Required Practicals / Edexcel / Practical 2
2 AS CP2

Determination of the Young modulus (CP2)

Determine the Young modulus of a metal by measuring the extension of a wire under increasing tensile load.

Apparatus

  • Long metal wire (~2 m, e.g. steel or copper) clamped at the top
  • Micrometer screw gauge
  • Metre rule and vernier scale (or travelling microscope)
  • Slotted masses and hanger
  • G-clamp and pulley

Safety

  • Wear safety goggles in case the wire snaps under overload.
  • Place a cushion or tray below hanging masses.

Method

  1. Clamp the wire vertically. Attach a small initial load to straighten it and record the zero extension reading.
  2. Measure the unstretched length L of the wire with a metre rule.
  3. Measure the wire diameter d at three positions; calculate mean $A = \pi(d/2)^2$.
  4. Add masses in 100 g steps. After each addition, record the extension x from the vernier scale.
  5. Unload and confirm the wire returns to its original length (elastic behaviour). Plot stress vs strain; gradient $= E$.

Key Variables

Independent Applied force F (stress $= F/A$)
Dependent Extension x (strain $= x/L$)
Controlled Temperature; Original wire length L; Wire cross-section A

Analysis and Results

  • $E = \frac{\text{stress}}{\text{strain}} = \frac{FL}{Ax}$.
  • Plot F vs x: gradient $= EA/L$, so $E = \text{gradient} \times L/A$.
  • Plot stress vs strain for a cleaner determination of E directly from the gradient.

Common Errors

  • Measuring diameter at only one position along the wire.
  • Overloading past the elastic limit so the wire deforms plastically.
  • Not accounting for the initial extension present before the first mass is added.

Exam-style questions on this practical. Click Show mark scheme to reveal the answer after attempting each question.

Q1 4 marks

A copper wire of length 1.8 m and diameter 0.60 mm extends by 0.90 mm when a load of 15 N is applied. Calculate the Young modulus of copper.

Q2 2 marks

Explain why the wire should be unloaded after the experiment and the extension checked.