Science — CBSE Class 10 board question

(i) Resistance of a conductor is given by $R = \rho \dfrac{l}{A}$. When length or area of cross-section changes, the ratio $l/A$ changes, so R changes. However, resistivity ($\rho$) depends only on the nature (material) of the conductor, not on its dimensions. Hence, resistivity remains unchanged even when shape or size changes.

(ii) The resistivity of an alloy is generally higher than that of its constituent pure metals. Also, alloys do not oxidise (burn) readily at high temperatures. Therefore, they can withstand the high temperatures produced in heating devices like toasters and electric irons without melting or oxidising, making them more suitable than pure metals.

(iii) SI unit of electric current is ampere (A).
One ampere is defined as the flow of one coulomb of charge per second through a cross-section of a conductor.
$$1 \text{ A} = \frac{1 \text{ C}}{1 \text{ s}}$$

Source: Chapter 11 – Electricity, Sections 11.5 and 11.7.1

---

• For part (i), the key is distinguishing resistance (geometric + material property) from resistivity (material property only). Always quote the formula $R = \rho\,l/A$ to justify your reasoning.
• For part (ii), two reasons are expected: high resistivity (produces more heat efficiently) and non-oxidising nature at high temperatures. Both points carry marks.
• For part (iii), simply naming the unit alone may not fetch full marks. Always add the definition (1 C/s) when 1 mark is allocated to a "define" question.