Topic 4: Materials — Edexcel A-Level Physics
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Topic 4: Materials explained
Force-extension graphs plot the applied force against the extension of a specimen; force-compression graphs plot force against the reduction in length, so the axes are labelled with the appropriate deformation.
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In the linear region the graph is a straight line through the origin, and its gradient equals the stiffness (force per unit extension). The limit of proportionality is the point beyond which force is no longer proportional to deformation, so the line starts to curve. The elastic limit is the point beyond which the material no longer returns to its original length when the force is removed. The yield point is where a small increase in force produces a large increase in deformation as plastic flow begins. Elastic deformation is reversible; plastic deformation is permanent.
Your focus
- Draw and interpret force-extension and force-compression graphs, identifying the linear region and its gradient.
- Define and locate the limit of proportionality, elastic limit and yield point on a graph.
- Distinguish elastic from plastic deformation using loading and unloading behaviour.
Topic 4: Materials exam tips
Marking Points
- States that the gradient of the linear section of a force-extension graph equals the stiffness (force per unit extension).
- Identifies the limit of proportionality as the point where the graph stops being a straight line through the origin.
- Distinguishes the elastic limit (no permanent deformation on unloading) from the limit of proportionality (end of proportionality).
- Identifies the yield point as where a small force increase causes a large increase in deformation.
- Explains that elastic deformation is reversible and plastic deformation is permanent.
- Describes unloading after plastic deformation as a line parallel to the elastic region, leaving a permanent extension.
Examiner Tips
- 💡Label both axes with the correct quantity and unit before reading any feature from the graph.
- 💡When asked to identify a point, quote the feature of the graph (straight line, curve, large deformation for small force) rather than just naming it.
- 💡For unloading questions, sketch the unloading line parallel to the elastic region to show permanent deformation.
Common Mistakes
- Confusing the limit of proportionality with the elastic limit: the correction is that proportionality ends when the line curves, while elasticity ends when permanent deformation remains after unloading.
- Assuming the gradient of a force-extension graph equals the spring constant in all cases: the correction is that the gradient gives stiffness for that specimen, and the spring constant applies to the specific spring being tested.
- Thinking a force-compression graph has extension on the horizontal axis: the correction is that the horizontal axis shows compression (reduction in length).
- Believing plastic deformation still returns to the original length: the correction is that plastic deformation is permanent.