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Convection

Heat and matter · level 3, lesson 4 of 6

A question to start

A radiator sits low down by the wall, yet it warms the whole room, even up by the ceiling. How does the heat get there?

The idea

In liquids and gases, heat can travel by convection: the warm fluid itself moves and carries the heat with it.

  1. The radiator warms the air touching it.
  2. Warm air spreads out a little, so it is lighter than the same amount of cold air. It rises.
  3. Cooler air sinks to take its place, and is warmed in turn.

This loop is a convection current.

warm air risescool air sinksback along the floor

Convection makes winds and sea breezes, and it stirs the water in a pan on the cooker.

On Mars, the air is so thin that it carries very little heat. At midday near the equator, the Sun can warm the ground to about 24 °C, but the air at the height of your head stays near 0 °C. A Mars breeze carries away far less heat than an Earth breeze of the same speed.

Air for carrying heat Mars Earth
Mass of 1 m³ of air at the ground about 0.016 kg about 1.2 kg

Worked examples

1. A hot-air balloon. A burner heats the air in the balloon. The warm air is lighter than the air outside, so the balloon rises.

2. A pan of water. Heated from below, hot water rises in the middle and cooler water sinks at the sides.

3. A fridge. Its cooling part is at the top. Cold air sinks and cools everything below it.

Try it

  1. Why does warm air rise?
  2. Why are radiators put low down and fridge cooling coils high up?
  3. Can convection happen in a solid? Why?
  4. Why does Mars's air carry so little heat?
Answers
  1. It spreads out, so it is lighter than the same amount of cold air.
  2. Warm air rises from a low radiator to fill the room; cold air sinks from a high coil to cool the whole fridge.
  3. No. The particles in a solid cannot flow around.
  4. There is so little of it: a cubic metre has less than a sixtieth of the mass of Earth's air.

Common mistake

"Heat rises." It is warm air or water that rises, carrying heat with it. Heat itself flows in every direction from hot to cold.

Where it leads