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Edexcel A Level (IAL) Physics-2.41 Core Practical 7: Investigating Resistivity- Study Notes- New Syllabus

Edexcel A Level (IAL) Physics -2.41 Core Practical 7: Investigating Resistivity- Study Notes- New syllabus

Edexcel A Level (IAL) Physics -2.41 Core Practical 7: Investigating Resistivity- Study Notes -Edexcel A level Physics – per latest Syllabus.

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Edexcel A level Physics-Study Notes- All Topics

CORE PRACTICAL 7: Determining the Electrical Resistivity of a Material

This practical determines the resistivity \( \rho \) of a wire using measurements of its resistance, length, and cross-sectional area. The key equation is:

\( R = \rho\dfrac{l}{A} \)

Rearranged to find resistivity:

\( \rho = R\dfrac{A}{l} \)

 Apparatus

  • Test wire of known material
  • Metre ruler
  • Micrometer screw gauge (for diameter)
  • Ammeter (A)
  • Voltmeter (V)
  • Power supply (low voltage)
  • Switch
  • Crocodile clips
  • Connecting leads

Method

  1. Measure the wire’s diameter at several points using a micrometer to account for irregularities.
  2. Calculate the average diameter and then calculate cross-sectional area:

    \( A = \dfrac{\pi d^{2}}{4} \)

  3. Attach the wire to a metre ruler and connect it into a circuit with voltmeter across the test length.
  4. Set a known length \( l \) (e.g., 20 cm, 40 cm, 60 cm…)
  5. Switch on the power briefly and record the voltage \( V \) and current \( I \).
  6. Calculate resistance using:

    \( R = \dfrac{V}{I} \)

  7. Repeat for several lengths \( l \) of the wire.
  8. Plot a graph of \( R \) (y-axis) against \( l \) (x-axis).
  9. The gradient of the line is:

    \( \text{gradient} = \dfrac{R}{l} = \dfrac{\rho}{A} \)

  10. Multiply gradient by \( A \) to obtain resistivity:

    \( \rho = \text{gradient} \times A \)

Why a Graph is Used

  • Reduces random error from single measurements.
  • Allows identification of anomalies.
  • Gradient gives a more reliable value of \( \rho \).

Safety Considerations

  • Use a low voltage supply to avoid overheating the wire.
  • Switch on for short periods only (minimises temperature rise → avoids resistance changes).
  • Handle micrometer carefully to avoid damaging the wire.

Sources of Error & How to Reduce Them

  • Wire not uniform thickness: measure diameter in several places and average.
  • Poor contact resistance: ensure crocodile clips grip firmly.
  • Heating of wire: use low voltage, short measurements.
  • Length measurement error: ensure wire is straight AND ruler aligned properly.

Conclusion

  • This practical allows accurate determination of material resistivity.
  • Graphical method improves reliability.
  • Careful measurement of diameter is crucial.

Example Calculation

You measure:

  • Average diameter: \( d = 0.50\ \mathrm{mm} = 0.00050\ \mathrm{m} \)
  • Length: \( l = 0.80\ \mathrm{m} \)
  • Voltage across wire: \( 1.20\ \mathrm{V} \)
  • Current through wire: \( 0.50\ \mathrm{A} \)
▶️ Answer / Explanation

Step 1: Calculate area

\( A = \dfrac{\pi d^{2}}{4} = \dfrac{\pi (0.00050)^{2}}{4} = 1.96\times10^{-7}\ \mathrm{m^{2}} \)

Step 2: Calculate resistance

\( R = \dfrac{V}{I} = \dfrac{1.20}{0.50} = 2.40\ \Omega \)

Step 3: Calculate resistivity

\( \rho = R\dfrac{A}{l} = 2.40 \cdot \dfrac{1.96\times10^{-7}}{0.80} \)

\( \rho = 5.88\times10^{-7}\ \Omega\mathrm{m} \)

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