In this blog post, we will examine how the 18th-century caloric theory was disproved and led to the 19th-century laws of thermodynamics and the concept of entropy.
In the 18th century, it was widely accepted that the substance of heat was “caloric,” a collection of massless particles that flowed from high-temperature to low-temperature areas. This was known as the caloric theory, which posited that when a cold object and a hot object were brought into contact, their temperatures became equal because caloric flowed from the hot object to the cold one. In this context, one of the major concerns for scientists was the issue of thermal efficiency in heat engines, such as the steam engine.
A heat engine is a device that absorbs heat from a high-temperature heat source and releases heat to the outside environment, such as the atmosphere, while performing work; thermal efficiency is defined as the ratio of the amount of work done to the amount of heat absorbed by the engine. In the early 19th century, Carnot addressed the problem of thermal efficiency in heat engines based on the caloric theory. Carnot noted that in hydraulic machines, such as waterwheels, when water flows from a higher to a lower elevation and performs work, the ratio of the volume of water to the amount of work done depends solely on the difference in elevation. Just as water moves due to a difference in elevation, caloric also moves from a high temperature to a low temperature while performing work; thus, the thermal efficiency of a heat engine depends solely on these two temperatures.
Meanwhile, efforts to improve the thermal efficiency of such heat engines became a critical challenge intertwined with the Industrial Revolution. The Industrial Revolution brought about rapid technological advancements centered on energy conversion and efficiency, leading to various studies aimed at improving the efficiency of heat engines. In this context, Carnot’s research laid the foundation for the First and Second Laws of Thermodynamics and became an important reference point for subsequent scientists.
In the 1840s, Joule conducted experiments to measure the amount of various forms of energy required to produce a specific quantity of heat. A prime example was the experiment on the equivalent of heat in work. This experiment did not involve a heat engine but instead used a falling weight to rotate a paddlewheel in water. Since the quantity of heat is expressed in calories, he measured the work-heat equivalence—the amount of work required to produce 1 kcal of heat—through precise experiments on the process by which mechanical energy (work) is converted into heat. Joule thus demonstrated that work and heat are physical quantities that are interchangeable, differing only in form, and therefore possess equivalence. He also discovered that when heat and work are converted into one another, the total energy—the sum of the energy of heat and work—remains constant. Subsequently, the law of conservation of energy was established, proving that not only heat and work but also chemical energy, electrical energy, and other forms of energy are equivalent and that the total amount of energy remains unchanged when they are converted into one another.
This understanding of heat and work led scientists to reexamine Carnot’s theory. In particular, Thomson pointed out that Carnot’s explanation of the heat engine, based on the caloric theory, contradicted Joule’s law of conservation of energy. According to Carnot’s theory, a heat engine performs work by releasing all the heat absorbed at a high temperature to a lower temperature. Since this contradicted the equivalence of heat and work and the law of conservation of energy proven by Joule, the idea that the substance of heat was caloric could no longer be sustained. However, Carnot’s theory regarding thermal efficiency was upheld by Clausius’s proof. Starting from the assumption that if Carnot’s theory were not valid, heat might flow from a lower temperature to a higher one, he proved Carnot’s theory that the thermal efficiency of a heat engine depends solely on the two operating temperatures at which the engine absorbs heat at a high temperature and releases it at a low temperature.
Clausius noted that in the natural world, heat flows only from high temperatures to low temperatures, and that there is an empirically observable directionality, such that the opposite phenomenon does not occur. He also noted the asymmetry in the direction of conversion: unlike when work is converted into heat, not all heat in a heat engine can be converted into work—that is, thermal efficiency cannot reach 100%. Discussions regarding this directionality and asymmetry gave rise to the concept of entropy, a new physical quantity capable of explaining these phenomena. By establishing the Second Law of Thermodynamics, Clausius used the concept of entropy to explain the irreversibility of natural phenomena and the limitations on energy conversion.
Through this process, thermodynamics evolved beyond the simple problem of energy conversion to become a crucial field of study for understanding the fundamental principles of natural phenomena. These principles are applied across various fields of modern science and engineering and form the foundation of the diverse technologies we use today.
