Stefan’s law:

Stefan’s law: the radiant energy emitted by a black body per unit area per unit time (i.e. emissive power) is directly proportional to the fourth power of its absolute temperature.

I.e. E & T⁴                      or,   E=σT⁴

Where σ is a constant called Stefan’s constant.

Change of state

Any material can remain in any of its three states (soli, liquid and gas). To change the substance from one state to other state is called change of state. For this either substance is heated or heat is extracted from the substance.

Change of state takes place at a fixed temp.

Fusion: The process by which a substance is changed from solid state to liquid state is called fusion. Fusion takes place at a fixed temperature called melting point (M.P)

Freezing: The process by which a substance is changed from liquid state to solid state is called freezing. Freezing takes at a fixed temperature called freezing point. (F.P) for a substance M.P = F.P.

M.P. of a substance changes with the change in pressure. Melting point of substances which contracts in the process of fusion (as wax) decreases with the increases in pressure. Melting point of substances which expands in the process of fusion (as wax) increases with the increase in pressure.

With the addition of impurity (as salt in ice), melting point of a substance decreases.

Vaporization: The process by which a substance is changed from liquid state to vapour state is called vaporization.

Vaporization takes place by two methods: (i) Evaporation & (ii) Boiling or Ebullition

Evaporation: the process of vaporization which takes place only from the exposed surface of liquid and those at all temperatures is an earthed pot gets cooled in summer.

Boiling: The process of vaporization which takes place at a fixed temperature and form whole part of liquid is called boiling.

The temperature at which boiling takes place is called boiling point.

Condensation: The process by which a substance is changed from vapour state to liquid state is called condensation.

     Boiling point of a liquid increases with the increases in pressure. 
     Boiling point of a liquid increases with the addition of impurity.

  Latent heat of heat of transformation

The amount of heat required to change the state of unit mass of substance at constant temperature is called latent heat.

If Q heat is required to change the state of a substance of mass m at constant temperature and L is the latent heat, then Q = m L.
S.I. unit of latent heat is Joule / Kilogram.
Any material has two types of latent heat.

(i) Latent heat of fusion: It is the amount of heat energy required to convert unit mass a substance from solid state to liquid state at its melting point. It is also the amount of heat released by unit mass of liquid when changed int osolid at its freezing point.

(ii)  Latent heat of vaporization: It is  the amount of heat required to change unit mass of a substance from liquid state to vapour state at its boiling point. it is also the amount of heat released when unit mass of a vapour is changed into liquid.