Convection is a mode of heat transfer by actual motion of a matter. It is possible only in fluids. Convection can be natural or forced. In natural convection, gravity plays an important part. When a fuild is heated from below, the hot part expands and, therefore, becames less dense. Because of buoyancy, it rises and the upper colder part replaces it. This again gets heated, rises up and is replaced by the colder part of the fluid.
The process goes on. This mode of heat transfer is evidently different from conduction. Convection involves bulk transport of different parts of the fluid.
In forced convection, materials forced to move by a pump or by some other physical means. The common examples of forced convection systems are forced- air heating systems in home, the human circulatory system, and the cooling system of an automobile engine. In the human body, the heat acts as the pump that circulates blood through different parts of the body transferring heat by forced con Convection and maintaining it at a uniform temperature.
Natural convection is responsible for many familiar phenomena. During the days the grounds heats up more quickly than large bodies of water do. This occurs both because the water has a greater specific heat and because mixing currents disperse the absorbed heat throughout the great volume of water. The air in contact with the warm ground is heated by conduction surroundings coolar air. As a result, the warm air rises (air currents) and other air moves (winds) to fill the space- creating a sea breeze near a large body of water.
Cooler air descends and a thermal convection cycle is set up, which transfer heat away from the land. At night the ground loses its heat more quickly, and the water surface is warmer than the land. As a result, the cycle is revesed.
The other example of natural convection is the steady surface wind on the earth blowing in from north- east towards the equator, the so called trade wind. A reasonable explanation is as follows: the equatorial and polar regions of earth 🌎 receive unequal solar heat. Air at the earth's surface near the atmosphere of the poles is cool.
In the absence of any other factor, a convection current would be set up, with the air at the equatorial surface rising and moving out towards the poles, descending and streaming in towards the equator. The rotation of the earth, however, modififies this convection current, because of this , air close to the equator has an eastward speed of 1699 km/h, while it is zero close to the poles. As a result, the air descends not at the poles but at 30° N (North) called trade wind.
