Required Practicals / Edexcel / Practical 3
3 AS CP3

Determination of resistivity (CP3)

Determine the resistivity of a metal wire by measuring its resistance at different lengths.

Apparatus

  • Resistance wire (e.g. nichrome) ~1 m long
  • Micrometer screw gauge
  • Ammeter and voltmeter
  • Low-voltage DC power supply
  • Crocodile clips, metre rule and connecting leads

Safety

  • Keep current low to avoid overheating the wire.
  • Check polarity of ammeter connections before switching on.

Method

  1. Measure the wire diameter d at three positions; calculate $A = \pi(d/2)^2$.
  2. Connect the ammeter in series and voltmeter across a measured length L of wire using crocodile clips.
  3. Record V and I for at least seven values of L from 0.10 m to 1.0 m; calculate $R = V/I$.
  4. Plot R vs L: straight line through origin with gradient $= \rho/A$.
  5. Calculate $\rho = \text{gradient} \times A$.

Key Variables

Independent Wire length L
Dependent Resistance R
Controlled Cross-sectional area A; Temperature (low current)

Analysis and Results

  • $R = \rho L/A$. Gradient of R vs L equals $\rho/A$.
  • A non-zero y-intercept indicates contact resistance at the clips.
  • Uncertainty in $\rho$ comes from uncertainties in the gradient and in A.

Common Errors

  • Heating the wire with high current, increasing resistivity and causing a non-linear graph.
  • Measuring total wire length rather than the length between crocodile clips.
  • Not zeroing the micrometer before measuring diameter.

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

Q1 3 marks

A resistance wire has diameter 0.38 mm and the gradient of its R vs L graph is $12.5$ $\Omega$ m$^{-1}$. Calculate the resistivity.

Q2 2 marks

The R vs L graph has a positive y-intercept. Suggest a reason for this.