Convection in Liquids and Gases

The movement of the liquid due to a difference in its density sets up a convection current.
When a beaker of water is heated under a hot bunsen burner, the water at the bottom of the beaker gets heated up and expands. It is now less dense than the the surrounding water and therefore rises to the top, and bring thermal energy upwards. The surrounding water in the upper part of the flask, being denser, will sink to replace the less dense water. This movement sets up a convection current in the beaker of water.



To see the convection current, place some potassium permanganate crystals at the bottom of the beaker, and one would be able to see the circulation of the purple streams of water.

Convection currents occur only in fluids such as liquids and gases but not solids, because convection involves the bulk movement of the fluids which can carry thermal energy with them. The particles of solids are strongly bonded together and not allowed to flow, thus, solids transfer thermal energy through vibrations, without any bulk movement of the particles themselves.

Convection

Convection

Definition: Transfer of thermal energy by means of currents in a fluid.
Fluid=liquids/gases
Heat is transmitted from one place to another by the movement of heated particles of a fluid.

Why is it fluid only?
In liquids and gases, the molecules are free to move about, unlike the molecules in solids which can only vibrate at fixed positions.Convection cannot take place in solid because the particles of solid are strongly bonded together and not allowed to flow.

Rmb: Solids can only transfer heat from one region to another through conduction and radiation due to its kinetic model.