A student stretches a copper wire by hanging different weights from it. At first, the wire returns to its original length when the weights are removed. After a certain weight is added, the wire does not return to its original length when that weight is removed. Explain what has happened to the copper wire at this point and describe the difference between the wire's behaviour before and after this change.

Model answer (5 marks)

Examiner tips

• Use the terms "elastic limit" and "plastic deformation" – examiners look for these exact words.
• Show the difference clearly: elastic = return to original length, plastic = permanent change.
• Mention the limit of proportionality as the point where Hooke’s law no longer applies.

Common mistakes

• Saying the wire is "broken" instead of "stretched beyond its elastic limit".
• Confusing elastic with plastic deformation – e.g. saying the wire still returns to its original length after the load is removed.
• Using vague phrases like "the wire is damaged" without specifying elastic/plastic behaviour.