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Convection and Radiation

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Convection and Radiation - Lesson Summary

Heat transfer by actual motion of matter from one region to another is known as “thermal convection”.
 
Since convection involves bulk transport of different parts of the matter, this mode of heat transfer is possible only in fluids.
 
Thermal convection can be “natural” or “forced”.
If the convection is caused by difference in density due to thermal expansion, then it is called “natural convection”.
 
The heating of water is an example of natural convection.
 
“Gravity” plays an important role in “natural convection”.
 
That is why, in natural convection, heating is always in the upward direction.
If the container is heated at the top, transmission of heat by convection does not take place in the downward direction. Natural convection is responsible for many day to day phenomena.
 


For example, in coastal areas, wind blows from the sea to land during the day. This is known as “ sea breeze”.
 
The sea water warms up slower than the land because of high specific heat capacity of water. As a result the air above the land becomes hotter and moves up creating a low pressure area.  So the air above the sea water moves towards the land and forms “sea breeze”.

During night time, exactly the reverse takes place.  Air moves from land to sea. This is known as “land breeze”.
 
 
 Trade winds are another example of natural convection.
 


 
 
Winds blowing from the north-east towards the equator are called the trade winds or the steady surface winds. These winds are the result of the atmospheric convection.
Different parts of the earth receive different amounts of solar energy. Equatorial region receives more energy than the polar region.
 
Air at the equatorial region is hotter and moves up. The cold air from polar region may move towards the equatorial region to form convection currents from north and south poles.
 
Due to the rotation of earth, air at the equatorial region moves with a speed of 1600 km/h in the eastward direction, whereas the speed of these winds is almost zero at the poles.
 
If the movement of the matter is caused by a pump or blower then the convection is called “forced convection”.
 
The third mode of heat transfer is “ thermal radiation”, where heat is transmitted, through even vacuum, in the form of electromagnetic waves.
The electromagnetic wave emitted by a body by virtue of its temperature is called “thermal radiation”. Heat energy received from the sun is in the form of radiation.
 
“Thermos flask” is designed to minimise heat loss by all these three modes of heat transfer.
This consists of a double walled evacuated glass vessel with its inner and outer walls coated with silver.
 
The vacuum between the two walls prevents heat loss by conduction and convection. The silver coating on the walls prevents exchange of heat by radiation.

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