Thermal expansion

When exposed to heat, (almost) every solid object expands. For example, a metal stick gets bigger when it is heated. As soon as it cools down again, it shrinks back to its original size. For reasons of simplification, this page only deals with changes in length.

What does this expansion depend on?

It has been observed in experiments and measurements that this expansion of length - for example of a metal stick - is linearly dependend on the temperature. This means that every change in temperature, eg. of 1°C, results in a particular change of length compared to the total length of the stick.

Therefore, a so called linear expansion coeffeicent α can be determined. This number is depending on the material and describes to which percentage an object changes its length per °C of gaining temperature. Hence the change of length equals the expansion coefficient multiplied with the length multiplied with the change of temperature:

How far do different materials expand?

The expansion coefficient differs from one material to another. Aluminium, for example, expands more than copper, whereas glas does not expand as much. Of course building material such as concrete, wood and steel expands, too. Architects always have to keep this in mind when they make plans.

By the way: Not only solid objects but also liquids expand. They even expand far more than solid objects.

A special case of application for thermal expansion is bimetal: In this case, two kinds of metals with different expansion coefficients are joint together. This results in the bending of a bimetal when it is exposed to heat. If you want to learn more about this, we recommend the task "The Electric Iron" in the category "Thermodynamics".

What exactly happens during thermal expansion?

Graphic: Atomic lattice of a solid body.
Move your cursor over the image to increase the temperature of the object.

Solid objects, like all matter, are made out of atoms. In solid objects these atoms are connected like a lattice. However, within this lattice atoms can vibrate. The more they fluctuate, the higher the temperature of the solid object.

When a solid object is heated up and the atoms move farther, they need more space. This means that the space between atoms increases. Eventually, the whole object expands.