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Temperature
thermometers
When we assume that the linear expansion
ΔL of a metal bar
of length L
is proportional to the change in temperature
ΔT,
according to
ΔL = α L ΔT
then we only need to set a temperature scale
in order to have a thermometer.
Celsius
selected 0oC when the metal bar was placed
in melting ice
and 100oC
when the bar was placed in boiling water.
For the
linear
thermal expansion coefficient
α Celsius obtained then
L(T=100oC) -
L(T=0oC) =
ΔL = α L ΔT =
α L (100-0) = 100α L
Tables
of linear thermal expansion coefficients
can be found in many places, like
in WikipediA, or in the
Engineering
ToolBox.
Kelvin
used the absolute
zero temperature of
-273.15oC
to define 0 K.
But the Kelvin unit of 1 K
is equal to the Celsius unit of 1oC.
As a consequence linear thermal expansion coefficients
are exactly the same in Kelvin units as in Celsius units.
Several types of thermometers and thermal controllers make use of
bimetallic strips.
A bimetal bends
under temperature changes which allows for temperature to be measured.
Other types of thermometers make use of a liquid column.
Thermal expansion is sometimes the cause of railroad accidents.
Heat expands the rail and when there are no joints to create any
extra room,
then thermal expansion creates pressure on the steel.
When the pressure becomes too much,
the track can buckle or bulge out considerably,
giving rise to so-called
sun kinks.
volumetric expansion