In liquids and gases heat is transmitted by convection. In this process heat is carried from one place to another by the actual movement of liquids and gases. If we heat a liquid in a vessel from below, the liquid at the bottom gets heated and expands. The hot liquid rises due to its lower density and its place is taken by cold liquid from above. 'Convection currents' are thus set up and the whole liquid gets heated to a uniform temperature. Convection currents are set up in gases and air in a similar way.
Heating elements in geysers and water heaters are fitted near the bottom so that the water can be heated by convection currents. Heating elements in electric ovens are fitted near the bottom to heat the entire enclosed air by convection. An element near the top of an oven would heat only the top layers of air, leaving the cool layers below almost unaffected.
The cooling unit (freezer) in a refrigerator is fitted near the top to cool the whole of the interior. The air near the top cools and decreases due to increased density. Its place near the top is taken by warm air and in this way convection currents are set up, which cool the entire interior.
Heating elements in geysers and water heaters are fitted near the bottom so that the water can be heated by convection currents. Heating elements in electric ovens are fitted near the bottom to heat the entire enclosed air by convection. An element near the top of an oven would heat only the top layers of air, leaving the cool layers below almost unaffected.
The cooling unit (freezer) in a refrigerator is fitted near the top to cool the whole of the interior. The air near the top cools and decreases due to increased density. Its place near the top is taken by warm air and in this way convection currents are set up, which cool the entire interior.
