Convection

In conduction, heat is transmitted through solids where energy is transferred from one molecule to another. The molecules vibrate with larger amplitude but do not leave their mean positions.

In convection, heat is transmitted from one point to another by the actual bodily movement of the heated (energised) molecules within the fluid. The molecules themselves physically travel to carry heat.

Convection is the process by which heat is transmitted through a substance from one point to another due to the actual bodily movement of the heated particles of the substance.

Consider water being heated in a vessel from below:

1. The water at the bottom is heated first.
2. Its density decreases – water molecules at the bottom are separated further apart.
3. These hot molecules have high kinetic energy and rise upward to the cooler region.
4. Molecules from the cool region come down to take their place.
5. Each molecule at the bottom gets heated, rises, then cools and descends.
6. This action sets up the flow of water molecules called convection currents.
7. The convection currents transfer heat to the entire mass of water.

Apparatus

• A beaker
• Potassium permanganate crystals
• A burner
• Water

Procedure

1. Take some water in a beaker.
2. Heat the beaker slowly on a gas burner.
3. Drop a few crystals of potassium permanganate into the water.
4. Watch the water in the beaker carefully.

Observation

Streams of water are seen going upwards and coming back to the bottom.

Due to the potassium permanganate, these magenta-coloured streams can be identified easily.

Explanation

• When heating begins, the water near the bottom becomes warm, and its density decreases.
• As a result, it moves upwards and is replaced by cold water coming from above.
• In this manner, heat is transferred by means of currents called 'convection currents'.
• This process is called convection of heat.

• Convection occurs only in fluids (liquids and gases) — not in solids.
• In conduction, molecules vibrate but stay in place.
• In convection, molecules physically move from one place to another.
• Heating reduces density → hot fluid rises; cool fluid sinks → a convection current is set up.
• Convection currents transfer heat to the entire mass of the fluid.
• Potassium permanganate makes convection currents visible as magenta-coloured streams.