Convection (Oxford AQA IGCSE Physics)

Revision Note

Convection

  • Convection is the main way that thermal energy is transferred through liquids and gases

    • Convection only occurs in fluids

      • Remember that fluids are gases or liquids

    • Convection cannot happen in solids

      • This is because convection requires the movement of particles

      • Particles are held in a fixed position in a solid

  • Descriptions of convection currents always need to refer to changes in temperature causing changes in density

    • A convection current can be created when temperature increases or decreases

  • When a liquid (or gas) is heated (for example by a radiator near the floor):

    • The molecules move apart, making the liquid/gas expand

    • This makes the hot liquid/gas less dense than its 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 current from a fire

    A fire heats the air particles around it causing them to move apart. This makes them less dense so this hot gas rises and is replaced by cooler denser gas which falls. This continues in a cycle
    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 closer together, making the liquid/gas contract

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

    • The cold liquid/gas falls and warmer liquid or gas moves 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

  • An example of a convection current is when potassium permanganate is added to a beaker of water and heated

    • The purple potassium permanganate rises and falls, eventually making the entire beaker purple

Heating potassium permanganate

Heat is applied to the bottom of a beaker of water containing potassium permanganate. The potassium permanganate rises and then falls due to the heating and cooling caused by convection  currents
Potassium permanganate placed inside a beaker of water is heated and demonstrates convection

Examiner Tip

If a question on energy transfers mentions liquids or gases, the answer will probably be about convection.

Heat does not rise. The hot gas or liquid that rises due to the change in density when it is heated.

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