Convection (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

• Convection is the main way that heat travels through liquids and gases
  • Convection only occurs in fluids
  • Convection cannot happen in solids
• In convection, the fluid near to the heat source is heated first
  • This heated fluid then rises and is replaced by cooler fluid
• The cooler fluid is, similarly, heated and then rises, being replaced by cold fluid
  • In this way, a convection current is formed and the entirety of the fluid is heated

Extended tier only

Density & convection

• Descriptions of convection currents always need to refer to changes in temperature causing changes in density
• The temperature may fall or rise, both can create a convection current 
• When a liquid (or gas) is heated (for example by a radiator near the floor):
• • The heated molecules vibrate and push each other apart, making the liquid/gas expand
  • This makes the hot liquid/gas less dense than the surroundings
  • The hot liquid/gas rises, and the cooler (surrounding) liquid/gas moves in to take its place
  • Eventually the hot liquid/gas cools and contracts, increasing in density, and sinks back down again
  • The resulting motion is called a convection current

Simple convection current diagram

When a liquid or gas is heated, it becomes less dense and rises

• When a liquid (or gas) is cooled (for example by an A.C. unit high up on a wall):
  • The molecules lose energy and collide less, making the liquid/gas contract
  • This makes the cold liquid/gas more dense than the surroundings
  • The cold liquid/gas falls, so that warmer, less dense, liquid or gas can move into the space created
  • The warmer liquid/gas gets cooled and also contracts and falls down
  • The resulting motion is called a convection current