Transfer of heat - Conduction, Convection, Radiation

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.

1.Conduction

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.

Applications

(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.

(ii)                   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.

(iii)                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 conductivity

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 heat.

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 gradient

If A = 1 m², and ∆T/∆x = unit temperature gradient

Then ∆Q / ∆t = K ? 1 ? 1

Or

K=∆Q/∆t

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 K^-1.

2.Convection

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.

Application

It plays an important role in ventilation and in heating and cooling system of the houses.

3. Radiation

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.

Thermal radiation

The energy emitted by a body in the form of radiation on account of its temperature is called thermal radiation.

It depends on,

(i) temperature of the body,

(ii)                   nature of the radiating body

The wavelength 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 through vacuum.

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

pass.

5.  They obey inverse square law.

Absorptive and Emissive power

Absorptive power

Absorptive 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 a_λ.

Emissive power

Emissive power 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 e_λ. Its unit is W m^-2.