Convection of heat
LIQUIDS AND GASSES ONLY
CONVECTION occurs when the more energetic particles MOVE from the HOTTER REIGION to the COOLER REGION - AND TAKE THEIR HEAT ENERGY WITH THEM
This is how immersion heaters in kettles and hot water tanks and convector heaters work.
Convection simply can't happen in solids because the particles cannot move.
The Immersion Heater
- Heat energy is transferred from the heater coils through to the water by CONDUCTION (PARTICLE COLLISIONS)
- The particles near the coils get more energy, so they started moving around faster.
- This means that is more distance between them, i.e. the water expands and becomes less dense.
- This reduction in density means that the hotter water tends to rise above the denser, cooler water.
- As the hot water rises it displaces the colder water out of the way, making it sink towards the heater coils.
- This cold water is then heated by the coils and rises - and so it goes on. You end up with convection currents going up, round and down, circulating the heat energy through the water.
Convection is most effiecient in roundish or squashed containers, because they allow the convection currents to work best.
Shallow, wide containers or tall, thin ones dont work quite so well.
Because hot water rises (because of low density) you only get convection currents in the water above the heater. The water below it stays cold because there is almost no conduction.
CONVECTION CURRENT are all about CHANGES IN DENSITY
The Radiation Example
Heating a room with a radiatior relies on convection currents too
Hot, less dense air by the radiator rises and denser, cooler air flows to replace it