(a) Write the relationship between resistivity and resistance of a cylindrical conductor of length $l$ and area of cross-section A. Hence derive the SI unit of resistivity. (b) Why are alloys used in electrical heating devices?

(a) The relationship between resistivity and resistance of a cylindrical conductor of length $l$ and area of cross-section A is $R = \rho \frac{l}{A}$. Hence, the SI unit of resistivity is $\Omega m$.<hr/>(b) Alloys are used in electrical heating devices because they have higher resistivity and do not oxidise readily at high temperatures.