A metal cooking pot is painted matt black on the outside. Explain why this coating is more effective at keeping food warm than a shiny silver coating would be.

AQA GCSE Physics (8463) — 4.6.3 Black body radiation (Phys only) · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A manufacturer claims that painting the outside of a cooking pot matt black helps food stay warmer for longer after cooking.

Model answer (4 marks)

A matt black surface is a good emitter of infrared radiation, so the pot radiates heat away rapidly.
A shiny silver surface reflects most infrared radiation and emits very little, so it loses heat more slowly.
Thus the black pot loses energy by radiation faster than the silver pot.
Therefore the silver coating keeps the food warmer for longer because less heat is transferred to the surroundings.

Examiner tips

  • Use the term "black body" or "good emitter" for the black surface. Mention that the silver surface reflects IR. Explain the consequence for heat loss. Keep the answer concise and use the exact wording from the mark scheme.

Common mistakes

  • Confusing absorption with emission. Saying the black pot keeps food warmer. Omitting the point that the silver surface emits less IR.

Mark scheme (4 marks)

  1. A black body / matt black surface is a better emitter of infrared radiation than a shiny/silver surface
  2. The shiny/silver surface emits less infrared radiation (in a given time)
  3. So the matt black pot loses energy (by radiation) at a faster rate / the shiny pot loses energy more slowly
  4. Therefore the shiny/silver coating is more effective at keeping food warm because less energy is transferred to the surroundings

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