Thermal energy, also called heat, is a form of energy which gives us sensation of hotness. Like other forms of energy its SI unit is Joule (J). Temperature is a measure of hotness of a body. It is measured in, ºF, ºC or K, with the help of a device called thermometer.
The three scales of temperature are related as:
$$ \frac{C}{100} = \frac{F - 32}{180} = \frac{K - 273}{100} $$
Different types of thermometer are designed for different purposes for measuring human body temperature doctors use a clinical thermometer, for measurement of temperature of bodies in laboratory, laboratory thermometers are used, for measuring the highest and lower value of atmospheric temperature meteorologists use maximum and minimum thermometer.
When heat is supplied to a substance and its state does not change, the temperature of the substance rises. The heat gained or lost by a substance is given by Q = mcθ where m = mass, c = specific thermal capacity and θ = rise or fall in temperature.
Specific thermal capacity of a substance (also called its specific heat) is defined as heat per unit mass per degree change in temperature. Its SI unit Jkg-1K-1.
When heat is supplied to a substance and it changes from solid to liquid or liquid to gaseous state (or vice-versa) there is no change in its temperature. The heat supplied during change of state is called Latent heat of the substance. The Latent heat of a substance is defined as heat required to change a unit mass of substance from one state to another without change in its temperature.
L = Q/m
There are two types of latent heats of a substance: Latent heat of fusion and latent heat of vaporization. SI unit of latent heat is J kg-1.
The constant temperature at which a solid changes to its liquid state is called melting point and the constant temperature at which a body changes from liquid to gaseous state is called boiling point. Melting point and boiling point are characteristic properties of the substance. Thus, the substance which melt at 0ºC and boils at 100ºC is H2O.
All the substances expand on heating. Expansivity of different materials is different. However, the expansivity of liquids is more than solids, where as, the expansivity of gases is very much more than even liquids.
Solids may increase in length, in breadth, as well as, in height, on heating. Therefore we define linear expansivity of a solid as the increase in length per unit original length per degree Celsius rise in temperature. The SI unit of expansivity is per Kelvin.
A bimetallic strip bands on heating due to difference in expansivities of the two metals it is made of. Bimetallic strips are used in temperature control devices, called thermostats.
While laying our structures we have to make provisions for thermal expansions or otherwise our structures will be damaged or get deformed to damage other things.