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Convection Material Flow Objects at different temperatures may not be in direct thermal contact. An intermediate mechanism is needed to transfer heat. If the heat transfer is through the motion of a fluid, then it is called convection. Fluid Motion Objects in a fluid have a force of buoyancy. Materials expand when they are heated. Fb = mg • Increased volume • Lower density Warm fluids generally rise. Fnet coolVg warmVg Circulation If one area in a fluid is heated that are will become less dense. As it moves away it transfers energy and cools Cool fluid replaces the warm fluid that left. Home Heating Convection causes heat to circulate in a house. • Water circulates in pipes • Air circulates around radiators. Forced circulation uses a fan to push heated air. • Faster exchange of heat Thunderstorm Warm air rises in the storm over a mass of colder air. The moisture condenses as the air cools. The cool air in the storm descends with precipitation. Thunderstorms move heat energy with convection. Temperature difference is between masses of warm and cold air. Global Circulation Warm air at the equator also rises. As it moves poleward it cools. The cool air descends. This forms a Hadley cell. Global Circulation The rotating earth breaks up the Hadley cell. Circulation moves heat in a complex pattern that controls the weather. More Convection Convection moves heat in many systems. • Inside the oceans – ocean currents • Inside the earth – continental drift • Inside the sun - sunspots next
