Convection Of Heat
WBBSE Class 11 Convection of Heat Notes
As mentioned earlier, the transmission of heat through fluids (liquids and gases) usually takes place by convection. In this process the heated molecules of the medium, by their actual displacement, transmit heat from hotter parts to cooler parts of the fluid.
- Liquids and gases, on being heated, expand and hence become less dense or lighter. So, the hot fluid rises up and the comparatively cooler part of the fluid, having a higher density or being heavier, flows downwards. In this way a current is set up in a fluid on heating it.
- This current is called a convection current in a fluid. Clearly, In this process, heat can only be transmitted upwards, neither sideways nor downwards. It is important to mention that, a convection current cannot be set up at a place where there is no gravity.
- In the absence of gravity, there is no difference in weight between the hotter part and the cooler part of the fluid. So a convection current is not possible.
As the molecules in a solid are strongly bound together, there is no heat transfer by convection in solids.
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Practical Applications of Convection of Heat: Convection can be seen in our daily lives and in nature.
Understanding Convection in Fluids
1. Working principle of a table lamp: A table lamp burns due to the convection current of air. The frame holding the chimney covering the flame is provided with holes.
When the flame burns, while hot air escapes through the upper open end of the chimney, fresh cool air enters through the holes maintaining the supply of oxygen for the flame. If the holes in the frame are covered, the convection current of air stops, and the flame extin¬guishes due to absence of oxygen.
2. Ventilation in a room: For air circulation in a room ventilators are provided just below the roof. Because of breathing process and also if fire is burning in the room, the air in the room gets heated and polluted.
This warm, polluted air rises up andescapes through the ventilator. Cold and fresh air enters the room through doors and windows maintaining the purity and continuity of the air flow in the room.
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3. Sea breeze and larid breeze: Sea breeze and; land breeze are set up due to the convection current of air. Land absorbs more heat from the sun than the sea does. Also, as the specific heat of land is less than that of water, land gets warm rapidly during the day.
Air in contact with land gets warm and light and moves upwards. Comparatively colder air in contact with the sea flows towards land to fill up the void created, setting up a sea breeze. It flows during daytime and its intensity is the highest just before sunset.
Types of Convection Explained
During night-time, seawater remains warm for a longer period due to higher specific heat of water while land loses heat fast and cools down. Air in contact with the sea remains hot and light and thus rises up. Air in contact with land is cooler. This air flows towards the sea to fill up the void created. Thus a land breeze is set up. It flows during the whole night and its intensity is the highest just before sunrise.
Real-Life Examples of Convection in Action
4. Trade winds: Temperature at the equatorial region is much higher than that at polar regions. As the equatorial region gets very heated by the sun, air in this region gets warm and light. Thus warm air from the equatorial region rises up and colder air from polar regions rush in to fill up the space.
Because of the rotation of earth about its axis from west to east, colder air in the northern hemisphere flows from N-E, and that in the southern hemisphere flows from S-E. These air currents are called north-east and south-east trade winds respectively. These winds once helped in the navigation of merchant vessels as they blow pretty regularly throughout the year. Hence they are called trade winds.