Unit Overview

Deformation of solids

CIE A-Level Physics· 5 min read 📊 n/a

1. Unit at a glance

This unit progresses from basic definitions of deformation parameters to interpreting how different materials behave under load, ending with calculation of energy stored during deformation. The learning arc builds sequentially: we start with core quantities to quantify deformation, then move to graphical representations of behaviour, distinguish between permanent and temporary deformation, and finally connect deformation to energy storage.

Work through the following sub-topics in order to build your understanding:

Stress, strain and Young modulus

Learn core definitions and experimental methods to calculate stress, strain and Young modulus.

★★⏱ 10 min

Force-extension and stress-strain graphs

Interpret key features of deformation graphs for brittle, ductile and polymeric materials.

★★⏱ 12 min

Elastic and plastic deformation

Distinguish between temporary and permanent deformation, and learn key concepts like yield point.

★★★⏱ 8 min

Strain energy

Calculate energy stored in a deformed solid using formulas and graphical methods.

★★⏱ 9 min

2. Common Pitfalls

Wrong move:

Confusing extension with total length when calculating strain or Young modulus

Why:

Strain depends only on the change in length from the original undeformed state

Correct move:

Always use $Δ L$ (extension) divided by original length $L_{0}$ to calculate strain

Wrong move:

Assuming all materials follow Hooke's law up to their breaking point

Why:

Hooke's law only applies in the proportional elastic region for most materials, and many materials never follow it

Correct move:

Only apply Hooke's law for deformation within the proportional limit of a Hookean material

3. Quick Reference Cheatsheet

Quantity/Concept	Formula / Key Definition
Tensile Stress	$σ = \frac{F}{A}$ , where $F$ = applied force, $A$ = cross-sectional area
Tensile Strain	$ε = \frac{Δ L}{L _{0}}$ , where $Δ L$ = extension, $L_{0}$ = original length
Young Modulus	$E = \frac{σ}{ε} = \frac{F L _{0}}{A Δ L}$ , a constant for a given material
Hooke's Law	$F = k Δ L$ , applies up to the proportional limit
Strain Energy (Hookean)	$U = \frac{1}{2} F Δ L = \frac{1}{2} k (Δ L)^{2}$

What's Next

Begin with the first sub-topic to master core definitions of stress, strain and Young modulus, which form the foundation for all other concepts in this unit. Work through each sub-topic in order to build up your understanding of material deformation step by step. Once you have completed all sub-topics in this unit, you can progress to the next unit introducing core properties of waves.