Thermal Expansion

The most easily observed examples of thermal expansion are size changes of materials as they are heated Figure 1. Representation of thermally induced change in a bimetallic strip made of iron and aluminum. The strip bends because the two materials do not expand or contract equally. Illustration by Hans & Cassidy. Courtesy of Gale Group. or cooled. Almost all materials (solids, liquids, and gases) expand when they are heated, and contract when they are cooled. Increased temperature increases the frequency and magnitude of the molecular motion of the material and produces more energetic collisions. Increasing the energy of the collisions forces the molecules further apart and causes the material to expand.