It is known that under the influence of heat particles accelerate their chaotic motion. If you heat a gas, then the molecules that make it up will simply scatter from each other. The heated liquid will first increase in volume, and then begin to evaporate. What will happen to solids? Not every one 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, it is possible to calculate the volumetric expansion coefficient, which makes it possible to predict the behavior of gases and liquids in changing external conditions. To get the same results for solids, the coefficient of linear expansion must be taken into account. Physicists have singled out a whole section for this kind of research and called 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, laying roads and pipes.
Gas expansion
Therm althe 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 the expansion of air, as it seemed to them a feasible task. They got down to business so zealously that they got rather contradictory results. Naturally, the scientific community was not satisfied with such an outcome. The accuracy of the measurement depended on which thermometer was used, the pressure, and a variety of other conditions. Some physicists have even come to the conclusion that the expansion of gases does not depend on changes in temperature. Or is this addiction incomplete…
Works by D alton and Gay-Lussac
Physicists would continue to argue until they are hoarse or would have abandoned measurements if not for John D alton. He and another physicist, Gay-Lussac, were able to independently obtain the same measurement results at the same time.
Lussac tried to find the reason for so many different results and noticed that some of the devices at the time of the experiment had water. Naturally, in the process of 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 instruments that he used to conduct the experiment, and to exclude even the minimum percentage of moisture from the gas under study. After all these manipulations, the first few experiments turned out to be more reliable.
D alton de alt with this issue longerhis colleague and published the results at the very beginning of the 19th century. He dried the air with sulfuric acid vapor and then heated it. After a series of experiments, John came to the conclusion that all gases and steam 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
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 middle of the 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 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. D alton decided to test all the calculations empirically, using a siphon barometer for this. Despite the fact that the temperature was not the same in all experiments, the results were very accurate. So he published them as a table in his physics textbook.
Evaporation theory
The thermal expansion of gases (as a physical theory) has undergone various changes. Scientists tried to get to the bottom of the processes by which steam is produced. Here again, the well-known physicist D alton distinguished himself. He hypothesized that any space is saturated with gas vapor, regardless of whether it is present in this reservoir(room) any other gas or steam. Therefore, it can be concluded that the liquid will not evaporate simply by 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 contributes to the formation of steam. But gravity continues to act on the vapor molecules, so the scientists calculated that atmospheric pressure has no effect on the evaporation of liquids.
Expansion of fluids
The thermal expansion of liquids was investigated in parallel with the expansion of gases. The same scientists were engaged in scientific research. To do this, they used thermometers, aerometers, communicating vessels and other instruments.
All experiments together and each separately refuted D alton's theory that homogeneous liquids expand in proportion to the square of the temperature to which they are heated. Of course, the higher the temperature, the greater the volume of the liquid, but there was no direct relationship between it. Yes, and the expansion rate of all liquids was different.
The thermal expansion of water, for example, starts at zero degrees Celsius and continues as the temperature drops. Previously, such results of experiments were associated with the fact that it is not the water itself that expands, but the container in which it is located narrows. But some time later, the physicist Deluca 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, he did not succeed due to neglectsome 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 mechanism of expansion 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 in the process of heating by dT (delta T is the difference between the initial temperature and the final temperature), expands by dL (delta L is the derivative of the coefficient of linear thermal expansion by object length and temperature difference). This is the simplest version of this law, which by default takes into account that the body expands in all directions at once. But for practical work, much more cumbersome calculations are used, since in reality materials behave differently from those modeled by physicists and mathematicians.
Thermal expansion of the rail
Physic engineers are always involved in laying the railway track, as they can calculate exactly what distance should be between the rail joints 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 is no exception. But there is one detail. Linear changefreely occurs 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 the overcoming of obstacles in the form of linear and butt resistances.
If a rail cannot change its length, then with a change in temperature, thermal stress increases in it, which can both stretch and compress it. This phenomenon is described by Hooke's Law.
