1. Key Definitions: Stress and Strain★★☆☆☆⏱ 10 min
When a force deforms a material, stress and strain normalise the effect of force to the size of the sample, letting us compare properties of different sized objects made of the same material.
2. Young Modulus: Definition and Calculation★★☆☆☆⏱ 12 min
Young modulus is an intensive material property that describes stiffness: a higher Young modulus means a stiffer material that deforms less for a given applied stress.
3. Experimental Determination of Young Modulus★★★☆☆⏱ 15 min
CIE frequently asks 5-6 mark questions describing this experiment, so you need to remember the method, measurements and error reduction steps.
Clamp a long metal wire to a rigid support, with a fixed ruler alongside the wire.
Add known masses (weights) to the free end, measure extension for each weight.
Measure original length $L_0$ from the clamp to a marker on the wire with a metre ruler.
Measure diameter at multiple points along the wire with a micrometer, calculate average diameter.
Plot a graph of force $F$ against extension $\Delta L$, find the gradient $m = F/\Delta L$.
Calculate Young modulus with $E = \frac{m L_0}{A}$, where $A = \pi (d/2)^2$.
4. Stress-Strain Graph Properties★★★☆☆⏱ 10 min
The gradient of a stress-strain graph in the elastic region is equal to Young modulus, since gradient = $\Delta \sigma / \Delta \varepsilon = E$. This is a common exam question.
Common Pitfalls
Why: This leads to Young modulus values 10⁶ times too large or small, which is a common exam error.
Why: Most students measure diameter directly and forget to convert to radius for the area formula.
Why: Students confuse extension (sample-dependent) with strain, which is normalised for sample size.
Why: Strain is calculated as a ratio of two lengths, so students incorrectly add units.
Why: Stress is no longer proportional to strain once plastic deformation starts.