Explain why placing a hot drink in a vacuum flask with a shiny inner surface helps to keep the drink warm for longer. In your answer, refer to conduction, convection and radiation.

Cambridge International IGCSE Physics (0625) — 2.3 Transfer of thermal energy · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The vacuum between the double walls stops heat transfer by conduction and convection because these processes need a material medium to carry energy.

The shiny inner surface is a poor emitter of thermal radiation; it reflects most of the infrared radiation that the drink emits back into the drink, so less energy is lost by radiation.

Thus the drink stays warm longer.

Examiner tips

  • Use the word "prevent" for conduction/convection and "reflect" for radiation
  • Show the link between vacuum and lack of medium
  • Mention the shiny surface as a poor emitter
  • Keep the answer concise and use UK spelling

Common mistakes

  • Saying the vacuum only stops conduction, ignoring convection
  • Claiming the shiny surface absorbs more radiation
  • Using vague terms like "insulation" instead of specifying conduction, convection, radiation

Mark scheme (4 marks)

  1. The vacuum (between the double walls) prevents thermal energy transfer by conduction and/or convection
  2. Conduction and/or convection require particles/a medium to transfer energy, which are absent in a vacuum
  3. The shiny inner surface is a poor emitter of thermal radiation
  4. The shiny inner surface reflects thermal radiation back into the drink / reduces energy lost by radiation

Key terms in this question

vacuum · conduction · convection · radiation

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