In this blog post, we will look at how the caloric theory of the 18th century was rejected and led to the laws of thermodynamics and the concept of entropy in the 19th century.
In the 18th century, it was believed that heat was a substance called caloric, which was a collection of particles with no mass that had the property of flowing from a higher temperature to a lower temperature. This was called the caloric theory, which states that when a cold object and a hot object are brought into contact with each other, the temperature of both objects becomes the same because caloric moves from the hot object to the cold object. In this situation, one of the major concerns of scientists was the issue of the thermal efficiency of heat engines such as steam engines.
A heat engine is a device that absorbs heat from a high-temperature heat source and releases heat to the outside of the device, such as the atmosphere at a low temperature, to do work. The heat efficiency is defined as the amount of work done divided by the amount of heat absorbed by the heat engine. In the early 19th century, Carnot addressed the issue of heat efficiency of heat engines based on the caloric theory. Carnot noticed that in a water-powered machine such as a water wheel, the amount of work done by the water as it flows from a higher place to a lower place depends only on the height difference. Similarly, the caloric value moves from high temperature to low temperature as water moves due to the height difference, and the thermal efficiency of the heat engine depends only on these two temperatures.
Meanwhile, attempts to increase the thermal efficiency of such heat engines became a very important task in conjunction with the Industrial Revolution. The Industrial Revolution brought about rapid technological development centered on the issues of energy conversion and efficiency, and various studies were conducted to improve the efficiency of heat engines. In this context, Carnot’s work laid the foundation for the First and Second Laws of Thermodynamics and became an important reference point for scientists.
In the 1840s, Joule conducted an experiment to measure the amount of energy required to obtain a certain amount of heat. A typical example was the experiment on the equivalent amount of heat. This experiment was not conducted on a heat engine, but on a water wheel that was rotated by dropping a weight. The amount of heat is expressed in calories, and he measured the equivalent amount of heat, which is the amount of work required to obtain 1 kcal of heat through a precise experiment on the process of converting mechanical energy into heat. He proved that work and heat are equivalent because they are physical quantities that can be converted into each other, and found that when heat and work are converted into each other, the sum of the energy of heat and work is conserved. Later, the law of conservation of energy was proved, which states that not only heat and work, but also chemical energy and electrical energy are equivalent and that the total amount of energy does not change when they are converted into each other.
This understanding of heat and work led to a reexamination of Carnot’s theory by scientists. In particular, Thomson pointed out that Carnot’s description of the heat engine based on the caloric theory violated the law of conservation of energy. According to Carnot’s theory, a heat engine works by releasing all the heat absorbed at a high temperature to a low temperature. This is contrary to the equivalence of heat and work and the law of conservation of energy, which Joule had proven, so the idea that heat is a substance called caloric can no longer be maintained. However, Carnot’s theory on heat efficiency was able to be maintained by Clausius’s proof. He proved Carnot’s theory that the heat efficiency of a heat engine is only related to the two operating temperatures when the heat engine absorbs heat at a high temperature and releases it at a low temperature, based on the assumption that if Carnot’s theory is not maintained, heat may flow from a low temperature to a high temperature.
Clausius noted that there is an empirically known directionality in the natural world, such as heat only flows from high temperatures to low temperatures and the opposite does not occur. He also noted that there is an asymmetry in the direction of conversion, that is, heat cannot be converted to work in a heat engine in full, that is, the efficiency of the conversion cannot be 100%, unlike when work is converted to heat. This direction and asymmetry gave rise to the concept of entropy, a new physical quantity that can explain this. Clauius established the second law of thermodynamics and explained the irreversibility of natural phenomena and the limitations of energy conversion through the concept of entropy.
Through this process, thermodynamics has become an important academic field that goes beyond the simple problem of energy conversion to understanding the fundamental principles of natural phenomena. These principles are applied across various fields of modern science and engineering and form the basis of the various technologies we use.
