Convection (Cambridge International IGCSE Physics)

Spec point — 2.3.3 · Know that convection is an important method of thermal energy transfer in liquids and gases Explain convection in liquids and gases

• in terms of density changes and
• describe experiments to illustrate convection

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 heated, it becomes less dense and rises

• 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

Diagram showing an experiment with potassium permanganate to demonstrate convection

• 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

Exam Hint Note that the presence of a liquid or gas does not automatically imply convection. You should check whether the fluid is free to circulate — if it is trapped or the heating is from above, the process is usually conduction.

Thermal energy does not rise – it is the hot gases or liquids which rise due to the change in density when they were heated.

Demonstrating Convection video

Spec point — 2.3.3 · nan