It is at the atomic level that we must seek the explanation of the thermal expansion of matter. The thermal energy contained in the atoms is expressed in the form of a vibration around their average position. This vibration depends on the temperature and the neighboring atoms. A variation in temperature has the effect of modifying the volume. This property applies to any type of material (solid, liquid, gas). It partly explains the rise of the sea level in the case of global warming, the flight of hot air balloons when heating the air they contain, and the inclination of the Eiffel tower several centimeters in the opposite direction of the Sun.
This property of the material is characterized by a coefficient of thermal expansion. The formula below corresponds to a linear expansion:
ΔL = α . L0 . ΔT
- ΔL the variation in length in meters (m);
- α the coefficient of linear expansion (in K-1 or in °C-1);
- L0 the initial length in meters (m);
- ΔT = T - T0 the temperature variation (in K or in °C).
The difference in the coefficients of expansion of two metals explains that the bimetallic strip curves on the side where the coefficient of expansion is the smallest. This property is exploited in many devices like the thermosensitive switch (see animation fire alarm). This twisting is all the more important as the bimetallic strip is long. It is this amplification that is sought with a spiraled shaped bimetallic strip like those used in certain thermometers.
This phenomenon of thermal expansion is not very perceptible on small objects. However, it should be considered on large structures such as bridges, buildings, or even a large mass of air or water.