Convection takes place in ONLY in both gases and liquids because, Convection INVOLVES the movement of the fluids which carry thermal energy with them. [They practically MOVEEES]
But, Solids don't. They stay about their fixed position and VIBRATE.
Have you ever notice a kettle?
Did you ever wonder WHY the heating coil is placed at the bottom of the kettle?
THAT's BECAUSE it makes use of the convection concept.
When water is heated in a kettle, it will expand and rise as it is less dense than the surronding water...
As it rise, the cooler water at the top would sink as it is denser..
The movement of the water is known as CONVECTION CURRENT.
**Note: Density= Mass / Volume. When it expands, it'll have more volume, hence, it becomes LIGHTER*
The diagram ABOVE describes how convection in air works.
*By the way, this is not found in the textbook.*
When you place the flame in tube 1, the flame will extinguish very quickly because the oxygen in the tube will soon be used up.
However if a glass rod is placed FULLY inside a tube, like tube 2, the flame will continue to burn and will not extinguish
why?
The air above the candle gets heated and expands. Since the air expands, it becomes less dense than the surrounding air. Hence, it rises out of the tube from one side of the glass. IN doing so, the surrounding air will then enter the tube from the other side of the glass, replenishing the oxygen used in heating.
One last example that I'll like to use is the illustration of sea breeze and land breeze at the beach.
Did you realise, the beach tends to be very windy?
If you sit facing the sea...
During the day, the air above land gets hotter than the sea. Hence, the air above land will rise and cool air above the sea will sink to replace the air above the land. Hence, the wind seems to be coming FROM THE SEA
During the night, the air above the land is cooler than the sea. Hence, the air above the sea will rise and the air above the land will sink to replace the air above the sea. This time, the wind seems to be coming FROM the LAND.
Now that we've come to the end of introducing convection, I'd like you to think
WHY IS IT THAT PEOPLE DONT BOIL AT THE SIDE OF THE PAN BUT AT THE CENTRAL BOTTOM?
Do you think it is because of the way the covection current moves?
APPLICATIONS OF CONVECTION
1. Kettle
- The coil of the kettle is placed at the bottom
2.Air-Conditioner
-It is ALWAYS placed near the ceiling of
3. Refridgerator
-The Freezer unit is at the top so that the cool air will sink and the hot air will rise. This saves electricity.
4. Oven
- It is similar to the kettle as the heating coil is at the bottom.
Here's a short videoclip from Youtube that summarises the whole chapter.
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