Heat Transfer by Convection
This mode of heat transfer is seen in liquids and gases in which the molecules are less densely packed and are free to move. Thus, convection is the primary mode of heat transfer in fluids.
Let us see the following example and learn how heat is transferred by convection.
The bottom of a vessel containing water is heated. After sometime, the water at the top of the vessel also becomes hot. How does this happen?
Fig. Hot water (red) rises up and cold water (blue) falls to the bottom.
If a vessel containing water is kept on a flame, the water at the bottom of the vessel gets hot, and as hot water is lighter than cold water, it rises up carrying the heat energy with it (Fig.). The colder and denser water on the top falls to the bottom of the vessel and gets heated in its turn. This process continues and results in a circulating stream of hot and cold water. As a result, the whole water in the vessel gets heated.
This method by which heat is transferred by mass movement of the liquid or gas itself is called convection.
Practical Applications of Convection
The principle of convection of heat can be used in many applications. When you want to warm a fluid, you should warm it from below. If a room has to be warmed with a room heater, the heater should be placed at a lower level, so that the room gets warmed evenly. Similarly, an air conditioner used for cooling a room should be placed at a higher level.
In earlier times, windows and ventilators were designed and placed so that the buildings were kept cool in summers. Next time you visit old buildings, look out for these special features.
Sea Breeze and Land Breeze
Convection plays a major role in maintaining a moderate temperature in places near the sea. Land masses (beach, coastal town, or city) heat up much faster than water bodies (sea, ocean) during daytime, and cool down much faster during the night. This difference in temperature sets up a wind pattern.
During the day, the air above land rises as it is warm and cooler air from over the sea flows in to take its place. This gives rise to a sea breeze (sometime in the afternoon) which cools the land. In the night, as land cools down much faster than the sea, the cooler air over land flows out to take the place of warmer air over the sea which rises, setting up a land breeze.
Aim: To show that heat transfer in water is due to convection
Materials needed: A glass beaker made of heat-resistant glass/glass test tube, candle, match box, potassium permanganate crystals/artificial food colouring, and a spoon
- Fill the beaker to the half way mark with clear water.
- Take a pinch of the potassium permanganate crystals/food colouring and drop it gently to the bottom of the beaker.
- Let it settle for a few minutes.
Observation: You will see that the water at the bottom of the beaker is coloured, but the water at the top is quite clear. Light the candle and pick up the beaker very gently and hold it on top of the candle flame. You will see columns of coloured liquid rise up in the beaker.
Conclusion: This is due to convection. Water molecules at the bottom get heated and rise up, carrying the heat energy with them.