Convection · Grade 8
In Grade 8 you get fluent with: convection currents; convection in kettles, rooms, fridges and breezes.
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?
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.
Answer: by setting up a convection current
Example 3
Why is the heating element at the bottom of a kettle?
Answer: water heated at the bottom rises, setting up a convection current
Example 4
Why can convection not happen in a solid?
Answer: the particles cannot move from place to place
Example 5
A freezer compartment is at the top of an old fridge. Why?
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?
Answer: warm air above the land rises and cooler air from over the sea flows in to replace it