CBSE Class 10 Science 2018 · Paper 31/1 · Question 13

(a) The resistance $R$ of a wire depends on its length $l$ ($R\propto l$), its area of cross-section $A$ ($R\propto \frac{1}{A}$), and the nature of its material, that is its resistivity $\rho$; together, $R=\rho \frac{l}{A}$. It also depends on the temperature of the wire. (b) Metals have a large number of free electrons that can move through the metal, so their resistivity is very low (copper: $1.62\times 10^{-8}\Omega$ m) and they are good conductors. In glass the electrons are held tightly by the atoms and there are no free electrons to carry charge, so its resistivity is very high ($10^{10}$ to $10^{14}\Omega$ m) and it is a bad conductor. (c) Alloys have a high resistivity, generally higher than that of their constituent metals, so the heating element has a high resistance and produces most of the heat ($H=I^{2}Rt$). Also, alloys do not oxidise (burn) readily at high temperatures, and the heating alloys used, such as nichrome, have a high melting point, so the element can stay red-hot for a long time.