Convection (Cambridge (CIE) IGCSE Physics)

Convection

• 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 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

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 as shown in the simple convection current diagram below – this compound is purple in water

• When the water is heated around the crystals, the potassium permanganate will dissolve in the heated water and rise along with the warmed water, revealing the convection current

• As the warmed water cools and becomes more dense, it will fall and carry the potassium permanganate with it

Diagram showing an experiment with potassium permanganate to demonstrate convection