Convection is the transfer of heat through the movement of fluids. Warm materials expand, become less dense, and rise, while cool materials descend. This circulation pattern transfers heat energy within systems through buoyancy. Examples given include home heating systems where water and air circulate and transfer heat, thunderstorms where warm moist air rises and condenses as it cools, and global wind patterns where warm equatorial air rises and cooler air descends forming Hadley cells. Convection occurs across many natural systems to move heat.

1. Convection

2. 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.

3. Fluid Motion
 Objects in a fluid have a
force of buoyancy.
 Materials expand when they
Fb = mg
are heated.
• Increased volume
• Lower density
 Warm fluids generally rise.
Fnet = ρ coolVg − ρ warmVg

4. 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.

5. 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

6. Thunderstorm
 Thunderstorms move heat
energy with convection.
 Temperature difference is
between masses of warm
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.

7. Global Circulation
 Warm air at the equator also
rises.
 As it moves poleward it
cools.
 The cool air descends.
 This forms a Hadley cell.

8. Global Circulation
 The rotating earth breaks up
the Hadley cell.
 Circulation moves heat in a
complex pattern that controls
the weather.

9. More Convection
 Convection moves heat in many systems.
• Inside the oceans – ocean currents
• Inside the earth – continental drift
• Inside the sun - sunspots
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