# Heat: Thermal Energy

Thermal energy, also called heat, is a form of energy which gives 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:

C/100 =  (F - 32)/180 = (K - 273)/100

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 J kg–1 K–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.

All the substances expand on heating. While laying structures we have to make provisions for thermal expansions or otherwise structures will be damaged.