Thermal expansion of solids and liquids

It is known that under the influence of heat, particles accelerate their chaotic motion. If you heat the gas, the molecules that make up it simply fly apart. The heated liquid will first increase in volume, and then it will begin to evaporate. And what will happen to solid bodies? Not each of them can change its aggregate state.

Thermal expansion: definition

Thermal expansion is the change in the dimensions and shape of bodies when the temperature changes. Mathematically, it is possible to 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 type of research and named it dilatometry.

Engineers and architects need knowledge about the behavior of different materials under the influence of high and low temperatures for the design of buildings, roads and pipes.

Expansion of gases

Thermal expansion of gases is accompanied by an expansion of their volume in space. Natural philosophers noticed this in ancient times, but only modern physicists could build mathematical calculations.

First of all, scientists were interested in the expansion of air, as it seemed to them a feasible task. They got so zealously engaged in the case that they got quite contradictory results. Naturally, the scientific community did not satisfy such an outcome. The accuracy of the measurement depended on the thermometer used, on pressure and on many other conditions. Some physicists have even come to the conclusion that the expansion of gases does not depend on temperature changes. Or this dependence is not complete ...

The works of Dalton and Gay-Lussac

Physicists would continue to argue until they were hoarse, or they would neglect measurements, except for John Dalton. He and another physicist, Gay-Lussac, at the same time independently from each other were able to obtain the same measurement results.

Lussac was trying to find the cause of so many different results and noticed that in some devices at the time of the experiment there was water. Naturally, during the heating process, it turned into vapor and changed the amount and composition of the gases being studied. Therefore, the first thing the scientist did was to carefully dry all the instruments that he used to carry out the experiment, and excluded even the minimum percentage of moisture from the gas being studied. After all these manipulations, the first few experiments proved to be more reliable.

Dalton dealt with this issue longer than his colleague and published the results even at the very beginning of the XIX century. He dried the air with sulfuric acid vapors, and then heated it. After a series of experiments, John came to the conclusion that all gases and vapors expand by a factor of 0.376. Lussac got the number 0.375. This became the official result of the study.

Elasticity of water vapor

Thermal expansion of gases depends on their elasticity, that is, the ability to return to the original volume. The first question began to explore Ziegler in the middle of the eighteenth century. But the results of his experiments were too different. More reliable figures were obtained by James Watt, who used a boiler for high temperatures, and a barometer for low temperatures.

At the end of the 18th century, the French physicist Prony attempted to derive a single formula that would describe the elasticity of gases, but it turned out to be too cumbersome and difficult to use. Dalton decided to experimentally test all calculations using a siphon barometer for this. Despite the fact that the temperature was not the same in all the experiments, the results were very accurate. Therefore, he published them in the form of a table in his textbook on physics.

Evaporation theory

The thermal expansion of gases (as a physical theory) has undergone various changes. Scientists have tried to get to the essence of the processes under which steam is produced. Here again distinguished already known to us physicist Dalton. He hypothesized that any space is saturated with gas vapors, regardless of whether any other gas or vapor is present in this reservoir (room). Consequently, it can be concluded that the liquid will not evaporate, simply coming into contact with atmospheric air.

The pressure of the column of air on the surface of the liquid increases the space between the atoms, tearing them apart and evaporating, that is, contributing to the formation of steam. But the force of gravity continues to act on the molecules of the vapor, so scientists considered that atmospheric pressure does not affect the evaporation of liquids.

Expansion of liquids

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 the experiments together and each 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 rate of expansion for all liquids was different.

The thermal expansion of water, for example, begins at zero degrees Celsius and continues with a decrease in temperature. Previously, these results of experiments were attributed to the fact that not water expands, but the capacity in which it is located narrows. But some time later the physicist Delyuk nevertheless came to the conclusion that the cause should be sought in the liquid itself. He decided to find the temperature of its greatest density. However, this failed because of neglect of certain details. Rumfort, who studied this phenomenon, found that the maximum density of water is observed in the range of 4 to 5 degrees Celsius.

Thermal expansion of bodies

In solids, the main mechanism of expansion is the change in the amplitude of the vibrations of the crystal lattice. To put it in simple terms, the atoms that make up the material and rigidly adhered to each other begin to "tremble".

The law of thermal expansion of bodies is formulated as follows: any body with linear dimension L in the process of heating by dT (delta T is the difference between the initial temperature and the final one) expands 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 behave differently than modeled by physicists and mathematicians.

Thermal expansion of rail

For the laying of the railroad track, physicists are always attracted, since they can accurately calculate the distance between the rails joints so that when heated or cooled the paths are not deformed.

As already mentioned above, thermal linear expansion is applicable for 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 friction. The rails are rigidly attached to sleepers and welded to adjacent rails, so the law that describes the length change takes into account the overcoming of obstacles in the form of running and butting resistances.

If the rail can not change its length, then with a change in temperature, the heat stress in it grows, which can both stretch and compress it. This phenomenon is described by Hooke's law.