It is known that under the action of heat particles accelerate their chaotic motion. If you heat the gas, then the molecules that make it up will simply fly apart. The heated liquid will first increase in volume and then begin to evaporate. And what will happen to solids? Not each of them can change its state of aggregation.
Thermal expansion: definition
Thermal expansion is a change in the size and shape of bodies with a change in temperature. Mathematically, one can calculate the volume expansion coefficient, which allows predicting the behavior of gases and liquids in changing external conditions. To obtain the same results for solids, it is necessary to take into account the coefficient of linear expansion. Physicists have identified a whole section for this kind of research and called it dilatometry.
Engineers and architects need knowledge of the behavior of various materials under the influence of high and low temperatures for the design of buildings, the laying of roads and pipes.
Gas expansion
The thermal expansion of gases is accompanied by the expansion of their volume in space. This was noticed by natural philosophers in ancient times, but only modern physicists managed to build mathematical calculations.
First of all, scientists became interested in air expansion, as it seemed to them a feasible task. They got down to business so zealously that they got quite conflicting results. Naturally, the scientific community did not satisfy such an outcome. The accuracy of the measurement depended on which thermometer was used, pressure and many other conditions. Some physicists have even come to the conclusion that gas expansion does not depend on temperature changes. Or this dependence is not complete ...
Works by Dalton and Gay Lussac
Physicists would continue to argue hoarsely or abandon measurements if not for John Dalton. He and another physicist, Gay-Lussac, at the same time, independently of each other, were able to obtain the same measurement results.
Lussac tried to find the reason for so many different results and noticed that in some instruments at the time of the experiment there was water. Naturally, during heating, it turned into steam and changed the amount and composition of the studied gases. Therefore, the first thing the scientist did was to thoroughly dry all the tools that he used to conduct the experiment, and even exclude the minimum percentage of moisture from the test gas. After all these manipulations, the first few experiments were more reliable.
Dalton has been dealing with this issue longer than his colleague and published the results at the very beginning of the XIX century. He dried the air with vapors of sulfuric acid, and then heated it. After a series of experiments, John concluded that all gases and steam expand by a factor of 0.376. Lussac got the number 0.375. This was the official result of the study.
Water vapor elasticity
The thermal expansion of gases depends on their elasticity, that is, the ability to return to their original volume. Ziegler was the first to investigate this issue in the mid-eighteenth century. But the results of his experiments varied too much. More reliable figures were obtained by James Watt, who used a cauldron for high temperatures, and a barometer for low temperatures.
At the end of the 18th century, the French physicist Proni attempted to derive a single formula that describes the elasticity of gases, but it turned out to be too bulky and difficult to use. Dalton decided to experimentally verify all the calculations using a siphon barometer. Despite the fact that the temperature was not the same in all experiments, the results were very accurate. Therefore, he published them as a table in his physics textbook.
Theory of Evaporation
The thermal expansion of gases (as a physical theory) underwent various changes. Scientists tried to get to the bottom of the processes in which steam is produced. Here again the distinguished physicist Dalton distinguished himself. He hypothesized that any space is saturated with gas vapor, regardless of whether any other gas or vapor is present in this tank (room). Therefore, we can conclude that the liquid will not evaporate, just coming into contact with atmospheric air.
The pressure of the air column on the surface of the liquid increases the space between the atoms, tearing them apart and evaporating, that is, it promotes the formation of steam. But gravity continues to act on the vapor molecules, so scientists have calculated that atmospheric pressure does not affect the evaporation of liquids.
Fluid expansion
The thermal expansion of liquids was investigated in parallel with the expansion of gases. Scientific research was carried out by the same scientists. To do this, they used thermometers, aerometers, communicating vessels and other instruments.
All experiments together and each one separately refuted Dalton's theory that homogeneous liquids expand in proportion to the square of the temperature at which they are heated. Of course, the higher the temperature, the greater the volume of liquid, but there was no direct relationship between it. And the expansion rate of all liquids was different.
The thermal expansion of water, for example, begins at zero degrees Celsius and continues with decreasing temperature. Previously, such experimental results were associated with the fact that not the water itself is expanding, but the capacity in which it is located is narrowing. But some time later, the physicist Deluke still came to the conclusion that the cause should be sought in the liquid itself. He decided to find the temperature of its highest density. However, he did not succeed due to neglect of some details. Rumfort, who studied this phenomenon, found that the maximum density of water is observed in the range from 4 to 5 degrees Celsius.
Thermal expansion of bodies
In solids, the main expansion mechanism is a change in the amplitude of vibrations of the crystal lattice. In simple words, the atoms that make up the material and are rigidly linked to each other begin to “tremble”.
The law of thermal expansion of bodies is formulated as follows: any body with a linear size L during heating by dT (delta T is the difference between the initial temperature and the final) is expanded by dL (delta L is the derivative of the coefficient of linear thermal expansion by the length of the object and by the difference temperature). This is the simplest version of this law, which by default takes into account that the body expands immediately in all directions. But for practical work, much more cumbersome calculations are used, since in reality materials do not behave as modeled by physicists and mathematicians.
Rail thermal expansion
Physicists are always involved in laying a railroad track, since they can accurately calculate how much distance there should be between the joints of the rails so that the tracks do not deform when heated or cooled.
As mentioned above, thermal linear expansion is applicable to all solids. And the rail was no exception. But there is one detail. A linear change occurs freely if the body is not affected by the force of friction. The rails are rigidly attached to the sleepers and welded to adjacent rails, so the law that describes the change in length takes into account overcoming obstacles in the form of running and butt resistances.
If the rail cannot change its length, then with a change in temperature, thermal stress builds up in it, which can both stretch and compress it. This phenomenon is described by Hooke's law.