Generated by claude-sonnet-4-6 · 2026-06-15 06:42 · grounding rag
Model Answer
(a) The resistance of a cylindrical conductor is given by:
$$R = \rho \frac{l}{A}$$
where $l$ = length, $A$ = area of cross-section, and $\rho$ = resistivity.
Rearranging: $\rho = \dfrac{RA}{l}$
SI unit of $\rho$ = $\dfrac{\Omega \times m^2}{m}$ = Ω m
(b) Alloys are used in electrical heating devices because:
- Their resistivity is higher than that of pure metals, so they produce more heat.
- They do not oxidise (burn) readily at high temperatures.
Source: Chapter 11, Section 11.5
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Explanation
- For part (a), always write the formula first, then rearrange to get ρ, then substitute SI units (Ω for R, m² for A, m for l) to derive Ω m. Examiners expect the derivation of the unit, not just stating it.
- For part (b), two points are needed but since it is 1 mark, one complete reason (high resistivity + no oxidation) suffices. Both points together make a stronger answer.