Required Practicals / OCR A / Practical 3
3 AS PAG 3

Determination of resistivity (PAG 3)

Find the resistivity of a wire by measuring resistance at different lengths.

Apparatus

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

Safety

  • Keep current low to prevent the wire heating up.
  • Confirm all connections before switching on.

Method

  1. Measure diameter d at three positions; calculate $A = \pi(d/2)^2$.
  2. Connect ammeter in series; voltmeter across length L of wire. Record V and I for six to eight values of L.
  3. Calculate $R = V/I$ for each L.
  4. Plot R vs L: gradient $= \rho/A$; calculate $\rho = \text{gradient} \times A$.

Key Variables

Independent Length L
Dependent Resistance R
Controlled Cross-section A; Temperature (low current)

Analysis and Results

  • $R = \rho L/A$. Gradient of R vs L equals $\rho/A$.
  • Line should pass through the origin; a y-intercept indicates contact resistance.
  • Percentage uncertainty in $\rho$ comes from uncertainties in the gradient and A.

Common Errors

  • Heating the wire with high current.
  • Measuring total wire length not the length between clips.
  • Not zeroing the micrometer.

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

Q1 3 marks

A wire of diameter $0.50$ mm has gradient of R vs L equal to $4.4$ $\Omega$ m$^{-1}$. Calculate the resistivity.

Q2 2 marks

Suggest two improvements to reduce uncertainty in the cross-sectional area measurement.