Convection

Convection transfers thermal energy through the movement of a liquid or gas. Liquids and gases are both fluids: substances that can flow. As warmer fluid moves, it carries thermal energy with it.

When a region of fluid is heated, it generally expands. The same mass then occupies a larger volume, so the heated fluid becomes less dense than the surrounding cooler fluid. The warmer fluid rises, while cooler, denser fluid sinks and moves in to replace it. If this movement continues, it forms a circulating flow called a convection current.

Convection in water

Water in a container can be gently heated near one side, with dye added to make its movement visible. Water near the heated region expands and rises, carrying the dye with it. The water then moves across the container and, as it cools, sinks. Cooler water moves towards the heated region to replace the rising water, producing a circulating current.

Convection in air

A convection box has two chimneys. A candle beneath one chimney heats the air inside the box. The warmer air rises and leaves through the chimney above the candle. Cooler air is drawn down through the other chimney to replace it.

Smoke introduced at the second chimney follows this airflow: it travels down into the box and is carried out through the chimney above the candle. The smoke makes the convection current visible.