Heat a pot of soup from below and the soup starts to turn over on its own. Warm soup comes up in the middle and cooler soup slides down the sides. This loop is convection, and it is driven by density.
Key idea
Warm fluid expands and becomes less dense, so cooler, denser fluid sinks and pushes it up. Heating below and cooling above keep the loop, a convection current, going.
Why it works
Heat makes particles move faster and spread apart, so the same mass takes up more space: lower density. Gravity pulls the denser, cooler fluid down harder, and it sinks under the warm fluid, shoving it upward. At the top the warm fluid loses heat, becomes denser and sinks in turn. Without gravity there would be no convection: on the space station, a candle flame doesn't rise.
- Air: sunlight warms the ground, warm air rises and cooler air moves in. This makes wind and many clouds.
- Ocean: cold, salty water near the poles sinks and creeps along the sea floor.
- Mantle: hot rock deep down slowly rises and cooler rock sinks, a few centimeters a year. This slow churning helps move the tectonic plates.
Worked example: Where does it rise?
A pan of water is heated by a burner under its center. Where does water rise and where does it sink?
- Water over the burner warms, expands and becomes less dense.
- Cooler, denser water near the sides sinks and flows in along the bottom, pushing the warm water up.
Answer: It rises over the burner and sinks at the sides
Worked example: A slow loop
Mantle rock in a convection loop creeps about 5 cm a year. About how long does it take to move 1,000 km?
- 1,000 km = 1,000,000 m = 100,000,000 cm.
- $100{,}000{,}000 \div 5 = 20{,}000{,}000$ years.
Answer: About 20 million years
Common mistake
Saying "heat rises," as if heat were a substance that floats up. Heat can spread in any direction. In convection, warm fluid rises only because the denser, cooler fluid around it sinks and pushes it up. Check: put the heater at the top of a pot instead, and the water barely circulates.