Convection and Radiation: Concept Map

Concept Map

Organise the vocabulary, quantities and relationships into a connected model.

This extension applies that lens specifically to Convection and Radiation.

Conduction is not the only way heat travels. There are two more.

Convection happens in liquids and gases, where the material itself can move. When part of a fluid is heated it expands, becoming lighter for its size, and so it rises. Cooler, heavier fluid sinks to take its place, is heated in turn, and rises as well. This circulation is a convection current — and unlike conduction, the material really does travel.

You can see it in a pot of water on the stove, and feel it in a room heater: warm air rises to the ceiling, cool air is drawn in along the floor. It is also why the top bunk is always the warm one.

Radiation is heat energy travelling as invisible waves. It needs no material at all — which is why it is the only one of the three that can cross the empty space between the Sun and the Earth. You feel it the instant you turn towards a braai fire, even though the air between you and the coals is cool.

Surfaces matter for radiation:

• dark, dull surfaces absorb radiation well (and give it off well);
• light, shiny surfaces reflect it away.

Worked example — two tins in the Sun. Two identical tins of water stand in the sun at Upington, one painted matt black and one shiny white. After 30 minutes the black tin reads 46 °C and the white tin 34 °C — a difference of $46 - 34 = 12$ °C. The black surface absorbed far more of the Sun's radiation.

That single result explains a solar water heater's dark panel, a white-painted roof over a hot Karoo classroom, and why you should not choose a black car for Upington.