A-Level Physics May/June 2025 Q7(a): A nichrome resistance wire has length 150cm, cross-sectional area 2.45 × 10⁻⁷m² and res…

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  The relationship between resistance $R$, resistivity $\rho$, length $L$ and cross-sectional area $A$ is given by the formula:

  $R = \frac{\rho L}{A}$ [1]

  First, convert the length from cm to the base SI unit of metres: $L = 150 \text{ cm} = 1.5 \text{ m}$

  Now, substitute the given values into the formula:

  $R = \frac{(1.12 \times 10^{-6} \text{ } \Omega\text{m}) \times (1.5 \text{ m})}{2.45 \times 10^{-7} \text{ m}^2}$ [1]

  $R = 6.85714... \text{ } \Omega$

  Rounding the final answer to three significant figures as requested:

  $R = 6.86 \text{ } \Omega$ [1]

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  How the marks are awarded

  • C1 — Correctly stating the formula for resistance, $R = \rho L / A$.
  • C1 — Correctly substituting all values into the formula, including the conversion of length from 150 cm to 1.5 m.
  • A1 — Calculating the final correct answer of 6.86 Ω, which is correctly rounded to three significant figures.

  Common mistakes

  • Forgetting to convert the length from 150 cm to 1.5 m, leading to an answer of 686 Ω, which would lose the substitution and final answer marks.
  • Incorrectly rounding the final answer, for example to 6.9 Ω (2 s.f.) or writing the full calculator display, which would lose the final accuracy mark.
  • Misremembering the formula, for example as $R = \rho L A$ or $R = \rho A / L$, which would result in losing most or all of the marks.
  • Forgetting to include the unit (Ω) for the final answer, which can lead to the loss of the final mark.

  Examiner tip: Always check that all quantities are in their base SI units before substituting them into a physics formula.

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