Convection Material Flow Objects at different temperatures may

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. • Increased volume • Lower density ] Warm fluids generally rise. Fb = mg

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