Heat flows from a region of higher temperature to a region of lower temperature.
Heat is transferred via three different processes: conduction, convection and radiation.
Conduction
It takes place mainly in solids. We usually ignore the effect of conduction in liquid and gas.
Heat is transferred via molecular vibration.
Commonly seen terms: poor conductor of heat, good conductor of heat, it minimises/reduces heat loss/gain via conduction.
Convection
It takes place in liquids and gases only. This is because it requires bulk movement of the molecules due to difference in density.
Standard phrasing for explaining convection current:
when fluid is heated, it expands. Then it becomes less dense and rises. The cooler, denser fluid sinks to replace the hotter fluid that rose up. Cycle repeats itself setting up convection current.
Radiation
heat can be transferred without the need of a medium.
shiny and silver surface is poor emitter of infrared radiation. It is also a poor absorber of infrared radiation.
dull and black surface is good emitter of infrared radiation. It is also a good absorber of infrared radiation.
