2. Heat phenomena

The most significant and most noticeable phenomena after mechanical motion are heat phenomena. In the simplest form you learned them in the first book.

Heat phenomena are usually related to the heating or cooling of bodies and changes in their temperature.

Changes in the temperature of the surrounding air by 10 - 20 degrees cannot go unnoticed. And if the temperature of our body changes by one or two degrees, we are already sick.

The motion of the body usually does not cause particularly significant changes in it. But heating or cooling the body can change it beyond recognition. By heating the water, which is transparent, but still visible and easily sensed by our senses, we can turn it into almost intangible steam. Strong cooling will make a piece of ice out of the water. The change in the heat quantity received from the sun by our earth changes its entire appearance. With the onset of spring, nature begins to awaken, forests are dressed in leaves and meadows are green. In winter, however, a variety of summer colours are replaced by a more monotonous background, and the lives of plants and many animals stop.

The normal mechanical properties of bodies, such as elasticity, change very strongly when heating or cooling. A piece of rubber tube, which will not suffer from the fact that you will hit it with a hammer at room temperature, becomes as brittle as glass when it is heavily cooled. A slight blow will turn the rubber tube into small pieces. Only when heated does the rubber regain its elasticity.

All these phenomena, as listed above, as well as many others, are subject to certain laws. These laws are as accurate and reliable as the laws of mechanics, but greatly differ from the laws of mechanics in content and form.

The first thing we have to do is to learn how to describe the heat processes that happen to macroscopic bodies. Find laws that could explain the reasons for the changes that occur with bodies when the bodies themselves do not move, i.e. when nothing happens to them mechanically. These laws describe a new form of motion of matter - heat motion. We will find that heat motion is inherent in all bodies, regardless of whether they move in space or not.

The section of physics that studies heat phenomena is called thermodynamics.

The course of heat processes is directly related to the structure of the substance and its internal structure. After all, for example, the fact that the heating of paraffin by a few tens of degrees makes it liquid, and the heating of the iron rod at the same value does not affect it in any noticeable way (it only begins to burn the fingers), is certainly due to the fact that the internal structure of these bodies is different. That is why heat phenomena can be used to determine the overall structure of the matter. Conversely, certain ideas about the structure of the matter can shed bright light on the physical meaning of heat phenomena and give their deep visual interpretation. This is exactly the task of molecular-kinetic theory.

Finally, it is very important that the discovery of the laws to which heat phenomena are subject makes it possible to apply these phenomena in practice and technology with maximum benefit. Modern heat engines, refrigeration systems, liquefaction systems, etc. are based on knowledge of these laws.

Now let's move on to the quantitative description of heat processes. The first, most important and quite difficult step is to introduce the concept of temperature - the main characteristic of all heat phenomena.