Explain why a metal railway track has a higher thermal expansion than a wooden railway sleeper when both are heated by the same temperature rise, and state one practical consequence of thermal expansion that is accounted for in railway track design.

Cambridge International IGCSE Physics (0625) — 2.2 Thermal properties and temperature · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

When heated the molecules in both the rail and the sleeper vibrate more and the material expands.
Metals have weaker inter‑atomic bonds and a larger coefficient of linear expansion than wood, so for the same temperature rise the rail lengthens more.
Railways are therefore laid with small gaps between sections of rail.
These gaps allow the rail to expand without buckling or bending when it is heated.

Examiner tips

  • Use the phrase ‘coefficient of linear expansion’ to show understanding of why metals expand more. Explain the purpose of the gaps in a concise way.
  • Mention that the gaps prevent the rail from buckling in warm weather.

Mark scheme (4 marks)

  1. When heated, molecules (in both materials) vibrate more / take up more space
  2. Metals / the rail expand more than wood because the intermolecular forces in metal allow greater expansion / metals expand more for the same temperature rise (accept: different materials have different rates/amounts of thermal expansion)
  3. Small gaps are left between sections of railway track
  4. So that the track does not buckle / bend when it expands (in warm weather / when heated)

Key terms in this question

thermal expansion

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