Convection: Heat transfer due to the bulk movement of molecules within fluids

Convection is defined as the method of heat transfer through a fluid from places of higher temperature to lower temperature by the movement of the molecules due to the difference in density.

For example, atmospheric circulation moves warm air to cool places, causing wind. Wind, in turn, can enter and cool a room if the window is open. The movement of the clouds, the ocean currents and many types of heaters are examples of convection.

Convection can happen naturally ("natural convection") or because of a moving device ("forced convection").

The fan is a device that produces the movement of the air artificially. The air in this case is moving because of the rotation of the fan. This is an example of "forced convection".

Natural convection happens because a fluid is lighter if is hot and it is heavier if is cold, so if a fluid has a hot part and a cold part, the hot part will naturally move upward and the cold part naturally moves downward. For example, if the water in a pot is hotter near the bottom because of the fire, it moves from the bottom to the surface. At the same time the water near the surface is colder so it moves to the bottom.

Convection currents occur when there are significant differences in temperature between two parts of a fluid. When this happens, hot fluids rise and cold fluids sink. This causes movements or currents in the fluid. Atmospheric circulation, for example, is made by convection currents.

• Heat transfer
• Heat conduction
• Thermal radiation