Convection

Convection carries energy through liquids and gases. Warm fluid expands, becomes less dense and rises; cooler, denser fluid sinks to take its place, setting up a convection current. That is why a kettle heats from the bottom, a radiator warms a whole room and sea breezes blow on sunny days. Convection cannot happen in a solid, because its particles cannot flow.

Key facts

  • Convection: warm fluid expands, becomes less dense and rises; cooler fluid sinks, making a current.
  • Convection cannot happen in a solid: the particles cannot move from place to place.

Worked examples

Example 1

Why does hot air rise?

  1. Heating air makes it expand.
  2. The same mass takes up more volume, so it is less dense.
  3. Less dense air rises above the cooler, denser air.

Answer: it expands, so it is less dense than the cooler air around it

Example 2

Explain how a radiator on one wall heats a whole room.

  1. Air next to the radiator warms up and expands. It is now less dense, so it rises.
  2. It spreads across under the ceiling, cooling as it goes.
  3. Cooler, denser air sinks on the far side…
  4. …and flows back along the floor to the radiator. The loop keeps going.

Answer: by setting up a convection current

Example 3

Why is the heating element at the bottom of a kettle?

  1. Water heated at the bottom expands and becomes less dense.
  2. It rises, and cooler water sinks to take its place.
  3. This convection current heats all the water.

Answer: water heated at the bottom rises, setting up a convection current

Example 4

Why can convection not happen in a solid?

  1. Convection needs the warm material itself to move.
  2. In a solid the particles are fixed in place.
  3. They cannot flow, so there is no convection.

Answer: the particles cannot move from place to place

Example 5

A freezer compartment is at the top of an old fridge. Why?

  1. Air cooled by the freezer becomes denser.
  2. Dense, cold air sinks down through the fridge.
  3. Warmer air rises to replace it, so a convection current cools the whole fridge.

Answer: cold air sinks, setting up a convection current that cools the whole fridge

Example 6

On a sunny day, the land heats up faster than the sea. Why does a breeze blow from sea to land?

  1. The land heats the air above it.
  2. That warm air expands and rises.
  3. Cooler air from over the sea flows in to replace it: a sea breeze.

Answer: warm air above the land rises and cooler air from over the sea flows in to replace it