Short Notes:

We can see convection taking place when we heat any fluid such as water or air. Heating causes the fluid to expand and makes it lighter (Density = Mass / Volume). From the above formula, we can understand why the heated region of the fluid becomes lighter. Its volume increases as it expands while its mass remains the same.

From the same principle, cooling a fluid makes it contract and become heavier (denser).

The combination of heating and cooling a fluid allows convection to take place. Convection is a process that transfers heat throughout large volumes of any liquid or gas.

We are seeing convection taking place when we heat a pot of water. When a pot of water sits on a burner, water over the flame absorbs energy. This energy causes the water molecules to move apart from one another, making the water less dense. The warmed water rises. At the same time, colder, denser water sinks from the top to replace the warm water. As the hot water rises, it loses energy to the water through which it passes and cools somewhat. Meanwhile, warmer water keeps rising, pushing aside the now-cooled water. Convection stops after the flame goes out and all the water reaches the same temperature.