Convection (Cambridge O Level Physics)

• Convection is the main way that heat travels through liquids and gases
  • Convection only occurs in fluids
  • Convection cannot happen in solids

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 molecules 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, contracts and sinks back down again
  • The resulting motion is called a convection current

Convection Currents

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 move together, making the liquid/gas contract
  • This makes the hot liquid/gas more dense than the surroundings
  • The cold liquid/gas falls, so that warmer liquid or gas can move into the space created
  • The warmer liquid or gas gets cooled and also contracts and falls down
  • The resulting motion is called a convection current

   

Demonstrating Convection Currents

• A simple demonstration of convection in liquids involves taking a beaker of water and placing a few crystals of potassium permanganate in it, to one side, as shown in the diagram above
• When the water is heated at that side, the potassium permanganate will dissolve in the heated water and rise along with the warmed water, revealing the convection current

Demonstrating Convection

Diagram showing an experiment with potassium permanganate to demonstrate convection