Convection (Oxford AQA IGCSE Combined Science Double Award)

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

  

• 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