ConvectionConvection

Material FlowMaterial Flow Objects at different temperatures may not be in direct Objects at different temperatures may not be in direct

thermal contact.thermal contact.

An intermediate mechanism is needed to transfer An intermediate mechanism is needed to transfer heat.heat.

If the heat transfer is through the motion of a fluid, If the heat transfer is through the motion of a fluid, then it is called then it is called convectionconvection..

Fluid MotionFluid Motion Objects in a fluid have a Objects in a fluid have a

force of buoyancy.force of buoyancy.

Materials expand when they Materials expand when they are heated.are heated.• Increased volumeIncreased volume• Lower densityLower density

Warm fluids generally rise.Warm fluids generally rise.

VgVgF warmcoolnet

Fb = mg

CirculationCirculation If one area in a fluid is If one area in a fluid is

heated that are will become heated that are will become less dense.less dense.

As it moves away it transfers As it moves away it transfers energy and coolsenergy and cools

Cool fluid replaces the warm Cool fluid replaces the warm fluid that left.fluid that left.

Home HeatingHome Heating Convection causes heat to Convection causes heat to

circulate in a house.circulate in a house.• Water circulates in pipesWater circulates in pipes• Air circulates around Air circulates around

radiators.radiators.

Forced circulation uses a fan Forced circulation uses a fan to push heated air.to push heated air.• Faster exchange of heatFaster exchange of heat

ThunderstormThunderstorm Thunderstorms move heat Thunderstorms move heat

energy with convection.energy with convection.

Temperature difference is Temperature difference is between masses of warm between masses of warm and cold air.and cold air.

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.

Global CirculationGlobal Circulation Warm air at the equator also Warm air at the equator also

rises. rises.

As it moves poleward it As it moves poleward it cools.cools.

The cool air descends.The cool air descends.

This forms a Hadley cell.This forms a Hadley cell.

Global CirculationGlobal Circulation The rotating earth breaks up The rotating earth breaks up

the Hadley cell.the Hadley cell.

Circulation moves heat in a Circulation moves heat in a complex pattern that controls complex pattern that controls the weather.the weather.

More ConvectionMore Convection Convection moves heat in many systems.Convection moves heat in many systems.

• Inside the oceans – ocean currentsInside the oceans – ocean currents

• Inside the earth – continental driftInside the earth – continental drift

• Inside the sun - sunspotsInside the sun - sunspots

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