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2.3.2Thermal Physics

Convection — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 8 parts Lesson 31 of 45 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

2.3.2 Convection — IGCSE PHYSICS notes: Explain convection currents in terms of changes in density and describe how to demonstrate them.Convection — an important method of thermal energy transfer in liquids and gasesFluids only — convection occurs only in fluids and cannot happen…

  • Thermal energy does not rise
  • Free circulation
  • Fluids only — convection occurs only in fluids and cannot happen in solids
  • Cause — changes in temperature cause changes in density
  • Heating — the molecules gain kinetic energy and move faster, pushing apart so the fluid expands
  • Density — the hot fluid is less dense than its surroundings

Part 1 of 6What convection is

Convection
  • Convection an important method of thermal energy transfer in liquids and gases
Fluids only
  • Fluids only convection occurs only in fluids and cannot happen in solids
Cause
  • Cause changes in temperature cause changes in density

Part 2 of 6Convection when heated

Heating
  • Heating the molecules gain kinetic energy and move faster, pushing apart so the fluid expands
Density
  • Density the hot fluid is less dense than its surroundings
Rising
  • Rising the hot fluid rises and cooler fluid moves in to replace it
Cooling
  • Cooling the fluid cools, contracts, becomes denser and sinks back down
Result
  • Result the circulating motion is a convection current

Part 3 of 6Convection when cooled

Cooling
  • Cooling molecules lose kinetic energy, move slower and collide less often, so the fluid contracts
Density
  • Density the cold fluid is denser than its surroundings
Sinking
  • Sinking cold fluid sinks and warmer, less dense fluid moves into the space
Example
  • Example an air conditioning unit high on a wall

Part 4 of 6Demonstrations

Liquid
  • Liquid place potassium permanganate crystals in a beaker of water and heat beside them
  • The purple dissolved crystals rise with the warm water
  • As the water cools and becomes denser it falls and carries the colour down
Gas
  • Gas candles under angled blades on a central column
  • Heated air becomes less dense and rises
  • The rising air pushes the blades sideways and they spin

Part 5 of 6Common points

1Thermal energy does not rise
  • Thermal energy does not rise it is the hot fluid that rises because its density is lower
2Free circulation
  • Free circulation the fluid must be free to circulate
  • If it is trapped or heated from above the process is usually conduction

Part 6 of 6More from the specification

1Convection
  • Heat transfer in fluids by bulk movement: warmer fluid expands, becomes less dense and rises while cooler denser fluid sinks.
2Convection currents
  • Radiators heat air near them, which rises and circulates around a room, carrying thermal energy.
3Radiation contrast
  • Infrared radiation needs no medium;
    • dull black surfaces emit and absorb best, while shiny metal surfaces reflect and emit poorly.
4Greenhouse effect
  • Gases in the atmosphere absorb infrared radiation emitted by the warm Earth surface and re-emit it, raising the surface temperature.
5Conduction contrast
  • In a metal object, free electrons carry energy by conduction, which differs from the fluid movement of convection.
6Absorbed
  • Taken in by an object, with light energy transferred to thermal energy.
7Lattice
  • The regular arrangement of atoms in a solid.
8Vibration
  • A repeated backwards and forwards movement.
9Greenhouse effect
  • Warming of the Earth because gases absorb and re-emit its infrared radiation
10Lattice vibrations
  • Vibrations of atoms or ions about fixed positions that pass energy on
11Thermal equilibrium
  • Constant temperature because energy is absorbed and lost at equal rates
12Radiation
  • Transfer by electromagnetic waves, requiring no medium
13Infrared
  • The part of the electromagnetic spectrum emitted by warm objects.
14Electron
  • Negatively charged particle in an atom.

Key terms

TermMeaning
ConvectionTransfer of thermal energy by the movement of a heated fluid
Convection currentCirculating flow of fluid caused by density differences
FluidA liquid or a gas
DensityMass per unit volume

Avoid these mistakes

Avoid these mistakes
Saying heat rises — hot fluid rises because it is less dense.
Saying convection occurs in solids — it needs a fluid.
Forgetting density in the explanation — always link temperature change to density change.

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