Convection process
The hot air from the hot radiator raises upwards and
the cool air from the windows enters the room and the cycle continues until
the room reaches a final constant temperature.
As the fluid taken in a vessel is heated from below, the molecules at the bottom of the vessel begin to move faster, expanding more and thus becoming less dense.
This expanded fluid is now lighter than the surrounding cooler fluid, therefore the buoyant forces cause it to rise. Some of the surrounding cooler fluid then flows so as to take the place of the rising warmer fluid, and the process then continues. Thus the thermal energy gets transported towards the upward direction by the process of convection.
Convection currents occur in the atmosphere, affecting the weather. When air is warmed, it expands. In doing so it becomes less dense than the surrounding air. Like a balloon, it is buoyed upward. When the rising air reaches an altitude at which its density matches that of the surrounding air, it no longer rises. This is evident when we see smoke from a fire rise and then settle off as it cools and its density matches that of the surrounding air. Rising warm air expands because less atmospheric pressure squeezes on it as it rises to higher altitudes and gets cooled.
convection involves the motion of mass – the overall motion of a fluid. Convection can occur in all fluids, whether liquids or gases. Whether we heat water in a pan or heat air in a room, the process is the same.