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Thermal convection of gases
Hot air tends to rise, and cold air tends to fall. The air around a heater gets warmer and rises. This favors air circulation in the environment. If the heater were near the ceiling, the air above would be heated and would not come down. The cold air, in turn, would accumulate in the lower part of the room, and thus the device would not fulfill its purpose: to warm the room.
Thermal convection is called the heat transfer process that happens thanks to the movement of a material.
Note that this is exactly what happens in the example shown. The material that moves around the room is air, and with the movement heat is distributed throughout the room. The warmer, cooler air movement creates the so-called convection currents.
And in the case of an air conditioner: in order to ensure efficiency in the air cooling of the room, is it better to place it on the top or bottom? On high !!
Thermal convection of liquids
We have just studied thermal convection involving the movement of a gaseous material: air. But it is not only in the case of gases that convection can occur. With liquids too can. When a pan of water is ignited, convection currents in that liquid.
Although we cannot observe these currents directly, we can evidence their occurrence if we throw some sawdust into the water. We will see the sawdust move along the path of convection currents.
How was said, convection is a heat transfer process that occurs thanks to the movement of a material. In solids, unlike gases and liquids, no pieces of material can be moved and therefore convection currents cannot be established.
Thermal convection in everyday life
The refrigerator is a good example to prove what has been studied so far about convection. If we lay our hands flat on the floor in front of the open refrigerator, we will feel the cold air coming down from the refrigerator.
Refrigerator manufacturers take into account the fact that hot air rises and cold air descends. Therefore, the freezer that is responsible for the internal cooling of the refrigerator is on the top. It cools the air near you. This cold air descends as the warm air below rises. This produces convection currents that keep the interior of the refrigerator constantly cooling.
If the freezer were at the bottom of the refrigerator, the cooled air would remain at the bottom. And the air above would remain warm, for it would not come down so that it could be cooled by the freezer.
In the air of cities we also find convection. Pollutant gases from vehicle exhaust and factory chimneys tend to rise as they are hot. This is an example in which convection currents favor the dispersion of pollutants.