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THERMAL PROPERTIES OF MATTER 217 K A T – T of water do. This occurs both because water has (iii)′= H = ′ ( 1 2) a greater specific heat capacity and because 2 L mixing currents disperse the absorbed heat (1.6 W m –K –) (0.02 m 2) × (73K–273K ) throughout the great volume of water. The air = in contact with the warm ground is heated by 2× (.1 m ) conduction. It expands, becoming less dense = 916.1 W ⊳ than the surrounding cooler air. As a result, the warm air rises (air currents) and the other air 10.9.2 Convection moves (winds) to fill the space-creating a sea breeze near a large body of water. Cooler air Convection is a mode of heat transfer by actual descends, and a thermal convection cycle is set motion of matter. It is possible only in fluids. up, which transfers heat away from the land. Convection can be natural or forced. In natural convection, gravity plays an important part. At night, the ground loses its heat more quickly, When a fluid is heated from below, the hot part and the water surface is warmer than the land. As a result, the cycle is reveresed (Fig. 10.17). expands and, therefore, becomes less dense. The other example of natural convection is Because of buoyancy, it rises and the upper colder part replaces it. This again gets heated, the steady surface wind on the earth blowing rises up and is replaced by the relatively colder in from north-east towards the equator, the part of the fluid. The process goes on. This mode so-called trade wind. A resonable explanation is as follows: the equatorial and polar regions of of heat transfer is evidently different from the earth receive unequal solar heat. Air at the conduction. Convection involves bulk transport of different parts of the fluid. earth’s surface near the equator is hot, while In forced convection, material is forced to move the air in the upper atmosphere of the poles is by a pump or by some other physical means. The cool. In the absence of any other factor, a convection current would be set up, with the common examples of forced convection systems are forced-air heating systems in home, the air at the equatorial surface rising and moving human circulatory system, and the cooling out towards the poles, descending and system of an automobile engine. In the human streaming in towards the equator. The rotation body, the heart acts as the pump that circulates of the earth, however, modifies this convection blood through different parts of the body, current. Because of this, air close to the equator transferring heat by forced convection and has an eastward speed of 1600 km/h, while it maintaining it at a uniform temperature. is zero close to the poles. As a result, the air Natural convection is responsible for many descends not at the poles but at 30 N (North) familiar phenomena. During the day, the latitude and returns to the equator. This is ground heats up more quickly than large bodies called trade wind. Fig. 10.17 Convection cycles. Reprint 2026-27