Most solids and liquids expand linearly with temperature. That is, the thermal expansion of lengths and volumes are directly proportional to the change in temperature.
Different materials expand by different amounts for a given change in temperature.
We define two constants for each material, the coefficient of linear expansion and the coefficient of volume expansion . They give the fraction of expansion per unit change in temperature. In SI units, they are expressed as . Bi-metallic strip and the expanding aluminum sphere. If you heat an object with a hole or cavity in it, the cavity expands along with the object.