IGCSE Physics / Thermal energy
Convection and convection currents
Edexcel International GCSE Physics (4PH1), chapter 15: Thermal energy, specification point 4.6, 4.7. Free lesson.
Read the revision notes · IGCSE Physics past papers
Convection transfers thermal energy through liquids and gases as warmer, less dense fluid rises and cooler, denser fluid sinks. Understanding this cycle explains how devices such as radiators, kettles and fridges heat or cool their contents.
How heating makes a fluid expand
When a gas is heated, its particles move faster and further apart. The particles do not get bigger. The gaps between them grow, so the gas takes up more space and expands, if it can. The same mass of gas spread through a larger space is less dense. Density describes how much mass there is in a given volume.
How a convection current forms
A convection current is a circulating flow in a liquid or gas. In a pan heated from below, the water near the bottom warms, expands and becomes less dense than the water around it. It rises. At the top it cools, its particles move closer together, and it becomes denser. It sinks, while cooler water flows in at the bottom to be heated. The repeated movement forms a loop that carries thermal energy through the fluid. Warm, less dense fluid rises; cool, denser fluid sinks.
How convection works in everyday situations
Convection explains how warm or cool fluid can spread through a room, appliance or surrounding area. In each case, fluid moves because heating or cooling changes its density. A radiator warms nearby air. The warm air rises, and cooler air sinks across the room to replace it, helping to heat the whole room. A kettle’s heating element is at the bottom. Heated water rises, so the water in the kettle mixes. A fridge’s freezer compartment is at the top. Air near it cools and sinks, while warmer air rises to be cooled. By day, land heats faster than the sea. Warm air above the land rises,…
Why convection does not happen in solids
Convection needs the heated material itself to flow. In a solid, particles are fixed in place and can only vibrate, so they cannot move from place to place as a fluid does. Convection happens only in liquids and gases, which are fluids.
Explain why a fridge freezer is at the top
A full-mark explanation links cooling to density, then describes how the moving air cools the rest of the fridge.