Thermal expansion is the tendency of matter to change in
volume in response to a change in
temperature.
[1] When a substance is heated, its particles begin moving and become active thus maintaining a greater average separation. Materials which contract with increasing temperature are rare; this effect is limited in size, and only occurs within limited temperature ranges. The degree of expansion divided by the change in temperature is called the material's
coefficient of thermal expansion and generally varies with temperature.
Common engineering solids usually have coefficients of thermal expansion that do not vary significantly over the range of temperatures where they are designed to be used, so where extremely high accuracy is not required, calculations can be based on a constant, average, value of the coefficient of expansion.
To more accurately calculate thermal expansion of a substance an
equation of state must be used, which will then predict the values of the thermal expansion at all the required temperatures and
pressures, along with many other
state functions.
For solid materials with a significant length, like rods or cables, an estimate of the amount of thermal expansion can be described by the
ratio of
strain:
is the length before the change of temperature and
is the length after the change of temperature.
For most solids, thermal expansion relates directly with temperature:
Thus, the change in either the
strain or temperature can be estimated by:
where
is the difference of the temperature between the two recorded strains, measured in degrees
Celsius or
kelvins, and
is the linear coefficient of thermal expansion in
inverse kelvins.
Materials with
anisotropic structures, such as
crystals and many
composites, will generally have different linear expansion coefficients
in different directions.
Isotropic materials, will by definition have the same
in every direction. For an isotropic material,
.[1]
A number of materials contract on heating within certain temperature ranges; this is usually called
negative thermal expansion, rather than "thermal contraction". For example, the coefficient of thermal expansion of water drops to zero as it is cooled to roughly 4 °C and then becomes negative below this temperature, this means that water has a maximum
density at this temperature, and this leads to bodies of water maintaining this temperature at their lower depths during extended periods of sub-zero weather.
Thermal expansion generally decreases with increasing bond energy, which also has an effect on the
hardness of solids, so, harder materials are more likely to have lower thermal expansion. In general, liquids expand slightly more than solids.
Absorption or desorption of water (or other solvents) can change the size of many common materials; many organic materials change size much more due to this effect than they do to thermal expansion. Common plastics exposed to water can, in the long term, expand many percent.
For an
ideal gas, the volumetric thermal expansivity (i.e. relative change in volume due to temperature change) depends on the type of process in which temperature is change. Two known cases are
isobaric change, where
pressure is held constant, and
adiabatic change, where no
work is done and no change in
entropy occurs. In an
isobaric process, the volumetric thermal expansivity, which we denote β
P, is:
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Genosida atau genosid adalah sebuah pembantaian besar-besaran secara sistematis terhadap satu suku bangsa atau kelompok dengan maksud memusnahkan (membuat punah) bangsa tersebut. Kata ini pertama kali digunakan oleh seorang ahli hukum Polandia, Raphael Lemkin, pada tahun 1944 dalam bukunya Axis Rule in Occupied Europe yang diterbitkan di Amerika Serikat. Kata ini diambil dari bahasa Yunani γένος genos ('ras', 'bangsa' atau 'rakyat') dan bahasa Latin caedere ('pembunuhan').
