When we heat the water in the vessel, water molecules at the bottom receive heat energy and move upward. Then the molecules at the top come down and get heated. This mode of heat transfer is called as convection. Air in the atmosphere also follows the same principle while heating. Thus, the form of heat transfer from regions of higher temperature to areas of lower temperature by the actual movement of molecules is called convection. Convection takes place in liquids and gases.
Convection is defined as the movement of fluid (liquid and gas) molecules from higher temperature regions to lower temperature regions.
Fig. - Convection
Fig. - Convection process
The following are some examples of convection in daily life.
- Boiling of water - The molecules with a high density move at the bottom (Cold water). In contrast, the molecules with less dense (Hot water) move upwards, resulting in the circular motion of the molecules so that water gets heated.
- The formation of land breeze and sea breeze is due to convection of the air.
- Wind flows from one region to another region by convection.
- In warm-blooded animals, blood circulation takes place with the help of convection which helps to maintain the body temperature.
- In hot air balloons, heat is transferred by convection, and so the balloon raises.
- In refrigerators, cool air moves downward and replaces the hot air because of convection.
By conduction, heat is transferred through solids. By convection, heat is transferred through liquids and gases, but by radiation, heat can be transferred through empty space, even through a vacuum. Heat energy from the Sun reaches the Earth by this form of heat transfer.
Radiation is defined as the way of heat transfer from one place to another in the form of electromagnetic waves.
The following are some examples of convection in daily life
- Heat energy from the Sun reaches the Earth by radiation.
- While standing near a fire, we feel the heat which is transferred as radiation.
- Black surfaces absorb heat radiation. So that the bottom of the cooking vessels are painted black.
- White colour reflects heat radiation. That’s why we are advised to wear white cloth during summer.
Note: Heat transfer by radiation is visible to our eyes. When a substance is heated to \(500\)°C, the radiation becomes visible to the eye as a dull red glow, and the skin is sensing the warmth. Further heating rapidly increases the amount of radiation, and its perceived colour becomes orange, yellow and finally white.