Final answer:
Thermal expansion is the term used to describe the change in size of a material due to temperature changes. It happens because of the increase or decrease in the kinetic energy of particles within the material. As temperature rises, substances expand in length, area, and volume, while they contract when cooled.
Step-by-step explanation:
When a material changes size as a temperature changes, it is called thermal expansion. Thermal expansion is the increase, or decrease, of the size (length, area, or volume) of a body due to a change in temperature. For most substances, an increase in temperature usually results in an expansion, while a decrease in temperature can cause a contraction. This property is significant because it affects various applications and structural designs, such as bridges and railroads which have expansion joints allowing them to freely expand and contract with temperature changes.
The fundamental cause of thermal expansion is the increase in the kinetic energy of the atoms or molecules within a substance as the temperature rises. As materials are heated, they tend to expand in all dimensions; their lengths, areas, and volumes increase. This is true for solids, liquids, and gases, with gases experiencing the largest expansion relative to their size. Similarly, when the temperature drops, substances generally contract as the kinetic energy of their particles decreases.