Carnot's ideas led other scientists to develop the second law of thermodynamics, which explains the flow of heat and energy.

Sadly, Carnot passed away early due to an illness called cholera when he was 36 years old. Since the sickness might live on his papers, many notes were buried along with him. Although he didn’t live long enough to realize how important his work was, other scientists like Rudolf Clausius, William Thomson, and Rudolf Diesel used his ideas to make big discoveries and better engines.

Because of all he did, Carnot is often regarded as the “Father of Thermodynamics.” His idea for an ideal engine set the stage for a device to measure energy, which opened the door to even more amazing science in the future.

References

1. American Society of Mechanical Engineers (ASME). "Nicolas Léonard Sadi Carnot."ASME.org, https://www.asme.org/topics-resources/content/nicolas-leonard-sadi-carnot. Accessed March 15, 2025.
2. O'Connor, J. J., and Robertson, E. F. Sadi Carnot. MacTutor History of Mathematics Archive, University of St Andrews, https://mathshistory.st-andrews.ac.uk/Biographies/Carnot_Sadi/. Accessed April 10, 2025.
3. Chodos, Alan. "June 12, 1824: Sadi Carnot Publishes Treatise on Heat Engines." APS News, American Physical Society, May 2009, https://www.aps.org/apsnews/2009/05/sadi-carnot-heat-engines. Accessed March 15, 2025.
4. New World Encyclopedia. "Nicolas Léonard Sadi Carnot." https://www.newworldencyclopedia.org/entry/Nicolas_L%C3%A9onard_Sadi_Carnot. Accessed March 15, 2025.
5. "​Nicolas Léonard Sadi Carnot (1796–1832)." Corrosion-Doctors.org, https://www.corrosion-doctors.org/Biographies/CarnotBio.htm. Accessed March 15, 2025.​
6. Mendoza, Eric. "Sadi Carnot." Encyclopaedia Britannica, Encyclopaedia Britannica, Inc., https://www.britannica.com/biography/Sadi-Carnot-French-scientist. Accessed March 15, 2025.
7. American Physical Society. "PhysicsQuest 2020: Force and Motion." APS Learning Center, https://www.aps.org/learning-center/learning-resources/physicsquest-2020. Accessed March 15, 2025.
8. Erlichson, Herman. "Sadi Carnot, 'Founder of the Second Law of Thermodynamics'." Department of the Geophysical Sciences, University of Chicago, 1998, https://geosci.uchicago.edu/~moyer/GEOS24705/Readings/Carnot_article_1998.pdf. Accessed March 15, 2025.
9. Sella, Andrea. "Carnot's Heat Engine and the Birth of Thermodynamics." Chemistry World, Royal Society of Chemistry, 23 Feb. 2024, https://www.chemistryworld.com/opinion/carnots-heat-engine-and-the-birth-of-thermodynamics/4018950.article. Accessed March 15, 2025.