A student compares two materials: a steel spring and a strip of polythene. The steel spring obeys Hooke's Law up to a certain point. The polythene strip stretches easily under a small force and does not return to its original length when the force is removed. Explain the behaviour of each material, using the terms elastic deformation, plastic deformation, and limit of proportionality in your answer.

AQA A-Level Physics (7408) — 3.4.2 Materials · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

The steel spring undergoes elastic deformation when the applied force is below its limit of proportionality. In this region the extension is directly proportional to the force, so the spring obeys Hooke’s Law. The limit of proportionality is the point at which the stress–strain relationship ceases to be linear; beyond this point the extension is no longer proportional to the force.

The polythene strip, on the other hand, undergoes plastic deformation. In plastic deformation the material is permanently deformed – the change in length does not recover when the force is removed, making the deformation irreversible.

Examiner tips

  • Use the terms elastic deformation, plastic deformation and limit of proportionality exactly as in the mark scheme.
  • Show the relationship between force and extension for the steel spring (Hooke’s Law) and explain why it stops being proportional beyond the limit of proportionality.
  • Explain that the polythene’s permanent change in length is a characteristic of plastic deformation.

Common mistakes

  • Confusing elastic with plastic deformation for the polythene; or saying the steel spring is plastic. Misstating the limit of proportionality as the yield point. Failing to mention that the polythene’s deformation is irreversible.

Mark scheme (5 marks)

  1. The steel spring undergoes elastic deformation (below the limit of proportionality)
  2. The extension of the steel spring is proportional to the force applied (up to the limit of proportionality / it obeys Hooke's Law)
  3. The limit of proportionality is the point beyond which extension is no longer proportional to the applied force
  4. The polythene strip undergoes plastic deformation
  5. Plastic deformation means the material is permanently deformed / the deformation is irreversible after the force is removed

Key terms in this question

elastic deformation · plastic deformation · limit of proportionality

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