Transfer of heat
There are three
ways in which heat energy may get transferred from one place to another. These
are conduction, convection and radiation.
Heat is transmitted through the
solids by the process of conduction only. When one end of the solid is heated,
the atoms or molecules of the solid at the hotter end becomes more strongly
agitated and start vibrating with greater amplitude. The disturbance is
transferred to the neighbouring molecules.
(i) The houses of Eskimos are made up
of double walled blocks of ice. Air enclosed in between the double walls
prevents transmission of heat from the house to the coldest surroundings.
Birds often swell their feathers in winter to enclose air
between their body and the feathers. Air prevents the loss of heat from the
body of the bird to the cold surroundings.
Ice is packed in gunny bags or sawdust because, air trapped in
the saw dust prevents the transfer of heat from the surroundings to the ice.
Hence ice does not melt.
Coefficient of thermal
Let us consider a metallic bar of
uniform cross section A whose one end is heated. After sometime each section of
the bar attains constant temperature but it is different at different sections.
This is called steady state. In this state there is no further absorption of
If ∆x is the distance between the two
sections with a difference in temperature of ∆T and ∆Q is the amount of heat conducted
in a time ∆t, then it is found that the rate of conduction of heat ∆Q/∆t is
(i) directly proportional to the
area of cross section (A)
(ii) directly proportional to the
temperature difference between the two sections (∆T)
(iii) inversely proportional to
the distance between the two sections (∆x).
∆Q / ∆t α A ∆T/∆x
∆Q / ∆t = K A ∆T/∆x
where K is a constant of proportionality called co-efficient of thermal
conductivity of the metal.
∆T/∆x is called temperature
If A = 1 m2, and ∆T/∆x
= unit temperature gradient
Then ∆Q / ∆t = K ? 1 ? 1
Coefficient of thermal conductivity of the material
of a solid is equal to the rate of flow of heat per unit area per unit
temperature gradient across the solid. Its unit is W m-1
It is a phenomenon of transfer of heat in a fluid
with the actual movement of the particles of the fluid.
When a fluid is heated, the hot
part expands and becomes less dense. It rises and upper colder part replaces
it. This again gets heated, rises up replaced by the colder part of the fluid.
This process goes on. This mode of heat transfer is different from conduction
where energy transfer takes place without the actual movement of the molecules.
It plays an important role in
ventilation and in heating and cooling system of the houses.
It is the phenomenon of transfer of heat without any
material medium. Such a process of heat transfer in which no material medium
takes part is known as radiation.
The energy emitted by a body in
the form of radiation on account of its temperature is called thermal
It depends on,
(i) temperature of the body,
nature of the radiating body
of thermal radiation ranges from 8 ? 10-7 m to 4 ? 10-4 m.
They belong to infra-red region of the electromagnetic spectrum.
Properties of thermal radiations
1. Thermal radiations can travel
2. They travel along straight lines
with the speed of light.
3. They can be reflected and
refracted. They exhibit the phenomenon of interference and diffraction.
4. They do not heat the intervening
medium through which they
5. They obey inverse square law.
Absorptive and Emissive power
power of a body for a given wavelength and temperature is defined as the ratio of the radiant energy absorbed
per unit area per unit time to the
total energy incident on it per unit area per unit time.
It is denoted by aλ.
of a body at a given temperature is the
amount of energy emitted per unit
time per unit area of the surface for a given wavelength. It is denoted by eλ. Its unit is W m-2.