Practical skills in core practicals 7 and 8

Core practical 7: resistivity

• Diameter with a micrometer at several places and orientations, correcting zero error. It is squared in A = πd²/4, so it usually dominates the uncertainty.
• At least six lengths over a wide range, measured with a metre rule along which the wire is taped.
• Plot R against l: gradient = ρ/A, so ρ = gradient x A.
• A positive intercept on the R axis shows contact resistance at the clips: a systematic error that does not change the gradient. Using the gradient rather than single values removes its effect.
• Small currents, switched off between readings, so the temperature (and so R) stays constant.

Core practical 8: e.m.f. and internal resistance

• Vary the external resistance and record pairs of V and I; at least six pairs over a wide range of current.
• Plot V against I: gradient = -r, y-intercept = ε. If the current axis starts at zero, read ε directly; otherwise calculate it from a point on the line.
• A protective resistor limits the current; open the switch between readings so the cell does not warm up or run down.

Worked example

For a 0.32 mm diameter wire, R against l has gradient 13.0 Ω/m. Find ρ.

1. A = π(0.32 x 10^-3)² / 4 = 8.04 x 10^-8 m²
2. ρ = 13.0 x 8.04 x 10^-8 = 1.05 x 10^-6 Ω m

Answer: ρ = 1.05 x 10^-6 Ω m

Check yourself

1. For a wire of diameter 0.27 mm, a graph of R against l has gradient 9.30 Ω/m. Find ρ, in units of 10^-7 Ω m, to 3 s.f.
   Show answer

   5.32 x 10^-7 Ω m. A = 5.73 x 10^-8 m²; ρ = 9.30 x A = 5.32 x 10^-7 Ω m.

2. In core practical 7, a micrometer of resolution 0.01 mm gives a wire diameter of 0.27 mm. Find the percentage uncertainty in the diameter, using half the resolution, to 2 s.f.
   Show answer

   1.9 %. Uncertainty = 0.005 mm; 0.005 / 0.27 x 100 = 1.9%.

3. A graph of R against l in core practical 7 has a small positive intercept on the R axis. What is the most likely cause?
   • The wire heating up
   • A random error in the lengths
   • The voltmeter having a high resistance
   • Contact resistance at the clips
   Show answer

   Contact resistance at the clips. Contact resistance adds the same resistance to every reading, raising the whole line.

4. A graph of V against I for a cell passes through (0.10 A, 1.47 V) and (0.50 A, 1.15 V). Find the e.m.f. in V.
   Show answer

   1.55 V. r = (1.47 - 1.15) / 0.40 = 0.80 Ω; ε = 1.47 + 0.80 x 0.10 = 1.55 V.

5. For that cell, find the internal resistance in Ω.
   Show answer

   0.80 Ω. r = -gradient = 0.32 / 0.40 = 0.80 Ω.

6. Why is ρ found from the gradient of R against l rather than from a single reading of R and l?
   • The gradient is unaffected by contact resistance and uses all the data
   • A single reading cannot be calculated
   • The gradient always has zero uncertainty
   • It avoids measuring the diameter
   Show answer

   The gradient is unaffected by contact resistance and uses all the data. A constant offset cancels in the gradient, and a line through many points reduces random error.

7. Why should the switch be opened between readings in core practical 8?
   • To reset the ammeter
   • To let the voltmeter cool
   • So the cell does not warm up or run down, which would change ε and r
   • To reduce contact resistance
   Show answer

   So the cell does not warm up or run down, which would change ε and r. ε and r must stay constant during the experiment.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics specification (XPH11/YPH11), Issue 3
• Pearson qualifications: International Advanced Levels Physics (2018)
• Pearson examiners' report: IAL Physics WPH13/01, October 2024

Last updated 8 November 2026. Spotted a mistake? Tell us.