Everyone, for sure, is familiar with the clatter of wagon wheels. Or the sound of tram wheels. Everyone knows that the reason for this is the gap at the junction between the rails. And why is it made? The simple answer is to compensate for rail expansion during heating. It is also a well-known fact that when heated, the bodies expand, and when cooled, they contract. A measure of this expansion or contraction is the coefficient of linear expansion.
The molecular theory of the expansion of a body during heating is explained by an increase in the speed of motion of atoms and molecules of matter. As a result, the amplitude of atomic vibrations in the crystal lattice increases and, as a result, the linear dimensions of the body increase. And how much the increase will occur can be determined using a formula in which the linear expansion coefficient is applied.
Now we need to explain the physical meaning of the coefficient. It shows how much body length will increase when heated by 1 ° C. This value is insignificant and for each material its own. Thus, the coefficient of linear expansion of steel is 0.000011 per 1 ° C. What a really similar quantity represents can be understood by a simple example. If the Earth is wrapped around the equator with an iron wire, the length of which will be 40,000 km, then with a temperature increase of 1 ° C, the length of the wire will increase by 400 meters.
The linear expansion coefficient is extremely important for any engineer. It allows you to take into account the change in body size with a temperature difference. So, if during the year the temperature in the city changes from plus fifty degrees Celsius to minus fifty degrees Celsius, this will cause significant changes in the length of the same rails. If they are continuous, the result will be their bend. Here, in order to avoid such a phenomenon, a gap is made between the rails when they are laid.
For different materials, the coefficient value will be different. For steel, its value has already been cited, and the linear expansion coefficient of aluminum will be 0.0000024 per 1 ° .
However, the above reasoning and examples suffer from some one-sidedness. When we talk about the increase in body size during heating, not only the length increases, but also the other dimensions - width and height. An increase in size will lead to an increase in volume, and then we can talk about the volume expansion of bodies. True, this concept is rather applicable not to solids, but to liquids.
A simple experiment that confirms this can be done by everyone. Place a kettle full of water on top of the fire. When the water heats up, it will increase in volume and "run away" from the kettle. But there is a positive use of this effect. Everyone is familiar with liquid thermometers - both outdoor and medical. They are also built on the effect of volume increase when heated.
In technology, sometimes ignoring such an increase leads to sad consequences. Special measures must be used to compensate for the increase. Many had to see a long row of pipes (pipeline) laid along the surface. And suddenly, out of the blue, the pipes form a huge zigzag. This is not a simple zigzag, its value is strictly defined, and the linear expansion coefficient was used in the calculation. A similar zigzag is made to compensate for the linear increase in the size of the pipes.
You can still give many examples of the use of linear and volume extensions in the technique, but the above examples are enough to understand the essence of the phenomenon. Of course, the abnormal behavior of some substances, of the same water, is very curious. When it freezes, its volume does not decrease, but increases. This will be another factor confirming the unique properties of water.
So, in this article, based on the simplest and most obvious life examples, such a concept as the linear expansion of bodies and the coefficient of linear expansion is defined. Examples of the use of extensions in technology and in everyday life are given, concepts are given on the order of magnitude of the mentioned coefficient.