what is convection?
- transfer of thermal energy through currents in a liquid or gas (convection currents)


how does convection work?
- convection current occurs only in fluids (liquids and gases) but not solids
why?
~ convection involves the bulk movement of fluids which carry the thermal energy with them
~ in solids, thermal energy is transferred through vibrations, not through bulk movement of the particles
- a convection current is the movement of the fluid caused by the change in density in various parts of the liquid
~ less dense- rise, more dense- sink



red arrows show the hotter region rising, blue arrows show the cooler regions sinking





cooler gas/ liquid ---> denser ---> sink
hotter gas/ liquid ---> less dense ---> rise




1. convection in liquids


this diagram shows what will be observed when a beaker filled with water and some potassium permanganate crystals at the bottom.



what will be observed:
~ circulation of purple stream of water, which represents convection currents
Ø when the water is heated, it expands and is then less dense than the surrounding water
Ø it starts to rise
Ø the cooler parts of the water, which is denser, sinks.


experiment~


- red is cold dye, green is hot dye
- the red cooler liquid will sink because it is denser

the results~



2. convection in gases



this diagram shows what will be observed when a glass box with 2 chimneys has a candle inside the box under one chimney, and a burning paper above the other chimney.



what will be observed:
~ smoke from the paper flow down from one chimney towards the candle and rises up from the other chimney at the other end.
Ø air above the candle is heated and expands , it is less dense than the surrounding air
Ø rises out of the chimney
Ø the cooler surrounding air is denser, it sinks through the chimney to replace the less dense air.


· the burning candle is used as a heat source to generate convection currents
· paper is burnt to produce smoke to help us observe the movement of convection current


experiment

the paper spiral will twirl when placed under a lighted candle because:
the candle heats up the air above it, the hot air becomes less dense and will rise