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.
