Elastic & Plastic Behaviour (AQA A Level Physics): Revision Note

Students often wonder about the section of graph between the limit of proportionality and the elastic limit. In this region, the material will still behave elastically (return to its original length) but it no longer obeys Hooke's law (a directly proportional relationship).

You will not be asked about this region on your exam. You do, however, need to be able to explain the behaviour of the material at all other points on the graph.

A sample of metal wire is subjected to a force which increases as a series of masses are added to the wire. The extension is measured and a force-extension graph of the data is plotted as shown below.

When the wire has been extended to Y, the point just before the wire fractures, the masses are removed one by one and the extension is re-measured.

(i) Describe the behaviour of the metal between X and Y.

(ii) On the graph, sketch the result obtained after the masses are removed and explain why the graph has this shape.

Answer:

Part (i)

• Since the graph is a straight line and the metal almost fractures, the point after X must be its elastic limit

• The graph starts to curve after this and would fracture beyond point Y [1 mark]

• This curve between X and Y denotes plastic behaviour [1 mark]

• Therefore, the metal exhibits plastic behaviour, but not elastic behaviour

Part (ii)

[1 mark]

• Plastic deformation has occurred which results in permanent extension

• As the load is decreased, the bonds in the metal are re-aligned hence the y-intercept is now not through the origin [1 mark]

• The gradient remains the same because the intermolecular forces (the forces between bonds) are identical to before [1 mark]

Avoid describing plastic deformation as 'does not obey Hooke's law'. Although this is mostly correct, it should be described as the material being permanently deformed.