Convection:

            Transfer of heat by actual movement of molecules from a hot place to a cold place is known as convection.

             Liquids and gases are poor conductors of heat. However, heat is transferred through fluids (liquids or gases) easily by another method called convection.

Experiment:

             Take a beaker and fill two-thirds of it with water. Heat the beaker by keeping a burner below it. Drop two or three crystals of potassium permanganate in the water. It will be seen that colored streaks of water formed by the crystals move upwards above the flame and then move downwards from sideways. These colored streaks show the path of currents in liquid. When the water at the bottom of the beaker gets hot, it expands, becomes lighter, and rises. While the cold but denser water moves downward to take its place.

Convection currents in the air:

              Gases also expand on heating; thus, convection currents are easily set up due to differences in the densities of air at various parts of the atmosphere.

Use of convection currents:

              Convection currents set up by electric gas or coal heaters help to warm our homes and offices. The central heating systems in buildings work on the same principle by convection. Convection currents occur on a large scale in nature. The day-to-day temperature changes in the atmosphere result from the circulation of warm or cold air that travels across the region. Land and sea breezes are also examples of convection currents.