By learning Physics Topics, we can gain a deeper appreciation for the natural world and our place in it.
What does a convection mean ? Give Example
We have just studied that the transfer of heat in solids takes place by the process of conduction. The transfer of heat in liquids and gases, however, takes place by the process of convection.
Convection is the transfer of heat from the hotter parts of a liquid (or gas) to its colder parts by the movement of the liquid (or gas) itself. The transfer of heat by convection can take place only in liquids and gases because the particles in liquids and gases can move about freely. In convection, the particles of a liquid (or gas) actually move from the hotter regions to the colder regions to transfer heat. The transfer of heat by convection cannot take place in solids because the particles in the solids are fixed at a place and hence cannot move about freely. The transfer of heat by convection also cannot take place in empty space (called vacuum) because there are no particles of any kind in empty space which can move and transfer heat.
We will now describe the transfer of heat by convection through the most common liquid around us called ‘water’ and the most common gas around us called ‘air’, one by one. Before we do that please note that hot water is lighter (less dense) than cold water. Similarly, hot air is lighter (less dense) than cold air.
Convection In Water
Water is a poor conductor of heat, so it cannot transfer heat by conduction. Water transfers heat by the process of convection. In order to heat water, we keep the vessel containing water over a gas burner. Now, though the burner is kept at the bottom of the vessel, but it still heats all the water in the vessel. Actually, when water is heated in a vessel, it transfers heat from its hotter parts (just above the flame of burner) to its colder parts by the process of convection. This happens as follows.
When a beaker containing water is kept over a burner, water at the bottom of beaker (near the flame) gets heated, expands and becomes lighter. This hot water (being lighter) rises upwards and carries heat alongwith it (see Figure). The cold water from above (being denser) sinks downwards to take the place of hot rising water (see Figure). This cold water then gets heated by the burner and also rises upwards carrying its heat upwards. And more cold water sinks downwards. This process of hot water rising upwards and cold water sinking downwards takes place again and again due to which the whole water in the beaker gets heated uniformly. The circulatory movements of water in the beaker in which hot water rises and cold water sinks again and again, are called convection currents. These convection currents transfer heat from water at the bottom of the beaker to the top of beaker rapidly. The arrows drawn in the beaker in Figure 11 show the direction of convection currents which take place during the heating of water. We can now say that water in a beaker is heated by convection currents.
To Show Convection Currents During Heating of Water
We can see the path of convection currents of hot and cold water taking place during the heating of water by dropping a crystal of a coloured substance called ‘potassium permanganate’ into the water to colour it. This can be done as follows.
We take a round-bottomed flask and fill it half with water. A small crystal of potassium permanganate is dropped carefully at the bottom of the flask containing water (see Figure).This potassium permanganate crystal dissolves slowly and forms a purple coloured solution around itself. Heat the water at the bottom of the flask by keeping a burner below it and observe the movement of this hot water (which has been coloured purple by dissolved potassium permanganate crystal). We will see the purple streaks of hot water rise from the bottom of flask up to the surface of water and then sink downward near the walls of the flask (as shown in Figure 12). These purple coloured streaks seen in the water of flask (which is being heated from below) show the convection currents taking place in the water of flask which transfer heat from the bottom to the top.
Water is a good convector of heat. Water can transfer heat by convection only in the upward direction because hot water (being lighter than cold water) rises upward in the vessel. This is why we heat a vessel containing water from below by keeping a burner at its bottom. The heating element of an electric kettle (used for boiling water to make tea) is also fixed at the bottom of the kettle. Please note that convection does not occur if the water is heated at the top (rather than at the bottom). If the water taken in a vessel is heated by a burner at the top, then the hot water (being lighter) stays at the top of the vessel. Since no hot water from top can sink downwards towards the bottom of the vessel, the water at the bottom of such a vessel (which is being heated at the top) will remain cold.
Convection In Air
Air is a very poor conductor of heat, so air cannot transfer heat by the process of conduction. Air transfers heat from its hotter parts to colder parts by the process of convection (by moving itself). In order to heat air in a room during winter, we keep the heater on the floor (usually in a corner of the room). Now, though the heater is placed on the floor at the bottom of the room but it still heats all the air in the room. Actually, when air is heated by the heater in one Heater part of the room, it transfers heat by convection. This happens as follows.
When the heater kept on the floor in a room is switched on, the air near the heater gets heated, expands and become lighter. This hot air (being lighter) rises above the top of the heater and carries its heat alongwith it (see Figure). The cold air from above (being denser) sinks downwards to the bottom of the heater to take the place of hot rising air (see Figure). This cold air also gets heated by the heater and rises upwards carrying its heat upwards. And more cold air sinks downwards towards the heater. This process of hot air rising upwards and cold air sinking downwards takes place again and again due to which all the air in the room gets heated uniformly after some time. The circulatory movements of air in the room in which hot air rises and cold air sinks again and again are called convection currents. Thus, a room heater heats all the air in a room by setting up convection currents in the air.
To Show the Transfer of Heat in Air by Convection
Fix a lighted candle on a table. Keep one hand at a safe distance above the flame of the candle and the other hand on the side of the flame (as shown in Figure). We will find that the hand kept above the candle flame feels quite hot but the hand kept on the side of the flame does not feel so hot. This can be explained as follows : The air just above the candle flame gets heated first. This hot air (being lighter) rises upwards carrying the heat alongwith it. The cold air from above (being denser) sinks downwards to take the place of hot rising air.
This cold air also gets heated by the flame and rises upwards. And more cold air sinks downwards to take its place. This process of hot air rising up and cold air moving down is repeated continuously. In this way, convection currents are set up in the air above the candle flame which carry more and more heat upwards. This transfer of heat by moving air makes our hand kept above the candle flame feel very hot.
Air can transfer the heat of a source by convection only in the upward direction (because hot air, being lighter, rises in the upward direction). Air cannot transfer the heat from a source by convection either on the sides or in the downward direction (below the source of heat). This means that no convection currents of hot air take place on the sides of candle flame (in the above activity) due to which the hand kept on the side of the flame does not feel so hot. Whatever small heat is felt by the hand kept on the side of the candle flame is due to the transfer of heat of candle flame by the process of radiation (which we will study after a while). Convection currents in air also occur in nature which lead to the blowing of sea-breeze and land-breeze in coastal areas.
Blowing of Sea-Breeze and Land-Breeze
Gentle wind is called breeze. The people living in coastal areas (sea-side areas) experience an interesting phenomenon called sea-breeze and land-breeze which are based on the transfer of heat in air by convection currents. This happens as follows.
During day time, when the sun shines, the solid land gets heated to a higher temperature much more quickly than the liquid sea-water. The hot air over the land rises upwards and cooler air from over the sea moves towards the land in the form of a cool breeze (to take the place of hot rising air) (see Figure). The breeze blowing from the sea towards the land is called sea-breeze. The cool sea-breeze blows only during the day time when the land is hotter than the sea. To receive the cool sea-breeze during the day, the windows of houses in coastal areas are made to face the sea.
At night time, the hot land cools much faster than the warm sea-water. Due to this, the land becomes cool very quickly but the sea-water remains warm for a much longer time. So, during night, the hot air over the warm sea
rises upwards and cooler air from the land blows in towards the sea in the form of a breeze (to take the place of hot rising air) (see Figure). The breeze blowing from the land towards the sea is called land-breeze. Land breeze blows only during the night when the sea-water is hotter than the land.