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PHYSICS · Stage 9 · Chapter 12: Energy Transfer

Physics 12.4 Energy transfer: convection — Fluid Density Changes and Convection Currents

5 min read 5 parts Lesson 4 of 7

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Physics 12.4 Energy transfer: convection — Fluid Density Changes and Convection Currents — Alhussan International Academy — Physics (Stages 7–9) notes: Describe how energy is transferred by convection and how convection currents form in everyday situations.Convection = the transf…

  • Fluids only
  • Conduction first
  • Cooling at the top
  • Fire in a house
  • Freezer compartment
  • Rising air

Part 1 of 5What convection is

  • Convection the transfer of energy by the movement of a heated liquid or gas.
  • Fluids onlyIt occurs only in liquids and gases that are free to move, where particles can move from place to place.
  • SaucepanHeating the bottom warms all the water, not just the bottom part.
  • Conduction firstEnergy passes through the solid bottom of the pan to the water by conduction.

Part 2 of 5How a convection current forms

  1. The water in contact with the pan is warmed.
  2. Its molecules move faster and further apart.
  3. The warmer water becomes less dense.
  4. The warmer water rises (floats up).
  5. Cooler, denser water moves down to take its place.
  6. Eventually all the water circulates up and down, forming a convection current.
  • Cooling at the top — Fluid that reaches the top cools, becomes denser and sinks.

Part 3 of 5Everyday examples

  • Fire in a houseHot smoky air expands and rises, so firefighters crawl along the floor where the air is cleanest.
  • RadiatorsThey are placed low in rooms so that warm air rises and cooler air flows down to be heated.
  • Freezer compartmentIt is at the top of a fridge so that cold, dense air sinks and cools the food.
  • ThermalsConvection currents in the air caused by the Sun heating the ground are used by birds to climb high.

Part 4 of 5Sea breezes

1Daytime3 points
  • The land heats faster than the sea, so the air above the ground gets hot.
  • The hot air expands and rises like hot water in a pan.
  • Cooler air from above the sea replaces it, so the breeze comes off the sea.
2Night3 points
  • The ground cools more quickly than the sea.
  • Air above the sea is now warmer and rises.
  • Cooler air from above the land replaces it, so the breeze comes off the land.

Part 5 of 5Convection in the atmosphere

  • EquatorThe Sun heats the Earth more at the equator than near the poles.
  • Rising airAir near the equator rises and is replaced by cooler air.
  • WeatherThis moves the air in the atmosphere and affects weather patterns across the world.
  • Key ideaWhen a gas or liquid is heated it expands and becomes less dense, and the less dense fluid rises.

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