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Heat, Thermodynamics & Kinetic Theory {6} Vinodkumar M, St. Aloysius H.S.S, Elthuruth, Thrissur.
are cooling system of automobile engines, heart that circulate blood throughout our body.
Radiation
In radiation, energy is transferred in the form of electromagnetic radiation called heat radiation.
It requires no medium for heat transfer. Earth receives energy from sun by means of radiation.
Thermal radiation : The electromagnetic radiation entitled by a body by virtue of its temperature is
called thermal radiation.
Q:The utensils for cooking purpose are blackened at the bottom. Why ?
This is to absorb maximum heat from fire and hence to fast up cooking.
Newtons laws of cooling
According to Newton’s law of cooling the rate of loss of heat is directly proportional to difference of
temperature between the body and its surroundings.
dQ
K(T T ) . T is temperature of surrounding medium and T is tempera-
2
1
dT 1 2
ture of body. K is constant that depends on nature of surface and area of ex-
posed surface.
Note: (1) The law is applicable for small temperature difference.
(2) For small temperature difference, cooling occurs
due to a combination of conduction, convection and radiation.
(3) The graph between difference in temperature and time is as shown in figure.
Blackbody radiation
A body which absorbs all the radiations falling on it is called a black body.
The energy of a blackbody radiation varies with wavelength . If is the wavelength for which energy is
m
maximum,. then this wavelength decreases with increase in temp T.
-3
i.e, T = constant. This is called Wien’s displacement law. The value of constant = 2.9 x 10 mK.
m
This law is used to find the surface temperature of celestial bodies like moon, sun and stars.
If a blackbody is a prefect radiator, then energy emitted per unit time H = A T .; where A = area, and TT
4
= absolute temperature of the body. This relation is called Stefan - Boltzmann law. is called Stefan -
4
-8
2
Boltzman constant = 5.6 x 10 W/m /K .
Q11. A cubical ice box of side 30 cm has a thickness of 5 cm. If 4 kg of ice is put in the box, estimate the
amount of ice remaining after 6 hrs. Outside temperature = 45 C. Thermal conductivity of material of box
0
= 0.01 W/m/K. Latent heat of fusion of ice = 335 x 10 J/kg. [3.687 kg]
3
Q12. A brass boiler has a base area 0.15 m and thickness 1 cm. It boils water at the rate of 6 kg/min when
2
placed on a gas stove. Estimate the temperature of the part of the flame in contact with the boiler. Thermal
0
conductivity for brass = 109J/s/m/K. Heat of vaporisation of water = 2256J/g. [238 C]
Thermodynamics
It is a branch of physics which deals with the study of heat, temperature and the interconversion of
heat energy into other forms of energy.
The various terms involved in thermodynamics are:-
(i) Thermodynamic System
A thermodynamic system consists of a large number of atoms or molecules at a particular temperature,
pressure and volume The system may exist in solid, liquid and gaseous state.
(ii) Surroundings
Everything outside a thermodynamic system is its surrounding.
(iii) Thermodynamic Variables or Parametres
These are quantities like pressure, volume and temperature which help us to study the behaviour of ther-
modynamic system.
(iv) Thermodynamic process
