A student places a hot drink in a polystyrene cup and notices it stays warm for much longer than when placed in a metal cup. Explain why polystyrene is a better thermal insulator than metal, and describe how adding a lid to the cup improves the situation further.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Polystyrene cups are often used to keep hot drinks warm. Metal cups allow drinks to cool quickly.
Model answer (4 marks)
Polystyrene has a very low thermal conductivity, so heat is conducted away from the hot drink very slowly, whereas metal conducts heat rapidly.
Because of this, the polystyrene cup acts as a thermal insulator and keeps the drink warm for a longer time.
Without a lid, heat is also lost from the surface of the drink by evaporation and by convection currents in the air above it.
A lid covers the opening, providing additional insulation at the top, reducing evaporation and convection, and therefore further slowing the rate of cooling.
Because of this, the polystyrene cup acts as a thermal insulator and keeps the drink warm for a longer time.
Without a lid, heat is also lost from the surface of the drink by evaporation and by convection currents in the air above it.
A lid covers the opening, providing additional insulation at the top, reducing evaporation and convection, and therefore further slowing the rate of cooling.
Examiner tips
- Use the term "thermal conductivity" and state that metal > polystyrene. Explain conduction, evaporation and convection. Mention the lid reduces heat loss from the surface.
- Include the effect of the lid on both evaporation and convection to show full understanding.
Common mistakes
- Saying the cup is a good insulator without mentioning thermal conductivity. Forgetting that the lid also stops evaporation. Using vague terms like "keeps it warm" without explaining the mechanisms.
Mark scheme (4 marks)
- Metal has a higher thermal conductivity than polystyrene, so energy is transferred by conduction more quickly through metal.
- Polystyrene acts as thermal insulation, reducing the rate of heat transfer from the drink to the surroundings.
- Without a lid, energy is also lost from the surface of the drink (by evaporation or convection currents in the air above the liquid).
- A lid provides thermal insulation at the top, reducing the overall waste energy transfer and slowing the rate of cooling.
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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