Convection (Cambridge (CIE) IGCSE Physics): Revision Note

Convection

• Convection is an important method of thermal energy transfer in liquids and gases

  • Convection only occurs in fluids

  • Convection cannot happen in solids

Density and 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 gain kinetic energy and move faster, pushing each other further apart and making the liquid or 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 cooled (for example by an air conditioning unit high up on a wall):

  • The molecules lose kinetic energy, move more slowly and collide less frequently, making the liquid or gas contract

  • This makes the cold liquid/gas more dense than the surroundings

  • The cold liquid/gas sinks, 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 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

Demonstrating convection in a liquid with potassium permanganate

• A simple demonstration of convection in gases involves a central column with angled blades on top and candles that sit underneath the blades

• When the candles are lit, the heated air around them becomes less dense

• The heated air rises and pushes the angled blades sideways, making them spin