Coefficient of thermal expansion

The coefficient of thermal expansion is used in two ways:

{1over V} left({partial V over partial T} ight)_{P}

where ho is the density, T is the temperature, V is the volume, derivatives are taken at constant pressure P; lpha measures the fractional change in density as temperature increases at constant pressure. The expansion of a crystalline material occurs only when the force field of the crystal deviates from a perfect quadratic. If the force field is perfectly parabolic, no expansion will occur.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Linear thermal expansion coefficient

The expression for linear thermal expansion coefficient is approximately

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:eta = {lpha over 3}

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The expansion and contraction of material must be considered when designing large structures, when using tape or chain to measure distances for land surveys, and when designing molds for casting hot material. Engineers usually employ the linear thermal expansion coefficient, which is the fractional change in length of a bar per degree of temperature change. It is typically measured in parts per million per Celsius degree. Some values for common materials:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Aluminum 23 (x10-6/°C)

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Brass 19

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Concrete12

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Copper 17

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Diamond 1

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Glass 8.5

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Glass, Pyrex 3.3

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Gold 14

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Iron or steel 12

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

lead29

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

nickel13

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Mercury 60

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Platinum 9

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Quartz, fused 0.59

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Silicon 3

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Carbon Steel 10.8

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Stainless Steel 17.3

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Tungsten 4.5

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

For ordinary materials, the linear thermal expansion coefficient is approximately 1/3 the volumetric coefficient.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Applications

For applications using the thermal expansion property, see bi-metal and mercury thermometer

Related Topics:
Bi-metal - Mercury thermometer

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Thermal expansion is also used in mechanical applications to fit parts over one another, e.g. a bush can be fitted over a shaft by making its inner diameter slightly smaller than the diameter of the shaft, then heating it until it fits over the shaft, and allowing it to cool after it has been pushed over the shaft, thus achieving a 'shrink fit'humpy.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

External Links

Argyle Diamonds -- Physical properties of Diamond.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~