Definition Of Convection
In liquids and gases (commonly known as fluids) the heat transfers as the less dense particles move up and more dense particles come down. This process is known as Convection.
Example 1: Take a beaker and fill it half with water. Drop one or two potassium permanganate crystals (to colour water). Heat the beaker by placing on a stand with the help of burner. The following points can be observed.
When the bottom of beaker is being gently heated in one place only, the water above the flame becomes warmer. It expands and becomes less dense.
It rises upwards as cooler, denser water sinks and displaces it.
The circulation of water can be clearly observed due to the colour of water.
This type of heat transformation is known as convection current
More About Convection
Convection also occurs in gases.
Example 2: Due to energy received from the sun air around us gets warmer, becomes less dense and rises up. The cool air moves from the sea to replace the same.
At night, the reverse happens. The sea will be warmed than the land. Land cools quickly.
Warmer air rises above the land and the cooler air replaced the same.
Example 3: Convection takes place in the refrigerator we use at our home.
There is a freezer compartment for the refrigerator on the top. It will be very cool. Cold air sinks below the freezer compartment. This sets up a circulating current of air which cools all the food in the refrigerator.
Worked Example
Question: Why "A radiator quickly warms all the air in a room, even though air is a poor thermal conductor"?
Answer:
o Due to the radiator the air around the radiator gets warmer.
o The warmer air becomes less dense and rises up.
o The cool air sinks down from the top of the room.
o Due to this convection all the air in the room quickly warms.