Why does material expand when heated

Thermal expansion

(Almost all) solids expand under the influence of heat. For example, a metal rod becomes longer and thicker when heated. When the metal rod cools down to its original temperature, it also shortens to its original dimensions. For the sake of simplicity, we only want to deal with changing the length in this entry.

What does the expansion depend on?

Experiments and measurements have shown that the linear expansion - e.g. of a metal rod - depends linearly on the temperature. This means that every change in temperature, around 1 ° C, causes a certain change in length in relation to the total length of the rod.

That is why you can use a so-called linear expansion coefficient α determine and specify. This depends on the material and indicates the percentage by which a material expands in its length per ° C increase in temperature. The change in length is therefore equal to the expansion coefficient multiplied by the length multiplied by the temperature change:

ΔL = α L ΔT

How much do different materials expand?

The expansion coefficient is different depending on the material. For example, aluminum expands more than copper. Glass, on the other hand, expands even less. Building materials such as concrete, wood and steel naturally also expand. Architects must always take this into account when planning.

By the way: Not only solids expand when heated - liquids do that too. They expand even more than solids.

This is a special application for thermal expansion Bimetal: Two metals with different expansion coefficients are firmly connected to one another here. The result is that the bimetal bends when heated. If you want to find out more about this, we recommend the task "The intelligent iron" under the tab "Warmth"!

What exactly happens with the expansion?

Picture: Atomic lattice of a solid.
Hover over the picture to increase the temperature of the solid.

Solid bodies, like all matter, are made up of atoms. In solids, these atoms are connected to one another in the form of a lattice. However, the atoms can vibrate within this lattice. The stronger they vibrate, the higher the temperature of the solid.

If the solid is now heated and the atoms in the lattice vibrate more strongly against each other, then they also need more space. This means that the distance between the individual atoms in the lattice increases. In this way, the entire solid body eventually expands.