convection is the main source of heat transfer for liquids and gases.
convection occurs when the more energetic particles move from the hotter region to the cooler region and take thier heat energy with them, cant happen insolids because the particles can't move.
Immersion heater example:
1. heat transfered from heater coils tot he water by conduction (particle collisons)
2. particles near coils get more energy and move around faster.
3. more distance betwwen them i.e. water expands and becomes less dense.
4. less dense hot water rises above the cooler water.
5. as it rises it moves the colder water down near the heater coils which is sinking.
6. the cold water is then heated by the heater coils and the cycle starts again, More effcient in roundish or squarish containers.Convection currents are all about changes in density.