- Thermal expansion refers to a fractional change in size of a material in response to a change in temperature.
- This includes…
- changes in length compared to original length (∆ℓ/ℓ0) called linear expansion
- changes in area compared to original area (∆A/A0) called areal expansion or superficial expansion
- changes in volume compared to original volume (∆V/V0) called volumetric expansion or cubical expansion
- For most materials, over small temperature ranges, these fractional changes…
- are directly proportional to temperature change (∆T) and
- have the same sign (i.e., materials usually expand when heated and contract when cooled)
- are larger for liquids than solids
- A coefficient of thermal expansion…
- is the ratio of the fractional change in size of a material to its change in temperature
- is represented by the symbol α (alpha) for solids and β (beta) for liquids
- uses the SI unit inverse kelvin (K−1 or 1/K) or the equivalent acceptable non SI unit inverse degree Celsius (°C−1 or 1/°C).
- tend to retain their shape when not constrained and so are best described by a linear coefficient of thermal expansion, α (alpha).
- have an areal expansion that is very nearly twice their linear expansion, 2α (since two perpendicular linear measurements describe an area)
- have a volumetric expansion that is very nearly three times their linear expansion, 3α (since three perpendicular linear measurements describe a volume)
- tend to take on the shape of their container and so are best described by a volumetric coefficient of thermal expansion, β (beta).
- have a thermal expansion that is best described using the ideal gas law described later in this book.
Equations of thermal expansion
||areal or superficial expansion
||volumetric or cubical expansion
||ideal gas law