Science (086) — AI-generated practice question

(i) Heating element vs. copper wires:

From $R = \rho l/A$, resistance depends on resistivity ($\rho$), length, and cross-sectional area. The heating element of a toaster is made of an alloy (e.g., nichrome) which has very high resistivity ($\sim 100 \times 10^{-6}$ Ω m), whereas copper has very low resistivity ($1.62 \times 10^{-8}$ Ω m). Since $P = I^2R$, for the same current $I$, power dissipated is directly proportional to resistance. The heating element therefore has much higher resistance and dissipates far more heat, becoming very hot, while copper wires have negligible resistance and remain cool.

(ii) Fuse wire — property and design:

The fuse wire is made of a metal or alloy (e.g., aluminium, lead) with a low melting point. During normal operation, the current is within the rated value, so heat produced is insufficient to melt it. During a fault, excess current increases heat generation ($H = I^2Rt$) rapidly, raising the fuse wire's temperature above its melting point — it melts and breaks the circuit, protecting the appliances.

(iii) Why copper is preferred for connecting wires:

Copper has very low resistivity ($1.62 \times 10^{-8}$ Ω m), so its resistance is extremely small. This means minimal power is wasted as heat ($P = I^2R$) in the connecting wires, ensuring efficient transmission of electrical energy to the appliance.

Source: Chapter 11, Sections 11.5 and 11.7.1

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• Examiners look for explicit linking of $R = \rho l/A$ → high $\rho$ → high $R$ → high $P = I^2R$ for part (i). Just saying "resistance is high" without linking to resistivity loses marks.
• For part (ii), the key phrases are low melting point and the contrast between normal vs. fault current — both must appear.
• For part (iii), keep it brief: low resistivity → low resistance → less heat wasted. Don't repeat part (i) at length.
• Alloys are used for heating elements (not pure metals) because alloys have higher resistivity and don't oxidise at high temperatures — useful to mention if the question asks, but here focus on the contrast with copper.