IGCSE Physics revision · Thermal physics
Heat transfer
Three mechanisms, endlessly examined through everyday objects: saucepans, radiators, vacuum flasks, greenhouses. Each mechanism has a fixed explanation script — learn the script and the six-mark question writes itself.
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What the syllabus demands
- —Describe conduction in solids in terms of lattice vibrations and (in metals) free electrons
- —Describe convection in liquids and gases in terms of density changes
- —Describe thermal radiation as infrared waves requiring no medium
- —Relate surface colour and texture to emission and absorption of radiation
- —Apply all three to applications: vacuum flasks, cooling fins, household insulation
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Conduction
- The transfer of thermal energy through a material by particle vibrations passed between neighbouring particles — and in metals, mainly by free electrons moving through the lattice.
- Convection
- The transfer of thermal energy in a fluid: the heated fluid expands, becomes less dense and rises, while cooler, denser fluid sinks to replace it, setting up a convection current.
- Thermal radiation
- The transfer of energy by infrared electromagnetic waves; it requires no medium and can travel through a vacuum.
Where the marks die
Common mistakes to check
- 01
Writing 'heat rises'. Heat does not rise — hot fluid rises, because it expands and becomes less dense. The density chain is the mark.
- 02
Explaining why metals conduct well without free electrons. Non-metals conduct only by lattice vibrations; metals conduct much better because free electrons transfer energy quickly through the material.
- 03
Forgetting that radiation is the only transfer that works through a vacuum — the entire point of the vacuum flask question, and of how the Sun's energy reaches Earth.
- 04
Reversing the surface rules: dull black surfaces are the best emitters and best absorbers; shiny silver surfaces are the worst emitters and best reflectors.
One worked example, done properly
Question
Explain how a vacuum flask keeps a hot drink hot. (4 marks)
Method
- 1.The vacuum between the double walls prevents conduction and convection, because both need a medium.
- 2.The silvered surfaces reflect infrared radiation back, reducing transfer by radiation.
- 3.The stopper prevents convection currents escaping from the top.
- 4.The insulating supports minimise conduction through the structure.
Four transfer routes named and blocked — one mark each.