Heat is a form of energy associated with the chaotic motion of atoms or molecules of matter.Heat is a mystical thing. You can take a piece of wood in your hand, and from it, the hand will not, by and large, neither warm nor cold. However, it is worth to throw it into the fire – and it will catch fire, will give off heat in large quantities. Where does the heat come from? Since ancient times, people believed that heat is a special fluid called phlogiston, or a heat, enclosed in a tree and other combustible substances and released during combustion. By the end of the 18th century, however, enough experimental data accumulated to convince oneself of the fallacy of such a theory.
One of the first modern ideas about the nature of warmth was proposed by Benjamin Thompson (Earl of Rumford). He was always distinguished by his technical mindset and was interested in science in connection with ballistics and weapons business, to which he devoted his life. Already living in Bavaria, he was the technical manager of the cannon factory. Roughly cast barrels were machined from the inside by a drill cutter to bring it to the desired caliber and give them proper smoothness. Rumford noticed that when boring, the trunks heat up, and the stronger, the dumber the milling cutter. Measuring the heat capacity of the metal shavings, he managed to show that the heat could not be stored until boring in the material of the trunk, and consequently, the heat is generated as a result of friction. It is said that he even put the reamer in the water and drilled it until the water boiled after a few hours.
Today we understand the heat (more precisely, heat or thermal energy) as a special form of energy associated with the motion of atoms or molecules, of which the material consists. With the influx of energy from outside, atoms or molecules warm up – that is, they begin to oscillate or move faster while cooling, the movement slows down. In liquids and gases, the speed of the chaotic Brownian motion and the frequency of collisions of atoms or molecules with each other increase. In solid bodies, atoms with a larger amplitude oscillate around their places in the crystal lattice. In both cases, however, what we perceive as heat or thermal energy is actually the kinetic energy of atoms or molecules. Like all other forms of energy, subject to the first law of thermodynamics, thermal energy can be transferred to other forms of energy, and this is used, for example, in internal combustion engines and electric generators.